Reference
Specific Gravity Test for Gold Coins: The 3-Minute Authentication
Specific gravity (SG) is the simplest non-destructive test for gold coin authenticity, requiring only a digital kitchen scale, a glass of water and a piece of thread. Pure 24-carat gold has a density of 19.32 g/cm³; 22-carat sovereign alloy is 17.71 g/cm³. Almost no other material lands close to these figures — lead is 11.34, brass is 8.5, copper is 8.96, steel is 7.85. The SG test catches 95%+ of counterfeit gold coin categories at home in three minutes, with no chemicals and no damage to the coin. The single weakness is tungsten (19.25 g/cm³, just 0.4% off pure gold) and high-quality struck-from-real-die forgeries. This guide covers the procedure, the formula, the tolerances, and the complementary tests that close the SG test’s blind spots.
Last updated: 22 June 2026
In brief. SG = W_dry / (W_dry − W_wet). Genuine UK sovereign (22-carat) =
17.65–17.78 g/cm³. Pure 24-carat gold (Britannia, Krugerrand) =
19.30–19.32. Tolerance: ±0.5%. Common fake alloys: lead 11.34, brass 8.5,
copper 8.96, steel 7.85 — all fail by a wide margin. Tungsten 19.25 (0.4% off pure gold)
is the SG test’s blind spot — combine with magnet, ring, edge and calliper tests for
full coverage. For coins £500+, professional grading remains the gold standard.
What is specific gravity?
Specific gravity is the density of an object relative to water. By the formal definition pure water at 4°C has SG = 1.000 exactly, and every other material is measured as a ratio against this reference. SG is dimensionless: a metal with SG 19.32 is 19.32 times denser than water.
The principle dates back to Archimedes of Syracuse (c. 250 BC) and is the original application of his "eureka" buoyancy discovery: an object submerged in water displaces a volume of water equal to its own volume, and the apparent loss of weight equals the weight of water displaced. Knowing the object’s mass in air and its apparent mass when submerged is enough to calculate its density without any direct volume measurement — ideal for irregular shapes like coins.
For coin authentication, the SG test is the single most useful non-destructive technique because the density of pure gold (19.32 g/cm³) and 22-carat gold alloy (17.71 g/cm³) are extreme enough to be hard to fake with any common counterfeit material except tungsten.
Reference SG values for coins and counterfeit alloys
| Material | SG (g/cm³) | Comment |
|---|---|---|
| Tungsten | 19.25 | Defeats SG test (0.4% off pure gold). Mainly an issue with bars and large coins. |
| Pure gold (24-carat / .9999) | 19.30 — 19.32 | Britannia, modern Krugerrand 24-carat, Maple Leaf, IoM angel |
| Platinum | 21.45 | Higher than gold; uncommon counterfeit material |
| 22-carat gold alloy (.9167) | 17.65 — 17.78 | UK sovereign, half sovereign, double sovereign, quintuple sovereign |
| 21-carat gold alloy | 17.0 — 17.4 | Some Krugerrand-era pieces |
| 18-carat gold alloy (.750) | 15.2 — 15.9 | Jewelry; not coinage |
| 14-carat gold alloy (.585) | 13.0 — 14.0 | Jewelry; not coinage |
| Lead (gold-plated) | 11.34 | Gap of 6+ from sovereign; immediate fail |
| Copper (gold-plated) | 8.96 | Gap of 8.7+ from sovereign; immediate fail |
| Brass / bronze (gold-plated) | 8.4 — 8.7 | Most common cheap counterfeit material; immediate fail |
| Steel (gold-plated) | 7.85 | Caught earlier by magnet test; SG fail also obvious |
| Aluminium | 2.70 | Theatrical "stage gold"; obvious fail |
| Pure silver | 10.49 | For reference; SG test also works on silver coins |
| Sterling silver (.925) | 10.36 | UK pre-1920 silver coinage |
| 0.500 silver (1920–1946) | 9.6 — 9.8 | UK wartime debased silver |
Reference values from the London Bullion Market Association and the Royal Society of Chemistry materials database. Carat purity uses the international assay convention.
The math: SG = W_dry / (W_dry − W_wet)
The single equation behind the entire test:
SG = Wdry / (Wdry − Wwet)
Where:
- Wdry — mass of the coin in air, measured directly on the scale (in grams).
- Wwet — apparent mass of the coin when fully submerged in water, suspended from the scale by a thread (in grams).
- Wdry − Wwet — the buoyant force on the coin, equal to the mass of water displaced. Because water’s density is 1 g/cm³, this also equals the volume of the coin in cubic centimetres.
- SG — the dimensionless density ratio. Multiply by 1.000 g/cm³ to get density in g/cm³.
Worked example: a sovereign weighs 7.988 g dry. Submerged in water, the scale reads 7.537 g (a 0.451 g loss to buoyancy). SG = 7.988 / (7.988 − 7.537) = 7.988 / 0.451 = 17.71 — squarely in the genuine 22-carat sovereign range.
Counter-example: a "sovereign" weighs 7.988 g dry but submerged reads 7.069 g (a 0.919 g loss). SG = 7.988 / 0.919 = 8.69 — the density of brass. The coin is gold-plated brass.
Tolerance and sources of error
For UK sovereigns the accepted tolerance is ±0.5% from the expected 17.71 figure, giving a working range of 17.62–17.80 g/cm³. Several factors influence practical precision:
- Surface tension on the suspension thread. The thread enters the water surface and creates a small downward pull from surface tension. This biases the apparent submerged weight downward and the calculated SG upward. Use a thinner thread (e.g. fishing line) to minimise.
- Air bubbles trapped on the coin surface. A typical sovereign with a milled edge can trap a few microlitres of air against the milling. These bubbles displace water and reduce the buoyant force, biasing SG upward. Tap the coin gently underwater to dislodge bubbles.
- Water temperature. Pure water at 4°C is exactly 1.000; at 20°C it’s 0.998; at 40°C it’s 0.992. The variation matters more for objects closer to water density; for gold (19+) the effect is <0.3% across the practical room-temperature range.
- Scale resolution. A 0.01 g scale on a 7.988 g coin gives 0.13% relative precision per reading. Compounded across both readings the SG calculation has approximately ±0.2% precision. A 0.1 g scale is too coarse for sovereign-sized coins; insist on 0.01 g resolution.
- Genuine wear and circulation loss. A heavily-circulated sovereign may have lost 0.05–0.1 g of metal to wear; the SG remains correct (alloy density is unchanged by wear), but the dry weight is reduced. Dry weight failure on an old circulated sovereign could mean wear, not a fake.
Kit needed
| Item | Specification | Approximate cost |
|---|---|---|
| Digital scale | 0.01 g resolution, 100 g+ capacity | £15 — £30 |
| Calibration weight | 5 g and 10 g brass | £5 — £10 |
| Glass of water | Tumbler, 200 ml+ | £0 (kitchen) |
| Thin thread / fishing line | Sub-0.5 mm; 6 lb test fishing line is ideal | £3 — £8 |
| Microfibre drying cloth | Lint-free, lens-cleaning grade | £3 — £6 |
| Calculator | Phone calculator app fine | £0 |
| Total | £25 — £55 |
Suitable scales are sold on Amazon UK as "milligram scale", "diamond scale" or "jewelry scale" by brands including Smart Weigh, Aweigh, Truweigh and ProAccurate. Always verify scale calibration with the included weight before testing — cheap scales drift over time, particularly after travel or temperature changes.
Step-by-step SG test procedure
The full procedure takes about 3 minutes per coin and is entirely non-destructive. Follow these steps in order; the order matters because dry weight needs to be measured first, before any contact with water.
- Weigh the coin in air. Place the coin directly on the digital scale at 0.01 g resolution. Record this reading as Wdry. For a genuine UK sovereign, expect 7.988 g ± 0.020 g. If the dry weight is out of spec, the coin has already failed the basic weight test and SG testing is unnecessary — reject the coin or escalate to professional grading.
- Set up the water suspension rig. Place a glass of clean (ideally distilled) water beside the scale, or on a platform above the scale’s pan if your scale has an under-pan hook. Tie a length of thin thread or fishing line through the coin (if the coin has a hole), or fashion a small thread loop around the coin’s edge. The thread should be long enough to suspend the coin while keeping the rest of the rig out of the water.
- Suspend the coin fully submerged. Lower the coin into the water until it’s completely submerged but not touching the walls or bottom of the glass. Tap the coin gently with the thread to dislodge any air bubbles clinging to the surface (particularly around the milled edge). Allow the water to settle for 5 seconds.
- Record the submerged weight. Read the digital scale display while the coin remains suspended in water. The reading will be approximately 0.45 g lower than the dry weight (for a genuine sovereign). Record this as Wwet. Repeat the measurement 2–3 times to confirm the reading is stable; if the reading drifts, you have an air-bubble or thread-tension issue.
- Calculate SG. Apply the formula SG = Wdry / (Wdry − Wwet). For a genuine 22-carat UK sovereign, expect a result of 17.65–17.78. Pure 24-carat gold reads 19.30–19.32. Anything below 17.5 or above 17.9 is outside the acceptable range and warrants further investigation.
- Dry the coin and verify. Remove the coin from water and dry thoroughly with the microfibre cloth, then air-dry for 2 minutes. Re-weigh the now-dry coin as a sanity check; the dry weight should match the original Wdry within 0.01–0.02 g. If the post-test weight differs more than that, you may have introduced contamination or moisture.
When the SG test fails
The SG test is necessary but not sufficient. Three scenarios where SG alone misleads:
- Tungsten-core gold-plated counterfeits. Tungsten (19.25) is just 0.4% off pure gold (19.32). A solid tungsten core gold-plated to .9999 fineness will test within the SG range of pure gold and pass an SG test alone. Practical note: tungsten-core fakes are mainly an issue with gold bars and large coins (1 oz+). Tungsten-core sovereigns specifically are rare because the small format makes the fabrication economics marginal — the cost of machining a tungsten core to within 0.05 g of the target weight, plus accurate gold plating, exceeds the typical £30–£50 numismatic premium of a bullion sovereign over melt. They do exist but are not the dominant counterfeit category.
- Cast-from-real-die forgeries struck in correct-fineness gold. A counterfeiter using legitimate 22-carat alloy and either casting from a captured Royal Mint die or striking from a high- quality fake die will produce a coin of correct weight, diameter and density. The SG test passes, but the coin is still counterfeit. Detection requires edge inspection (mould seam on cast pieces), die analysis under magnification, and ultimately professional grading.
- Genuine coins with adhered foreign material. A genuine sovereign with hardened wax, mounting epoxy, solder residue or polish on the surface will displace more water than the gold alone, biasing SG downward. The coin reads as a fake when it’s genuine but contaminated. Always inspect visually under magnification and gently clean any visible residue (warm soapy water only, never abrasive) before SG testing.
Combine SG with magnet, ring, edge and calliper tests
The SG test is one of five complementary authentication tests that together cover every counterfeit category except struck-from-real-die forgeries (which require professional grading). The full sequence:
- Magnet test (5 seconds). Hold a strong neodymium magnet near the coin. Gold and copper are non-ferrous; any pull means a steel or iron core. Catches plated-steel fakes immediately.
- Calliper diameter (10 seconds). 22.05 mm ± 0.05 mm for a sovereign. Cast counterfeits typically run 0.1–0.3 mm undersize because of cooling shrinkage in the cast.
- Edge inspection at 10x magnification (30 seconds). Cast counterfeits show a faint horizontal seam where the two halves of the mould met; struck genuine pieces never do. This is the single most reliable cast-fake detector.
- Ring / sound test (10 seconds). Balance the coin on a fingertip and tap with a second coin or pencil. A genuine sovereign rings clearly with a high-pitched silvery tone for ~2 seconds. Counterfeits typically thud or have a much shorter decay. Tungsten-core gold-plated coins ring noticeably differently from solid gold — sharper but die faster.
- Specific gravity test (3 minutes). The procedure above. Confirms metal density.
Combined, these five tests catch every counterfeit category except very high-quality struck-from-real-die forgeries. For coins worth £500+, escalate to PCGS, NGC or CGS UK professional grading, which adds expert die analysis, provenance research and encapsulation.
Featured sovereigns on MyCoinage
½ Sovereign - Elizabeth II 3rd portrait; Proof
up to £7,155
1 Sovereign - Elizabeth II Gold Sovereign Anniversary
up to £2,500
1 Sovereign - Elizabeth II 4th portrait; Golden Jubilee
up to £1,792
1 Sovereign - Charles III Bullion
up to £1,300
1 Sovereign - Elizabeth II 5th portrait; Platinum Jubilee
up to £1,078
1 Sovereign - Charles III
up to £1,057
Related guides
- How to Spot a Fake Gold Sovereign — the six-test sovereign authentication framework that includes the SG test.
- How to Authenticate a Coin — the wider authentication workflow for any UK coin.
- How to Spot Fake British Coins — the five-test framework for non-gold UK coinage.
- Sovereign Weight & Specifications — the dry-weight reference table the SG test starts from.
- Gold Sovereign Values UK — what authenticated sovereigns are actually worth.
- Junk Silver UK Coins — the SG test also works for silver authentication (see the table above).
- 1920 Silver Transition UK — sister technique guide on identifying pre-1920 sterling vs post-1920 .500 silver via density and other signals.
Frequently asked questions
What is specific gravity in coin authentication?
Specific gravity (SG) is the density of an object relative to water. It is calculated by comparing the object’s mass in air to its mass in air minus its apparent mass when fully submerged in water. The number is dimensionless: pure water at 4°C is 1.000 by definition, pure gold is 19.32, and copper is 8.96. For coin authentication, SG is the single most useful non-destructive test for identifying counterfeit gold coins because very few materials have a density close to gold’s 19.32 g/cm³. The only common counterfeit material that defeats the SG test is tungsten (19.25 g/cm³), which differs from gold by just 0.4%; everything else — lead, brass, gold-plated copper, gold-plated steel, gold-filled jewelry — fails by a margin large enough to detect with kitchen-scale precision.
What is the SG of a genuine gold sovereign?
A genuine UK sovereign is 22-carat gold (.9167 fineness) alloyed with copper. The expected SG is 17.65 to 17.78 g/cm³, centred around 17.71 g/cm³. Pure 24-carat gold is 19.32 g/cm³, and the addition of ~8.3% copper (8.96 g/cm³) lowers the alloy density to the 17.7 range. Tolerance: a sovereign that tests at 17.55 to 17.85 is within acceptable bounds for normal manufacturing variation and minor wear. A sovereign testing below 17.5 or above 17.9 is suspect and warrants further investigation. Pure 24-carat gold coins (Britannias, modern Krugerrands at 24-carat, Maple Leafs) test at 19.30–19.32 instead. Each fineness has its own expected SG band and confusing them produces false positives or negatives.
What density should I expect from common counterfeit alloys?
Five common counterfeit material categories and their densities. Pure gold (24ct): 19.32 g/cm³ (genuine reference). Tungsten: 19.25 g/cm³ (uncomfortably close, 0.4% off; defeats most SG tests; mainly an issue with gold bars and large coins). 22-carat gold alloy: 17.71 g/cm³ (genuine sovereign reference). Lead (gold-plated): 11.34 g/cm³ (huge gap, easy to detect). Brass / bronze (gold-plated): 8.4–8.7 g/cm³ (massive gap, immediate fail). Steel core (gold-plated): 7.85 g/cm³ (also defeated by simple magnet test before SG even matters). Copper (gold-plated): 8.96 g/cm³. Of all these, only tungsten requires further authentication beyond the SG test. The other four fail the test by a wide margin.
How do I do an SG test at home?
You need three pieces of kit: a digital kitchen or jewelry scale with 0.01 g resolution (around £15–£30 from Amazon, sold as "diamond scales" or "milligram scales"); a small container of water (a tumbler glass works); and a thin thread or fishing line long enough to suspend the coin. The procedure is: weigh the coin dry on the scale, record the value as W_dry. Suspend the coin from the scale’s above-pan hook (or balance the scale on a raised platform with the cup of water below) so the coin hangs fully submerged in water without touching the cup walls or bottom. Record the new "submerged" reading as W_wet. Calculate SG = W_dry / (W_dry − W_wet). For a genuine 22-carat sovereign, expect 17.65–17.78. The full step-by-step is below in the procedure section.
What tolerance should I accept on the SG result?
For UK sovereigns the accepted tolerance is ±0.5% from the expected 17.71 figure, which gives a working range of 17.62 to 17.80 g/cm³. Several factors widen the practical tolerance: (1) Surface tension on the thread can lift the apparent submerged reading and bias SG slightly low (use a thinner thread to reduce). (2) Water temperature variations; water at 20°C is 0.998 instead of 1.000 (very minor effect on a coin-sized object). (3) Air bubbles trapped on the coin surface displace water and reduce the calculated SG (tap the coin gently underwater to dislodge). (4) Scale resolution: at 0.01 g resolution on a 7.988 g coin, your maximum precision is about 0.13% per reading, so SG precision is around ±0.2%. With these in mind, accept 17.55–17.85 as "passes for further consideration"; reject anything outside.
Can the SG test detect tungsten-core fakes?
No — this is the single biggest limitation of the SG test. Tungsten has a density of 19.25 g/cm³ against pure gold’s 19.32; a ratio of 0.9964. A solid tungsten core gold-plated to .9999 fineness will test within the SG range expected for a pure-gold piece, and an SG test alone will not catch it. Tungsten-core fakes are mainly an issue with gold bars and large coins (1 oz+) where the difference matters and the labour cost of fabrication is justified. Tungsten-core sovereigns specifically are rare because the small (7.988 g) format makes the fabrication economics marginal: the cost of machining a tungsten core to within 0.05 g of the target weight, plus accurate gold plating, exceeds the £30–£50 numismatic premium of a bullion sovereign over melt. They do exist, however, and for high-value sovereigns the SG test should be combined with at least one other authentication test.
What other tests should I combine with SG?
Four complementary tests cover the gaps in SG. (1) Magnet test: gold and copper are both non-ferrous; any magnetic pull from a strong rare-earth (neodymium) magnet means a steel or iron core. Catches plated-steel fakes that the SG test would also catch, but the magnet is a 5-second screen before bothering with water. (2) Ring / sound test: balance the coin on a fingertip and tap; a genuine sovereign rings clearly with a high-pitched silvery tone for ~2 seconds; counterfeits typically thud or have a much shorter decay. Tungsten-core gold-plated coins ring noticeably differently from solid gold (they ring sharper but die faster) — an experienced ear can pick this up. (3) Edge inspection at 10x magnification: cast counterfeits show a horizontal seam where two halves of the mould met; struck genuine pieces never do. (4) Calliper measurement: 22.05 mm ± 0.05 mm; cast counterfeits typically run undersize because of cooling shrinkage. Combined, these tests catch every counterfeit category except very high-quality struck-from-real-die fakes, which require professional grading.
When does the SG test fail to catch a fake?
Three failure modes. (1) Tungsten-core gold-plated fakes (described above) match the SG of pure gold to within 0.4%. (2) Cast-from-real-die forgeries struck in correct-fineness gold. If a counterfeiter sources legitimate 22-carat alloy, casts (or strikes) the coin from a real Royal Mint die, and produces a coin of correct weight, diameter and density, the SG test will pass. The forgery is detectable only through edge inspection (mould seam on cast pieces), die analysis under magnification (subtle wear or irregularities), or professional grading. (3) Genuine coins with adhered foreign material — a genuine sovereign with hardened wax, mounting epoxy or solder residue stuck to one face will test below the expected SG range and could be mistaken for a fake. Always inspect visually and clean any visible residue before SG testing. The SG test is therefore necessary but not sufficient: use it as the first non-destructive screen, then escalate doubtful results to professional grading.
How accurate does the scale need to be?
For sovereign-sized coins (~8 g) you want a scale with at least 0.01 g resolution. A 0.1 g resolution scale gives only 1.25% precision per reading, which compounds to 2.5%+ uncertainty in the SG calculation — not enough to discriminate 17.71 (genuine 22-carat) from 19.25 (tungsten) reliably enough for confidence at the boundaries. A 0.01 g resolution scale gives 0.13% per reading, compound 0.25%, well within the acceptable tolerance. Suitable scales are widely sold on Amazon UK as "milligram scale", "diamond scale" or "jewelry scale" for £15–£30; brands include Smart Weigh, Aweigh, Truweigh and ProAccurate. Always verify scale calibration with a 5 g or 10 g calibration weight before testing — unverified scales drift over time. For coins above £1,000 value, consider professional weighing on a calibrated trade scale or sending to a grader.
Is the SG test destructive?
No. The SG test is entirely non-destructive: the coin is briefly submerged in clean water, then dried and returned to its holder. No chemicals, no abrasion, no permanent change to the coin. This is one of the SG test’s primary advantages over older authentication techniques like the acid-test (which permanently scars the coin) or the touchstone test (which leaves a small abrasion mark). For collectible coins the SG test is the recommended first authentication step precisely because it leaves no trace. Two practical tips: (1) use distilled water rather than tap water if you’re testing a particularly delicate proof or toned coin, to avoid mineral residue; (2) dry the coin thoroughly with a soft microfibre cloth and air-dry afterwards to prevent any water-mark on the surface.
Should I do the SG test before or after weighing?
Always weigh in air first. The standard procedure is: weigh dry, record W_dry; then weigh submerged, record W_wet; then calculate SG. Doing it in this order means you only handle the coin twice (once to place on scale dry, once to suspend in water), and you avoid the risk of water residue affecting your dry weight if you reverse the sequence. If your dry weight is itself out of spec (a sovereign should be 7.988 ± 0.020 g) you have already failed the basic weight test and the SG test isn’t needed — the coin is rejected. If the dry weight is in spec, proceed to the submerged measurement. After the submerged reading, return the coin to its holder before drying, then weigh dry one final time as a sanity check that nothing has changed. The whole procedure takes 3 minutes per coin including drying.
How does SG testing compare to XRF and professional grading?
SG testing is free after the £15–£30 scale, takes 3 minutes per coin, is entirely non-destructive, and catches 95%+ of counterfeit categories (every category except tungsten-core and high-quality struck forgeries). XRF (X-Ray Fluorescence) testing available at some Royal Mint-authorised dealers and most jewellers costs £15–£30 per coin, takes under a minute, identifies the actual metal composition rather than just density, and catches tungsten-core fakes that defeat SG. XRF cannot read more than ~50µm below the surface, however, so a thick gold plating over a tungsten core can defeat it. Professional grading (PCGS, NGC, CGS UK) costs £25–£50 per coin, takes 4–12 weeks, includes expert visual examination of dies, edge profile, weight, dimensions and provenance research, and is the only test method that catches struck-from-real-die forgeries. For coins worth £500+ professional grading is the appropriate level. For everyday bullion screening, SG is the right tool. See our how to authenticate a coin guide for the wider authentication workflow.
Read next