Alcoholmeter and proof hydrometer: A proof and tralle hydrometer reads alcohol strength directly in both proof and percentage on the same scale, which removes the need to convert by hand during a run.
The Quick Answer
In the United States, proof is exactly twice the alcohol by volume percentage. To convert proof to ABV you divide by two. To convert ABV to proof you multiply by two. That is the whole rule.
So 100 proof is 50% ABV, 80 proof is 40% ABV, and 40% ABV is 80 proof. If you only need the US conversion, you can stop here. The rest of this guide covers worked examples, a full chart, and the difference between the US and the older British proof system, which uses a different ratio.
The Conversion Formula
The US definition fixes proof as a simple multiple of ABV. This means no measurement or lookup table is strictly necessary. The arithmetic works in both directions with nothing more than doubling or halving.
Converting proof to a percentage: divide the proof figure by two.
Converting a percentage to proof: multiply the ABV by two.
Because the relationship is exact and linear, the conversion is the same whether you are working with a cask strength spirit at 130 proof or a diluted bottling at 80 proof. There is no rounding or correction involved in the US system.
Worked Examples
The values below are the ones distillers and drinkers most often look up. Each uses the same divide by two or multiply by two rule.
Proof to ABV
A spirit at 100 proof is 100 ÷ 2 = 50% ABV. A spirit at 138 proof is 138 ÷ 2 = 69% ABV. A spirit at 150 proof is 150 ÷ 2 = 75% ABV. A standard 80 proof bottle is 80 ÷ 2 = 40% ABV.
ABV to proof
A spirit at 40% ABV is 40 × 2 = 80 proof. A spirit at 55% ABV is 55 × 2 = 110 proof. A spirit at 70% ABV is 70 × 2 = 140 proof. A spirit at 75% ABV is 75 × 2 = 150 proof.
If you would rather not do the arithmetic during a run, the proof converter handles any value instantly and shows both US and UK proof at once.
Conversion Chart
The chart below covers the full range from low strength to near azeotrope, using the US definition where proof equals twice the ABV.
| ABV (%) | US proof |
|---|---|
| 5% | 10 proof |
| 10% | 20 proof |
| 20% | 40 proof |
| 30% | 60 proof |
| 40% | 80 proof |
| 43% | 86 proof |
| 46% | 92 proof |
| 50% | 100 proof |
| 55% | 110 proof |
| 57.1% | 114.2 proof |
| 60% | 120 proof |
| 62.5% | 125 proof |
| 65% | 130 proof |
| 69% | 138 proof |
| 70% | 140 proof |
| 75% | 150 proof |
| 90% | 180 proof |
| 95% | 190 proof |
US Proof vs UK Proof
The simple divide by two rule only applies to the US system. The older British proof system, sometimes called degrees proof or sikes proof, used a different reference point. Under that system, 100 British proof was approximately 57.1% ABV rather than 50%.
The two systems therefore give very different numbers for the same spirit. A spirit at 40% ABV is 80 US proof but only about 70 degrees British proof. The UK moved to labelling by ABV percentage and no longer uses the proof system for official purposes, but British proof values still appear on older bottles, in historical recipes, and occasionally in writing about spirits.
| ABV (%) | US proof | British proof (approx) |
|---|---|---|
| 40% | 80 | 70.0 |
| 50% | 100 | 87.5 |
| 57.1% | 114.2 | 100.0 |
| 60% | 120 | 105.0 |
If a value is labelled simply as proof with no country given, it is almost always US proof in a modern context. British proof has not been in official use for decades.
Why Proof Exists
Proof predates accurate instruments for measuring alcohol content. It began as a method of verifying that a spirit was strong enough to be taxed at the higher rate for stronger liquor. In Britain the test involved soaking gunpowder in the spirit and attempting to ignite it. Spirit that allowed the powder to burn was considered to have been proved, which is the origin of the term.
That gunpowder threshold corresponded to roughly 57.1% ABV, which is why the British system fixed 100 proof at that strength. The United States later adopted a cleaner definition, setting proof at exactly twice the ABV, which removed the awkward ratio and made conversion trivial. Today proof is a labelling and marketing convention rather than a measurement method, since alcohol strength is measured directly with a hydrometer or alcoholmeter and expressed as ABV.
Frequently Asked Questions
In the US system, 100 proof is 50% ABV. US proof is exactly twice the alcohol by volume percentage, so you divide the proof by two to get the ABV. 100 divided by 2 is 50, so 100 proof equals 50% ABV.
In the US system, divide the proof by two. A spirit at 80 proof is 40% ABV, and one at 120 proof is 60% ABV. To go the other way, multiply the ABV by two to get the proof. This simple relationship only applies to US proof. The historical UK proof system uses a different ratio.
No. US proof is defined as twice the ABV, so 100 US proof is 50% ABV. The older UK or British proof system defined 100 proof as roughly 57.1% ABV, based on a gunpowder test rather than a simple multiple. The UK system is no longer used for official labelling, but you may still see British proof values on older bottles or in historical recipes.
In the US system, 150 proof is 75% ABV. Divide the proof by two: 150 divided by 2 is 75. A 150 proof spirit is a high strength spirit, well above standard bottling range and close to the strength at which many spirits come off the still.
Proof originated as a way to verify the alcohol content of spirits for taxation, long before accurate instruments existed. In Britain, spirit was tested by soaking gunpowder in it and checking whether the powder would still ignite. Spirit that passed was proved. The US later simplified its definition so that proof is exactly twice the ABV percentage.