ABV vs proof: 40% ABV is 80 US proof, 70 old UK proof

The gunpowder test is a story, not a measurement

The story told about the word is that spirit was poured on gunpowder and lit. If the powder still burned, the spirit had proved itself. If it fizzled, the spirit was under proof.

It makes a good story and a poor instrument, because how damp powder burns depends on the powder and the conditions as much as the spirit. The Excise was using hydrometers long before that. The Scotch Whisky Association, in answers reproduced at the University of Edinburgh, a secondary copy, records Customs and Excise taking up Clark’s hydrometer in the 1740s, and a more accurate one by Bartholomew Sikes replacing it. A hydrometer is a weighted float. Alcohol is lighter than water, so the stronger the spirit, the deeper the float sinks.

Sikes’s hydrometer made a little over 57% the definition of 100 proof

Difford’s Guide says Bartholomew Sikes invented an accurate hydrometer in 1816 and that his scale was adopted into law in 1818. The Oxford Companion to Spirits and Cocktails also gives 1818, and the Scotch Whisky Association names the Hydrometer Act of 1818 for Sikes’s instrument, which stayed in standard use until 1980. The Customs and Excise Act 1952 defined proof spirit by weight: at 51°F, it weighed exactly twelve thirteenths as much as the same volume of distilled water.

That mixture is a little over 57% alcohol by volume: 57.1% in the Scotch Whisky Association’s answer and 57.142857% in Difford’s Guide, which defines it as four sevenths alcohol by volume. It was fixed as 100 degrees proof.

Stronger spirit was over proof and weaker spirit under proof. The scale is proportional, so 70 degrees proof holds seven tenths of the alcohol in proof spirit, which is 40% ABV. That is where the multiplier of 1.75 comes from.

Britain switched to ABV on 1 January 1980

On 1 January 1980 Britain adopted the system recommended by the International Organisation of Legal Metrology, as the rest of the European Community did: strength as a percentage of alcohol by volume at 20°C. It replaced the Sikes system after more than 160 years.

Degrees proof survives on older bottles, in older books and in naval history. The Royal Navy’s daily rum issue ended with the last tot on Black Tot Day in 1970.

Most spirits are bottled at 40%, and some at the 37.5% minimum

Most spirits are bottled at 40% ABV. Rum, gin and vodka can legally go down to 37.5%, and some brands bottle at exactly that, because UK duty is charged on the pure alcohol in the bottle and every point below 40 lowers the bill. The difference is small on paper and shows in the glass, most of all once ice or a mixer starts to dilute the drink.

Navy strength is a trade term, not a legal one

Gin sold as navy strength is usually 57% ABV, the old 100 degrees proof. The Oxford Companion to Spirits and Cocktails says the Royal Navy in fact bought its spirits at 5.5 degrees under proof, 54.5% ABV, and Difford’s Guide says labels at 57% have confused English proof with navy strength. Navy strength gin is bottled at 57%, as Difford’s lists Plymouth Navy Strength. Rum sold as navy strength has no figure set in the rum category of Regulation (EC) No 110/2008.

The usual explanation ties the strength to gunpowder stowed near the rum on wooden ships: spirit at proof, spilled on powder, would not stop it firing. Like the proof test itself, it is naval tradition rather than record.

Overproof first meant stronger than 100 degrees proof, above 57.14% ABV. It is now used loosely for anything well above standard strength. Jamaican white overproof rum is usually 63%, and the 151s are 75.5%.

Cask strength means bottled without adding water

Cask strength, or barrel proof in America, means the spirit was bottled at the strength it reached in the barrel, without dilution to a standard figure. That usually lands between 50 and 65% ABV, depending on the cask and the climate. Add water a few drops at a time until it sits where you want it.