Proof and ABV, and how to measure your spirit

ABV: alcohol by volume

ABV is the share of a liquid's volume that is pure ethanol, measured at a standard temperature (20 °C in most of the world). A 700 mL bottle at 40% ABV holds 280 mL of pure alcohol. It is the measure used on labels almost everywhere, and the one to use in your own records.

You may also meet ABW - alcohol by weight. Because ethanol is lighter than water, the ABW of a spirit is lower than its ABV; it's rarely used outside science and some regulations.

Proof: two different scales

US proof is simply twice the ABV: 40% ABV is 80 proof, and 100 proof is 50% ABV.

British proof, used in the UK until 1980, came from an old test: spirit was "proved" if gunpowder soaked in it would still ignite. On that scale 100° proof is about 57.15% ABV, so British proof is roughly 1.75 times the ABV - and 40% ABV is 70° proof. You'll still meet it in older books and on some naval rums.

Measuring a wash: the hydrometer

A hydrometer measures density - specific gravity - not alcohol. Dissolved sugar makes a wash denser; as yeast turns sugar into alcohol, which is lighter than water, the gravity falls. So the strength of a wash comes from two readings: the original gravity (OG) before fermentation, and the final gravity (FG) when it has finished.

• Take readings in a tall trial jar, with the hydrometer floating freely, and read at the level of the liquid's surface.
• Degas a fermenting sample first: bubbles clinging to the hydrometer push it up.
• Most hydrometers are calibrated at 20 °C (some at 15.6 °C / 60 °F); correct warmer or cooler readings, or cool the sample first.
• A sugar wash typically finishes around 0.990–0.995. Washes with unfermentable solids, such as molasses, finish higher.

Measuring spirit: the alcoholmeter

An alcoholmeter looks like a hydrometer but is scaled directly in ABV for mixtures of alcohol and water. It reads spirit straight off the still, a jar of hearts or a finished bottle - but only if there is nothing else dissolved in it. Sugar, as in a wash or a liqueur, makes it read wrong.

• Use a trial jar tall and wide enough for the alcoholmeter to float without touching the sides.
• Spin it gently to dislodge bubbles, and read at eye level.
• Many distillers float one in a small cylinder under the still's outlet - a parrot - to watch the strength of the spirit as it runs.

Correcting for temperature

Alcohol expands more than water when warmed, so a warm sample reads high and a cool one reads low. At drinking strengths the error is roughly a third of a percentage point for every degree away from the calibration temperature - small on one reading, but enough to push a bottle off its label. Use the correction table supplied with your alcoholmeter, or bring the sample to its calibration temperature (usually 20 °C) before reading.

Diluting to bottling strength

Spirit is usually collected well above the strength you want to drink it at, so it needs diluting - "proofing down". Use clean, soft water: distilled, deionised or a low-mineral spring water, since minerals can cloud the spirit.

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