ABV calculator: a wash’s strength from its gravity

Enter a wash’s starting and final hydrometer readings for its strength, by a formula that holds at the high gravities distilling washes start at.

How it works

Yeast turns sugar into alcohol and the gravity falls, so the drop between the two readings gives the strength. The usual homebrew rule, (starting − final) × 131.25, is good to about 7% but reads low above that: a wash from 1.120 to 0.990 is 18.8% here but 17.1% by the rule. This calculator uses the alternate formula after M. L. Hall (Zymurgy, 1995):

ABV = 76.08 × (OG − FG) ÷ (1.775 − OG) × FG ÷ 0.794

OG and FG are the starting and final readings. The ×131.25 figure is shown beside it, to compare with recipes and posts that use it.

Gravity drop is how much of the starting gravity has gone (apparent attenuation). Alcohol is lighter than water, so a wash fermented dry finishes below 1.000 and the drop goes over 100%.

Worked example

From 1.080 down to 0.995: 76.08 × 0.085 ÷ 0.695 × 0.995 ÷ 0.794 = 11.7% ABV. The ×131.25 rule gives 11.2%, and the gravity drop is 106.3%.

Good to know

• Correct both readings for temperature first: the hydrometer calculator does that.
• It suits sugar fermentations starting up to about 1.150 and finishing at 0.980 or above.
• Spin the hydrometer to shake off bubbles: gas from a fermenting wash lifts it and reads high.