The math

How to calculate the proof of a whiskey blend

2 minute read · The Blending School

Blend proof is one formula: each pour's volume times its proof, summed, divided by total volume. Everything else in blending math is this formula wearing different hats.

A worked example

4 ounces of a 90 proof bourbon and 2 ounces of a 125 proof barrel-proof pour:

Notice the blend lands closer to 90 than to 125, because there is twice as much of the softer whiskey. The average is weighted by volume, always. A blend can never land outside the range of its parts: mix 90 and 125 and you will get something between 90 and 125, no exceptions. That fact is a useful sanity check on any calculation.

Pouring out changes nothing

Here is the counterintuitive one: drinking a glass does not change the proof. The pour you remove has the same composition as the bottle, so the ratio of alcohol to water in what remains is untouched. Volume falls, proof holds. Only adding something at a different proof, including water, moves the number.

Adding a pour to an existing bottle

Same formula, treating the whole bottle as one ingredient. A 20 ounce bottle at 106.7 plus 2 ounces at 124.9:

The fine print

Strictly speaking, ethanol and water contract slightly when mixed, so proof by volume is a hair off from proof by mass. For whiskey into whiskey the error is negligible, well below what any home measurement can detect. It matters slightly more when adding plain water, which is covered in dilution. For an infinity bottle, the volume-weighted average is the right tool, and it is the math Thief & Blend runs live before every pour is committed.