What formula is used for US Proof to UK Proof?

Both scales are normalized through ABV percent, then converted using a fixed ratio.

Is the mirror direction available?

Yes. Use the switch button or open the UK Proof to US Proof page.

Can I use decimal inputs?

Yes. Decimal strength values are supported for lab, labeling, and mixology use cases.

proof (US) to proof (UK)

1 US Proof = 0.874891 UK Proof · fixed factor via canonical reference constants · no offset

Direct Answer

1 US Proof equals 0.874891 UK Proof

This conversion uses a fixed factor based on canonical reference constants.

For 0.25 US Proof, the result equals 0.218723 UK Proof.

Converter Calculator

US Proof (proof (US))

1 US Proof (proof (US)) =
0.874891 UK Proof (proof (UK))

Explanation

Multiply by 0.87489063867 because 1 proof (US) corresponds to 0.5% ABV and 1 proof (UK) corresponds to 0.5715% ABV. US proof is defined as 2 times ABV. 100 UK proof equals 57.15% ABV.

From US Proof to UK Proof, the calculator applies one stable fixed-factor model derived from canonical reference constants.

The same constants drive calculator output and common-value rows, so quick checks and manual calculations remain consistent.

Method & Reference (Verified)

Method basis: US Proof to UK Proof uses the page-specific conversion model shown in Direct Answer.
Applied factor: 1 US Proof = 0.87489063867 UK Proof (as defined for this route).
Consistency rule: calculator output and table values use the same constants and rounding policy.

Common Conversion Values

US Proof (proof (US))	UK Proof (proof (UK))
0.25	0.218723
0.5	0.437445
1	0.874891
5	4.374453
12	10.498688
20	17.497813
40	34.995626
57.15	50
80	69.991251
100	87.489064