US proof is exactly twice the alcohol by volume. To read ABV from proof, divide by 2. To read proof from ABV, multiply by 2. Pick a common value below, or open the interactive converter for any number, including conversions to the separate UK proof scale.
These reference values cover the proof and ABV figures most commonly printed on spirit labels and referenced in recipes. For anything outside this list, or for UK Sikes proof conversions, the interactive converter linked above handles any value directly.
Proof to ABV
ABV to Proof
Confirm the real strength: proof printed on a label is one thing, the spirit in your jar is another. A 0 to 200 proof alcoholmeter reads it directly at 20°C.