110 Proof equals 55% ABV
US scale. US proof is exactly twice the alcohol by volume.
What 110 proof means
In the United States, proof is set at exactly twice the alcohol by volume. A spirit labelled 110 proof therefore contains 55% ABV. To go the other way, multiply the ABV by 2.
The same figure is sometimes written as "110 proof to percent", which asks the same question. The percent here is percent alcohol by volume, so the answer is still 55%.
The UK once used a separate proof scale where 100 degrees proof sat at roughly 57.1% ABV. Spirits sold today are labelled in ABV almost everywhere, so the US two to one rule is the one you will use most.
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.