Inch Fraction Calculator
Decimal inches to fractions like 5/16, and adding feet-and-inch measurements.
Good to know. Tape measures mark sixteenths because 1/16 inch is about the width of a pencil line.
How it works
In plain words
Multiply the decimal part by the denominator you want (16 for sixteenths), round to a whole number, and simplify: 0.3125 × 16 = 5, so 5/16. To add measurements, turn each into inches (feet × 12 plus inches), add them up, and turn the total back.
The formula
fraction = round(decimal × denominator) ÷ denominator inches = feet × 12 + inches · 1 in = 25.4 mm
Worked example
0.3125 × 16 = 5, so 0.3125 inches is 5/16″, which is 7.94 mm.
Sources
No outside data or rates: this is plain arithmetic.
6 automated tests check its answers on every change.
About the inch fraction calculator
US and British tape measures, rulers and drill bits are marked in fractions of an inch, while plans, calipers and calculators give decimals or millimetres. This calculator translates between them: type a decimal like 0.3125 or a metric size like 8 mm, and it gives the nearest fraction on a tape measure, 5/16″.
Choose how fine to round. Sixteenths suit most carpentry, eighths rougher work, and thirty-seconds or sixty-fourths machining. The calculator shows how far the fraction is from the exact value, so you know whether the rounding matters.
The second mode adds and subtracts measurements in feet, inches and fractions, the way you would on a cut list: 5′ 3½″ plus 2′ 7¾″ minus 1′ ⅛″. Type each measurement on its own line, in forms like 5' 3 1/2", 5 ft 3-1/2 in or just 63.5, and start a line with a minus to subtract it. The total comes out in feet and inches with a fraction.
An inch is exactly 25.4 mm. Handy equivalents: 1/16″ ≈ 1.6 mm, 1/8″ ≈ 3.2 mm, 1/4″ ≈ 6.4 mm, 1/2″ = 12.7 mm, and 3/4″ ≈ 19 mm, the thickness of a typical board.
Questions people ask
What is 0.3125 as a fraction of an inch?
5/16″. Multiply by 16 to get 5, so it is five sixteenths.
What fraction of an inch is 8 mm?
About 5/16″ (7.94 mm). 8 mm is 0.315 inches.
How do I add feet and inches?
Add the inches and the feet separately, then carry every 12 inches into a foot: 5′ 9″ + 2′ 7″ = 7′ 16″ = 8′ 4″.