Plans give 3.3125 in, the tape measure reads 3-5/16 and the metric drawing says 84.1 mm. This converter moves between all three in either direction, accepts feet-and-inches input like 5′ 3-5/16″, and rounds to the nearest 1/8, 1/16, 1/32 or 1/64 while telling you how far off the rounded value is. The chart below covers every sixteenth of an inch.
Fraction ↔ decimal calculator
How it works
- Fraction to decimal: whole number + numerator ÷ denominator; feet × 12 are added first
- Decimal to fraction: multiply by the denominator you want (16, 32, 64), round to the nearest whole number, then reduce the fraction
- Millimetres: 1 in = 25.4 mm exactly (NIST SP 811)
The “off by” figure is the rounding error. At 1/16 it can be up to 1/32 in (0.8 mm); at 1/64 up to 0.2 mm, which is finer than most tapes can be read.
Worked example
A cabinet opening measured at 5 ft 3-5/16 in:
- Convert: Fraction or feet-inches to decimal and mm
- Value: 5′ 3-5/16"
- Round fractions to: nearest 1/16 (tape measure)
The calculator gives:
- Nearest 1/16: 63-5/16" (exact)
- Decimal inches: 63.3125 in
- Millimetres: 1,608.14 mm
- Feet and inches: 5′ 3-5/16"
- Exact fraction check: Value is an exact 1/16
That is 63.3125 in, or 1,608.14 mm. Going the other way, a plan dimension of 0.8 in becomes 13/16 in on the tape, which is 0.0125 in (about 1/3 mm) over: fine for carpentry, worth noting for joinery.
Inch fractions to decimal and millimetres
| Fraction | Decimal in | mm |
|---|---|---|
| 1/16" | 0.0625 | 1.588 |
| 1/8" | 0.125 | 3.175 |
| 3/16" | 0.1875 | 4.763 |
| 1/4" | 0.25 | 6.35 |
| 5/16" | 0.3125 | 7.938 |
| 3/8" | 0.375 | 9.525 |
| 7/16" | 0.4375 | 11.113 |
| 1/2" | 0.5 | 12.7 |
| 9/16" | 0.5625 | 14.288 |
| 5/8" | 0.625 | 15.875 |
| 11/16" | 0.6875 | 17.463 |
| 3/4" | 0.75 | 19.05 |
| 13/16" | 0.8125 | 20.638 |
| 7/8" | 0.875 | 22.225 |
| 15/16" | 0.9375 | 23.813 |
| 1" | 1 | 25.4 |
Notes and limits
- Type fractions with a hyphen or a space: 3-5/16 or 3 5/16. Decimals with or without a leading zero work (.75).
- Feet can be written 5′, 5 ft or 5 feet; inches as 3″, 3 in or nothing.
- For woodworking, 1/32 is a practical limit for a tape; 1/64 is for machinist rules and calipers.
- When a plan mixes fractions and decimals, convert everything to decimal inches, do the arithmetic, and convert back once.
Frequently asked questions
What is 5/16 as a decimal?
0.3125 in, which is 7.94 mm.
What is 0.75 as a fraction of an inch?
3/4 in exactly (19.05 mm).
How do you read 1/16 on a tape measure?
The smallest marks on most tapes are sixteenths: 16 to the inch. Count from the last inch mark; 5 small marks past 3 is 3-5/16 in.
How many millimetres in 1/8 inch?
3.175 mm. A 3 mm allowance is a hair under 1/8 in; 3.2 mm is a hair over.
What is 84 mm in inches?
3.307 in, which rounds to 3-5/16 in on a tape (3.3125 in, about 0.14 mm over).
Sources
Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.