Metric to Standard Converter
Convert metric millimeters to the nearest standard inch fraction, or inches to millimeters. Enter a size in mm or inches and choose how fine to round the fraction.
Example: with Size 13 · Entered in Millimeters (mm) · Round to Nearest 1/16 in → Standard size: 1/2 in (standard).
- Exact conversion0.5118 in (13 mm)
- RoundingRounded to 1/16 in, 0.30 mm off
Computed by the calculator below using its default values. Change any input to see your own numbers.
One inch is exactly 25.4 mm. Millimeters convert to a decimal inch, then round to the nearest chosen fraction (1/16 by default) reduced to lowest terms.
Millimeters to standard inches
In the US, standard measurements are inches broken into halves, quarters, eighths, sixteenths, and thirty-seconds. Metric sizes are decimals of a millimeter. To read a metric size as a standard one, convert to a decimal inch — divide millimeters by 25.4 — then round to the nearest fraction on your ruler.
How fine you round changes the answer. A 13 mm size is 0.512 inch, which rounds to 1/2 inch at sixteenth resolution but to 33/64 at a finer step. Sixteenths are the everyday choice for tape measures and wrenches; thirty-seconds suit machining and drill bits. The tool shows both the fraction and the exact decimal so you can judge how much you are giving up to the rounding.
How it’s calculated
Inch = mm ÷ 25.4 (1 in = 25.4 mm exactly). The decimal inch is rounded to the nearest 1/16, 1/32, or 1/8, then the fraction is reduced to lowest terms. Reverse conversion multiplies inches by 25.4. The rounding gap is reported in mm.
The fraction is a nearest-rounded approximation. For precision work, use the exact decimal rather than the rounded fraction.
Metric to standard (nearest 1/16 in)
| Metric | Standard | Decimal in |
|---|---|---|
| 3 mm | 1/8 in | 0.118 |
| 6 mm | 1/4 in | 0.236 |
| 10 mm | 3/8 in | 0.394 |
| 13 mm | 1/2 in | 0.512 |
| 19 mm | 3/4 in | 0.748 |
| 25 mm | 1 in | 0.984 |
Computed as mm ÷ 25.4, rounded to the nearest 1/16 inch (1 in = 25.4 mm exact).
Common mistakes
- Reading the rounded fraction as exact — 13 mm is 0.512 inch, a hair over 1/2 inch, not exactly 1/2.
- Rounding to sixteenths when the job needs thirty-seconds or the exact decimal.
- Using 25 mm as one inch; the exact value is 25.4 mm, so 25 mm is about 0.984 inch.
Frequently asked questions
What is 13 mm in standard inches?
13 mm ÷ 25.4 = 0.512 inch, which rounds to 1/2 inch at sixteenth resolution. The exact value is a little over 1/2 inch.
What is the formula for mm to inches?
Inches = millimeters ÷ 25.4. To reverse it, multiply inches by 25.4 to get millimeters.
Why does the fraction change when I pick a finer step?
Rounding to 1/32 lets the fraction land closer to the true decimal than 1/16 does, so a size may show as 33/64 instead of a round 1/2.
How many millimeters is one inch?
Exactly 25.4 mm. That definition, set in 1959, makes every inch-to-mm conversion exact.