mm to Inches Converter
Convert millimeters to inches and back in one step, with the exact decimal result plus the nearest fractional inch (down to 1/64 in) and the rounding error. Just type your value in either field.
Convert Millimeters to Inches — and Inches Back to Millimeters
mm to Inches Formula
Where:
- \(\text{millimeters}\) — length in mm
- \(\text{inches}\) — the same length in inches
- \(25.4\) — the exact number of millimeters in one inch
Both formulas are exact, because the inch has been defined as exactly 25.4 mm by international agreement since 1959.
mm to Inches – Calculation Example
Example: you measure a bolt shaft as 25 mm long and need to express it in inches, including the nearest 1/64-inch fraction.
\[ 25 \div 25.4 = 0.984252 \text{ in} \]
Then round to the nearest 1/64 in: \(0.984252 \times 64 \approx 63\), so 25 mm ≈ 63/64 in (0.984375 in).
Result: 25 mm = 0.984252 in exactly = 63/64 in to 1/64 precision; the fraction overshoots the exact value by about +0.000123 in (+0.00313 mm).
How to use the calculator
- Type your length into the Millimeters (mm) field or the Inches (in) field — the converter follows whichever field you edited last and fills in the other one automatically.
- Set Fractional-inch precision to 1/8, 1/16, 1/32 or 1/64 in to control how the fractional-inch result and its rounding error are rounded.
- Read the Conversion Results: the value in both units, the exact inches, the nearest fraction (e.g. 63/64 in instead of 0.984252 in), and how far the fraction is from the exact decimal.
- Scroll down for the ready-made tables: mm 1–100 in inches, fractional inches 1/64–1 in in mm, and common drill bit and wrench sizes in both systems.
Millimeters and inches explained
The millimeter belongs to the metric system and the inch to the imperial/US customary system, so drawings, tools and parts from different countries often mix the two. The link between them is fixed by definition: one inch equals 25.4 mm exactly. That makes conversion a simple multiplication or division with no hidden rounding — dividing by 25.4 takes you from mm to inches, multiplying by 25.4 takes you back.
The decimal result is the mathematically exact answer, and the calculator shows it with up to six decimal places. In practice — machining, woodworking, construction — lengths are often given in fractional inches instead, because rulers, tapes and calipers in inch countries are graduated in fractions such as 1/8, 1/16 or 1/64 in. That is why the tool also reports the nearest fraction at your chosen precision.
The fraction rounding error tells you how much you give up by using the fraction instead of the exact decimal. In the default example, 63/64 in is 0.000123 in (about 3 micrometers) longer than the true value of 25 mm — negligible for woodworking, but possibly significant in precision machining. If the error exceeds your tolerance, switch to a finer precision (e.g. from 1/8 to 1/64) or work with the decimal figure directly.
For everyday reference: 1 mm ≈ 0.03937 in, 10 mm ≈ 0.3937 in (close to 25/64 in), 25 mm ≈ 1 in minus a hair, and 100 mm ≈ 3.937 in. Working in the cm to inches converter can be handier for longer measurements; for heights and room dimensions see the feet and inches calculator or the meters to feet converter.
Assumptions, limitations and practical tips
- Exact factor: all results use 1 in = 25.4 mm exactly (international yard and pound agreement, 1959). Older US survey units differ minutely; for everyday work this is irrelevant.
- Non-negative lengths only: the tool accepts values from 0 up to 1,000,000,000 mm (about 393.7 miles) or 40,000,000 inches.
- Two-way field: the converter tracks which field you edited last. If both change at once, retype the value in the field you care about to set the direction.
- Fractions are approximations: most decimal inch values have no exact fraction, so the nearest fraction almost always carries a small rounding error — always check it against your tolerance.
- Wrench and drill sizes: nominal metric and inch sizes are close but not interchangeable (13 mm vs 1/2 in = 12.7 mm). Always match the tool to the fastener or drill to the marked job.
- Kitchen-variety conversions: when precision matters, prefer the decimal result; when laying out lines with a ruler graduated in fractions, use the nearest-fraction value.
Frequently Asked Questions
How do I convert mm to inches without a calculator?
Divide the millimeter value by 25.4, since one inch is exactly 25.4 mm. For a quick estimate, multiply mm by 4 and divide by 100 — for example, \(50 \times 4 = 200\), so 50 mm is about 2 inches (exactly 1.9685 in). Another rule of thumb: 10 mm \(\approx\) 0.4 in, so 80 mm is roughly 3.2 in. Reverse direction: multiply inches by 25.4.
What is 100 mm in inches?
100 mm equals \(100 \div 25.4 = 3.937007\) inches. In fractional form that is about 3 15/16 in (0.062994 in = 1.6 mm short of the exact value) or nearly 3 63/64 in if you need 1/64 precision. For quick mental math, remember 100 mm ≈ 4 inches, since 4 in = 101.6 mm.
How many mm are in a quarter, half, and full inch?
1/4 in = 6.35 mm, 1/2 in = 12.7 mm, and 1 in = 25.4 mm — all exact values, not approximations. Other common ones: 3/4 in = 19.05 mm and 1/8 in = 3.175 mm. These exact equivalents come in handy when reading drawings that mix metric and imperial dimensions.
Why is 25.4 the conversion factor — is it exact?
Yes, it is exact by definition. Since the international yard and pound agreement of 1959, the inch has been defined as exactly 25.4 mm, replacing slightly different national values used before. Only the US survey inch (used in some historical land surveys) still differs, by about 3 mm per mile, which is irrelevant for everyday measuring, machining, or DIY work.
How do I convert mm to feet and inches?
First divide mm by 25.4 to get total inches, then divide by 12: the whole part is feet, the remainder is inches. For example, 1700 mm \(\div\) 25.4 = 66.929 in, which is 5 feet 6.93 in (about 5 ft 6 15/16 in). For a dedicated tool with combined feet-and-inch output, use a feet and inches calculator.
How do I read a decimal inch like 0.875 in as a fraction?
Multiply the decimal by the denominator you want and round to the nearest whole number. For example, \(0.875 \times 64 = 56\), so 0.875 in = 56/64 in, which simplifies to 7/8 in. The most common ruler graduations are 1/8, 1/16, and 1/64 in — finer markings let you read a length more precisely, but most decimal values still won't land exactly on a fraction line.
Is a 13 mm wrench the same as a 1/2 inch wrench?
No — they are close but not interchangeable. 1/2 in = 12.7 mm, so a 13 mm socket is 0.3 mm larger. A 1/2 inch wrench on a 13 mm bolt head is loose and may slip and round off the corners; a 13 mm wrench may not fit a true 1/2 in fastener. The closest practical pairs: 13 mm ≈ 17/32 in and 12 mm ≈ 15/32 in, but always match the tool to the fastener's actual marked size.