mm to inches Converter

The international inch is defined exactly as 25.4 mm. For mm to inches conversion use:

1 mm = 0.0393701 in · 1 in = 25.4 mm

Step-by-step conversion (example: 100 mm)

1. Start with the length in millimeters: 100 mm.
2. Divide by 25.4 (exact mm per inch): 100 ÷ 25.4 = 3.9370 in.
3. Reverse check: 3.9370 × 25.4 = 100 — the conversion round-trips cleanly.

Nearest 1/64 for 100 mm: 3 15/16 in (an approximation of the exact decimal — see “Why isn't the conversion exact?” below).

Use the bidirectional calculator at the top of this page. Type any millimeter value, or an inch value as a decimal or a fraction (1/16, 3 1/2, 4 7/8) — decimal inches, the nearest 1/64 fraction, and feet + inches update as you type. No separate calculator engine: this page shares the same conversion math as the homepage and drill-bit charts.

Decimal inches are exact. Fractional inches on rulers and bit charts are usually marked in 64ths. To match those ticks, convert mm → decimal inches, then round the fractional part to the nearest 1/64. That fraction is an approximation — the same rule the converter disclaimer uses: The inches fraction result is rounded to the nearest 1/64 fraction.

Worked example: 100 mm → nearest 1/64

1. Divide by 25.4 (exact mm per inch): 100 ÷ 25.4 = 3.9370 in (3.937008 before display rounding).
2. Split whole + fractional part: 3 + 0.937008.
3. Multiply the fractional part by 64: 0.937008 × 64 = 59.9685 → round to nearest integer 60 (the numerator over 64).
4. Simplify 60/64 by dividing by their greatest common divisor → 15/16.
5. Combine with the whole inches: 3 15/16 in. That is ≈ 3.9370 in, not exact — see Why isn't the conversion exact? (fraction error here: +0.012 mm).

Quick second example: 5 mm → 13/64

1. 5 ÷ 25.4 = 0.1969 in.
2. Fractional part 0.196850 × 64 = 12.5984 → rounds to 13.
3. Nearest 1/64 = 13/64 in (+0.159 mm from exact). Same result as 5 mm to inches and the chart row for 5 mm.

Tip: if you only need decimal inches, stop after step 1. Use the fraction steps when reading a tape, fractional drill chart, or socket set marked in 64ths (or coarser ticks that still sit on the 1/64 grid).

Fractional inches on rulers and bit charts are usually marked in 64ths. One millimeter is 0.0394 in ≈ 2.52/64 — not an integer tick — so the nearest 1/64 is always an approximation. That matters when you are matching drill bits, sockets, tubing, or sheet stock to an imperial chart.

Common “is it equal?” checks (and how far off they are):

• 5 mm ≠ 1/4 in. 5 mm = 0.1969 in, while 1/4 in = 6.35 mm — a difference of 1.35 mm. See 5 mm to inches.
• 14 mm ≠ 1 in. 14 mm = 0.5512 in — about 11.4 mm short of a full inch.
• 25 mm ≈ 1 in, but not exactly. 25 mm to inches = 0.9843 in — about 0.4 mm (≈1/64 in) short of 1 inch.
• 1 mm ≈ 3/64 in as the nearest 1/64. Exact decimal is 0.0394 in; 3/64 = 0.046875 in — the fraction is slightly large on purpose (nearest-tick rounding).

Why do mm fractions look ugly? Because 1 mm ≈ 2.52/64 is not an integer grid. Only a few sizes come close to a stock fraction (for example 3 mm ≈ 1/8 in, 0.175 mm small; 8 mm ≈ 5/16 in, 0.06 mm large). Most other millimeter sizes sit between ticks — which is why decimal and nearest-1/64 disagree on this page, and why the converter disclaimer exists.

Shop charts stock fractional-inch bit and socket sizes. A metric size like 5 mm or 10 mm rarely lands on a neat 1/64 tick, so picking the closest imperial bit means knowing both the decimal and how far the nearest fraction sits from the exact millimeter. Use this hub for conversion math; use the scene page for the full decision table.

Drill bit & socket mm to inches chart — full mm ↔ fractional-inch picks and “how far off” notes. The complete chart stays on that page so this hub does not become a second thin duplicate.

This short lesson from Math with Mr. J walks through the international definition (1 in = 25.4 mm) and worked decimal examples for inches ↔ millimeters. It is not produced by Length Tools and covers decimals only — the nearest-1/64 fractions, how-far-off checks, and the chart are above.

Conversion on this page uses the international inch (exactly 25.4 mm). Primary sources:

1. NIST — SI Units: Length — states that since the 1959 yard agreement, the standard inch is exactly equivalent to 25.4 mm.
2. Federal Register Notice 59-5442 (PDF) — July 1, 1959 announcement of the international yard and the inch = 25.4 mm exactly.
3. NIST Guide to the SI, Appendix B — conversion factors based on the international foot/inch (1 in = 0.0254 m exactly).