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MM to inches converter

Convert millimeters to inches (and back) with the exact 25.4 definition — plus the fraction table (1/16ths) that hardware and jewelry sizes actually use.

1 mm = 0.039370079 in

1 mm = 0.039370079 in · multiply by 0.039370079

Conversions run instantly on your device — nothing is sent anywhere.

Maintained by ToolsNexus

Convert millimeters to inches — and back — using the exact definition (1 in = 25.4 mm), with the fraction arithmetic explained for the hardware-store cases decimals don’t answer. Loads with mm → in preselected.

The formula

inches = mm ÷ 25.4 · mm = inches × 25.4. For fractional inches, convert the decimal remainder against sixteenths: 1/16 in = 1.5875 mm exactly.

Fraction table (the hardware sizes)

mm inches (decimal) nearest fraction
1.588 0.0625 1/16
3.175 0.125 1/8
6.35 0.25 1/4
9.525 0.375 3/8
12.7 0.5 1/2
19.05 0.75 3/4
25.4 1.0 1

Screws, sockets, straps, and stones

Small-measurement domains are split down the middle: fasteners and tools come in metric (M6, 10 mm) and SAE (1/4″, 3/8″) families that almost interchange — the FAQ lists the near-miss pairs that round off bolt heads. Watch straps, ring widths, and gemstones are quoted in mm globally. Rainfall is metric in most of the world (25 mm of rain = about an inch — exactly 0.984″). For anything body-or-furniture-sized, centimeters read more naturally — that’s the cm to inches converter, same definition, one decimal over.

Private by design

The math runs on your device as you type — nothing is uploaded. The full unit converter covers all six categories.

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Frequently asked questions

What is the exact mm to inches formula?
Divide millimeters by 25.4 — exact by definition since 1959. So 10 mm = 0.3937 in. Reverse: inches × 25.4 = mm. This is the same 2.54 factor as cm-to-inches shifted one decimal place, but small measurements are usually quoted in mm, hence this page.
How do I get a fraction of an inch instead of a decimal?
Multiply the decimal remainder by the denominator you care about (usually 16): 30 mm = 1.1811 in; 0.1811 × 16 = 2.9, so 30 mm ≈ 1 3/16 in. Rule of thumb at typical tolerances: each 1/16 inch is 1.5875 mm.
Is a 10 mm socket the same as a 3/8 inch socket?
Close but not the same: 3/8 in = 9.525 mm. A 3/8 wrench on a 10 mm bolt (or vice versa) will slip and round the head. The near-misses that cause the most damage: 13 mm vs 1/2 in (12.7 mm), 19 mm vs 3/4 in (19.05 — this one is usually safe), and 11 mm vs 7/16 in (11.11).
What sizes are jewelry and watch parts quoted in?
Millimeters, worldwide: ring bands (2–8 mm wide), watch lugs (18, 20, 22 mm straps), stud earrings (3–6 mm stones). A 20 mm watch strap is 0.787 in — US sellers list both, but the mm figure is the definitive one.

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