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Inches to MM Converter

Type inches, including fractions like 3/8 or 1 1/4, to get millimeters, then see the length at actual size on your screen.

Nearest 1/16 inch
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At actual size (top: cm, bottom: inches)

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Inches to mm

How to convert inches to mm

One inch is exactly 25.4 millimeters, so to convert inches to millimeters, multiply by 25.4. For example, 3/8 in = 0.375 × 25.4 = 9.525 mm. The table below lists common inch fractions in millimeters.

Related guides: Drill Bit Size Chart: Inch, Metric and Tap Drill Sizes · Wrench Size Chart: SAE vs Metric, Sockets and Allen Keys

How to use it

  1. Type a length in inches; fractions such as 3/8 or 1 1/4 work.
  2. Read the millimeters and centimeters below.
  3. Tap Calibrate and match a card to see the length at real size.

Good to know

  • 1 in = 25.4 mm exactly, and 1/16 in = 1.5875 mm.
  • 1/8 in = 3.175 mm, 1/4 in = 6.35 mm, 1/2 in = 12.7 mm, 3/4 in = 19.05 mm.
  • Inch drill bits and wrenches rarely match a metric size exactly; compare in millimeters to find the closest one.

Conversion table

InchesMillimetersCentimeters
1/161.58750.15875
1/83.1750.3175
3/164.76250.47625
1/46.350.635
5/167.93750.79375
3/89.5250.9525
7/1611.11251.11125
1/212.71.27
5/815.8751.5875
3/419.051.905
7/822.2252.2225
125.42.54
1 1/238.13.81
250.85.08
376.27.62
4101.610.16
512712.7
6152.415.24
8203.220.32
1025425.4
12304.830.48

Common inch fractions in mm

The fractions you see on US rulers, drill bits and wrenches, in millimeters:

InchesDecimalmmWhere you see it
1/160.06251.59Smallest mark on most US tape measures
1/80.1253.18A common small drill bit size
1/40.256.35Camera tripod screw (1/4-20 thread)
3/80.3759.53Common socket wrench drive size
1/20.512.70Socket drive size used for lug nuts
3/40.7519.05Close to a 19 mm wrench
1125.40Exactly 25.4 mm, by definition

See the length at actual size

The rulers on this page are drawn at real size once your screen is calibrated. Tap Calibrate and match any bank card (accurate to about ±0.5%), or choose your phone or laptop model. It takes about 20 seconds and is saved for next time.

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FAQ

Frequently asked questions

Exactly 25.4 mm. The inch has been defined as 25.4 millimeters since 1959.

Divide the top number by the bottom number, then multiply by 25.4: 5/8 in = 0.625 × 25.4 = 15.875 mm. Or type 5/8 into the converter.

1/4 inch is 6.35 mm, a little more than 6 mm.

1/2 inch is 12.7 mm, just under 13 mm.

1/16 inch is 1.5875 mm, about 1.6 mm. It's the smallest mark on most US tape measures. 1/32 inch is 0.79 mm.

1/8 inch is 3.175 mm, about 3.2 mm. 3/8 inch is 9.53 mm and 3/4 inch is 19.05 mm.

Multiply by 25 and add a little: 2 in × 25 = 50, and the exact answer is 50.8 mm. For an exact result, multiply by 25.4.

Yes, an inch is 25.4 times longer than a millimeter.