Mm to Inches Converter

Q: How many inches is 100 mm?

3.9370 inches, or 3 15/16" to the nearest sixteenth. It's the default value here, so you can verify it without typing.

Q: Is 25 mm the same as 1 inch?

No — 25 mm is 0.9843", and one inch is 25.4 mm. The 0.6% difference is invisible in rough work and fatal in threaded or press-fit parts.

Q: What is 1 mm in inches?

0.03937 inches, about 1/25 of an inch; to the nearest sixteenth it's 1/16". A 1 mm feeler gauge is thinner than the smallest mark most tapes show.

Q: How do I convert inches back to millimeters?

Multiply by 25.4. A 2" pipe is 50.8 mm; a 3/4" fitting is 19.05 mm.

Millimeters (mm)

Inches

3.9370 in

Closest 1/16 inch(what a tape measure actually reads): 3 15/16"
Inches (2 decimals): 3.94 in
Centimeters: 10.00 cm

Common millimeter sizes in inches

Millimeters	Inches (decimal)	Nearest 1/16"
1 mm	0.0394 in	1/16"
5 mm	0.1969 in	3/16"
10 mm	0.3937 in	3/8"
20 mm	0.7874 in	13/16"
25 mm	0.9843 in	1"
50 mm	1.9685 in	1 15/16"
100 mm (yours)	3.9370 in	3 15/16"
150 mm	5.9055 in	5 7/8"
200 mm	7.8740 in	7 7/8"

Exact conversion: 1 inch = 25.4 millimeters by international definition, so millimeters ÷ 25.4 never needs rounding until the last step.

100 mm is 3.9370 inches, which on a tape measure is 3 15/16". Millimeters convert to inches by dividing by 25.4 — an exact definition, not an approximation, so the only rounding happens at the end. Enter any millimeter value below for the decimal inches and the closest 1/16-inch fraction.

How to convert mm to inches

inches = millimeters ÷ 25.4

One inch has been defined as exactly 25.4 millimeters since the 1959 international yard and pound agreement, which is why this conversion never needs a fudge factor. Worked example, 100 mm: 100 ÷ 25.4 = 3.93700787 in, usually written 3.937 in.

To get the fraction a tape measure reads, multiply the leftover by 16: 0.937 × 16 = 14.99 sixteenths, which rounds to 15/16 — so 100 mm is 3 15/16". That rounding is where the practical answer comes from; 3.937" is not a mark you can find on a ruler.

Notice how close 25 mm and 25.4 mm are: 25 mm is 0.9843", about 1.6% short of an inch (0.0157" — the width of a few sheets of paper). That's why "25 mm" and "1 inch" get swapped in casual speech — fine for a rough fit, not fine for a threaded fitting or a bearing.

Which number to actually use

Decimal inches for CAD, spec sheets, and machining tolerances.
Fractions (1/16") for anything you cut, drill, or measure by hand — a 3 15/16" dimension is findable on a tape; 3.937" is not.
Millimeters when you're matching a metric part or an ISO print. Converting to inches and back again only adds rounding.

Comparing two sizes rather than converting one? The percentage increase calculator handles the "how much bigger is this" question directly, and kg to lbs does the same for weight.

How to use this converter

Type the measurement in millimeters (100 mm is filled in by default).
Read the decimal inches — that's your spec value.
Use the 1/16" line when you're holding a tape measure or picking a drill bit.
Open the chart for common sizes from 1 mm to 200 mm.

Frequently asked questions

How many inches is 100 mm?

3.9370 inches, or 3 15/16" to the nearest sixteenth. It's the default value here, so you can verify it without typing.

Is 25 mm the same as 1 inch?

No — 25 mm is 0.9843", and one inch is 25.4 mm. The 0.6% difference is invisible in rough work and fatal in threaded or press-fit parts.

What is 1 mm in inches?

0.03937 inches, about 1/25 of an inch; to the nearest sixteenth it's 1/16". A 1 mm feeler gauge is thinner than the smallest mark most tapes show.

How do I read mm on a US tape measure?

Divide by 25.4 for decimal inches, then take the remainder to the nearest 1/16. For 50 mm: 1.9685" → 1" plus 15.5 sixteenths → 1 15/16".

How do I convert inches back to millimeters?

Multiply by 25.4. A 2" pipe is 50.8 mm; a 3/4" fitting is 19.05 mm.

Data sources: The conversion uses the 1959 international definition of the inch as exactly 25.4 millimeters. Every decimal and fraction on this page was produced by this page's converter — enter the same millimeter value to reproduce it. No estimated or third-party figures were used. Related tools: kg to lbs, percentage increase calculator, all math calculators, concrete calculator.