mm to inches calculator

MM to inches calculator formula

An mm to inches calculator multiplies millimeters by 0.0393701. The formula is inches = millimeters × 0.0393701, which is the same as millimeters ÷ 25.4.

Enter a length in millimeters and you get inches to three decimals and centimeters to two. The default of 100 mm shows 3.937 inches and 10.00 cm. For a tape measure reading, take the decimal and find the closest sixteenth. 0.937 is very nearly 15/16.

Worked examples

Example 1 is a plywood panel listed as 19 mm.

• 19 × 0.0393701 = 0.748 inches
• Centimeters: 19 × 0.1 = 1.90 cm
• A nominal 3/4 inch is 0.750, so the panel is about 0.002 inch thin.

Example 2 is a piece of metric framing lumber, 38 mm thick.

• 38 × 0.0393701 = 1.496 inches
• Centimeters: 3.80 cm
• A US nominal 2x lumber is 1.5 inches thick, so 38 mm fits it closely.

Small differences like these matter in slots, rabbets and hinge mortises. When a close fit matters, measure the piece with calipers.

Common conversions

Millimeters	Inches	Centimeters	Closest fraction
3	0.118	0.30	1/8
6	0.236	0.60	1/4
8	0.315	0.80	5/16
10	0.394	1.00	13/32
12	0.472	1.20	15/32
15	0.591	1.50	19/32
18	0.709	1.80	23/32
19	0.748	1.90	3/4
25	0.984	2.50	1
50	1.969	5.00	1 31/32
89	3.504	8.90	3 1/2
1,000	39.370	100.00	39 3/8

The closest fraction column is rounded to the nearest 32nd, so each entry is within about 1/64 inch. That is fine for rough cuts and too loose for tight cabinet joints.

Where millimeters show up

• Sheet goods. Metric plywood and MDF are sold in 6, 9, 12, 15, 18 and 19 mm thicknesses.
• Lumber. Metric framing can be 38 × 89 mm, which is about 1.5 × 3.5 inches, the actual size of a US 2×4.
• Hardware. Bolts, screws and anchors use millimeter diameters such as M6 and M8, and these are not interchangeable with 1/4 and 5/16 inch sizes.
• Pipe and fittings. Many imported pipes are labeled by outside diameter in millimeters. Check the dimension, because the nominal size in inches may be different.
• Plans from outside the US. Details, clearances and wall build-ups are often in millimeters.

How to measure in millimeters

• Use the metric side of a tape or a digital caliper. Start from the zero mark, not the end of the hook, unless the hook is the datum.
• Read the closest millimeter for general work. For parts that fit together, use a caliper that reads to 0.1 mm.
• Measure in several places, since sheet goods can vary slightly across the sheet.
• Write the unit beside the number. A note that says 25 without a unit is easy to misread.

Common mistakes

• Dividing by 2.54 instead of 25.4. That gives a result ten times too large, because 2.54 is the centimeter factor.
• Mixing up 19 mm and 3/4 inch for a precise fit. The two are close, but not equal.
• Treating M-sized bolts as inch bolts. A 6 mm bolt is 0.236 inch, which is close to 1/4 inch but has a different thread.
• Rounding to one decimal in a tolerance-critical job. 3.937 and 3.9 inches differ by about 1/32 inch.
• Rounding early on a long run. 5,000 mm is 196.85 inches, and rounding that to 197 puts you off by 0.15 inch. Keep the decimals, then use the inches to feet calculator for the last step.

Related calculators

For centimeters, use the cm to inches calculator, and for the reverse direction the feet to inches calculator. Sheet goods are covered by the plywood calculator.