40 mm to inches, about 1 9/16 on a tape.

40 mm is 1.5748 inches. Divide by 25.4: 40 ÷ 25.4 = 1.574803. To four decimals that is 1.5748. On a tape marked in sixteenths it is 1 9/16 inches. At thirty-seconds it is 1 9/16, and at sixty-fourths it is 1 37/64. In centimeters it is simply 4 cm.

How to use the converter

1. The Millimeters (mm) box opens on 40. Type another length there and the Inches (in) box follows. Fractions such as 3 3/4 work too.
2. Read the big result, then The working under it. It shows the exact sum with your number, so you can check it on paper.
3. Choose Inch fractions in the side column to match your tape or your drawing.
4. Use Copy result or Copy link to keep the answer. The ⇄ button turns the sum around, and Back to 40 returns to this page's number.

Where the factor comes from

NIST Special Publication 811, Appendix B.8 marks 1 inch = 25.4 mm as exact (checked 24 Oct 2026). A millimeter is one thousandth of a meter, and a centimeter is one hundredth, as the NIST list of metric (SI) prefixes lists (checked 24 Oct 2026). So mm to inches is a division by 25.4, and the only rounding is the digit you stop at.

Which fraction should you use

Each finer step is closer to the true value. At 1/16 the answer 1 9/16 is 0.31 mm too short. At 1/32 it is 1 9/16, 0.31 mm too short. At 1/64 it is 1 37/64, 0.08 mm too long. For woodwork a fraction of a millimeter rarely matters. For machined parts, work from the millimeter figure and keep the decimal inches.

Sizes from 35 to 50 mm

Worked examples

• A 40 mm wide part in a 1 1/2 inch slot. The slot is 38.1 mm wide, so the part is 1.9 mm too wide to fit.
• Four parts of 40 mm. 4 × 40 = 160 mm, which is 6.2992 inches or 16 cm.
• 40 mm in thousandths of an inch. A mil is 0.001 inch, and 1.5748 inches is 1,574.8 mil.
• 40 inches in mm. Going the other way, 40 × 25.4 = 1,016 mm. The reverse of this page is a very different size.

A quick check in your head

An inch is about 25 mm, so 40 mm is about 40 ÷ 25 = 1.6 inches. The exact answer, 1.5748, is slightly lower because the true inch is 25.4 mm. The 0.4 mm difference per inch makes the quick answer 0.025 inch too high. For 40 mm that is small, which makes this a good rule for a first look.

Write the sum as a unit chain

A unit chain keeps the units honest. Write the length, then a fraction that equals 1 and has the unit you want on top: 40 mm × 1 in / 25.4 mm = 1.5748 in. The millimeters cancel and inches are left. Going through centimeters gives the same answer. 40 mm × 1 cm / 10 mm = 4 cm, and 4 cm × 1 in / 2.54 cm = 1.5748 in. If the result is bigger than the start, the fraction was upside down.

Whole inches first, then what is left

This is how a tape works. Count the whole inches that fit, then read the rest. 1 inch is 1 × 25.4 = 25.4 mm, which leaves 40 − 25.4 = 14.6 mm. That part is 14.6 ÷ 25.4 = 0.574803 inch. Together: 1 + 0.574803 = 1.5748 inches, which is 1 9/16 on a sixteenth tape.

Chart from 33 to 48 mm

Millimeters in steps of 1, with the same length in centimeters, decimal inches and the nearest sixteenth on a tape. A fraction is a rounding, so the decimals are the reference.

Several pieces of 40 mm

Multiply the length by the number of pieces, then convert once. That avoids adding rounding errors piece by piece.

How many decimals to keep

The answer never ends, so you must stop somewhere. The last column shows the length you give up at each stop, in millimeters.

Mistakes to avoid

The common slip is 40 mm read as 40 cm. 40 cm is 15.748 inches, ten times the size. Check the unit on the drawing or the label before you type. A second slip is rounding early. If you round 1.5748 to 1.6 and then use it in another sum, the error grows. Keep the digits until the last step. The MM to inches converter has the same sum with a longer table.