Home › Converters › Millimeters to Inches Converter
Millimeters to Inches Converter
Convert millimeters to inches with our free mm to inches converter — or flip it and turn inches into millimeters. One inch is exactly 25.4 mm, so every result on this page is exact, not an estimate.
Common sizes
| Millimeters | Inches |
|---|---|
| 10 mm | 0.394 in |
| 25.4 mm | 1 in |
| 50 mm | 1.969 in |
| 100 mm | 3.937 in |
| 152.4 mm | 6 in |
| 304.8 mm | 12 in (1 foot) |
How it works
Millimeters are the small metric unit used around the world for precise measurements — a paperclip is about 30 mm long, a standard pencil is about 175 mm, and a credit card is 85.6 mm wide. Inches are the American unit for the same job: a credit card is about 3.37 inches wide. When a product arrives sized in millimeters and your tools, shelves, or plans are in inches, you need to convert.
The link between the two is exact and never changes: 1 inch equals exactly 25.4 millimeters. That definition was agreed internationally in 1959, so there is no rounding or approximation in the factor itself. Any rounding happens only in how many decimal places you choose to display.
To turn millimeters into inches, divide by 25.4. To turn inches into millimeters, multiply by 25.4. For everyday use, three decimal places are plenty: 100 mm is 3.937 inches, and 50 mm is 1.969 inches. For woodworking or machining, keep more decimals, because a difference of a tenth of a millimeter can matter when parts must fit together.
This conversion comes up constantly in American homes and workshops. A camera lens listed as 50 mm, a bicycle tire marked 28 mm wide, a 3D printer nozzle of 0.4 mm, a drill bit sold in metric sizes, or a piece of imported furniture with metric dimensions — all of them need an inch figure before you can compare them with American tools, drill sets, or shelf space. The same factor also converts centimeters: 1 inch is 2.54 cm, so 10 mm is 1 cm and 0.394 inches.
Worked example
Maria in Denver is buying a wall shelf online. The listing says the mounting bracket is 100 mm wide, and she needs to know whether it will fit a 4-inch bracket slot on her wall.
Step 1: Write down the length — 100 mm.
Step 2: Use the conversion factor — 1 inch = 25.4 mm.
Step 3: Divide: 100 ÷ 25.4 = 3.93700787.
Step 4: Round to three decimal places — 3.937 inches.
That is the exact result this converter gives for 100 mm: 3.937 inches, which fits comfortably inside her 4-inch slot. The reverse check works too: 25.4 mm ÷ 25.4 = 1 inch exactly, which is why the converter starts with that value as its example.
FAQ
How many millimeters are in 1 inch?
Exactly 25.4 millimeters are in 1 inch. This is an exact definition, so 25.4 mm always equals 1 inch.
How many inches is 100 mm?
100 millimeters equals 3.937 inches. Divide 100 by 25.4 to get 3.93700787, which rounds to 3.937 inches.
How do I convert inches back to millimeters?
Multiply inches by 25.4. For example, 2 inches × 25.4 = 50.8 mm. Use the swap button on this page to convert either direction.
When would I need mm to inches in the US?
When a product, tool, or part is sized in metric — for example camera lenses, bicycle parts, 3D printer nozzles, drill bits, and imported furniture — and you need the size in inches to match American tools or shelving.