A millimeter ruler for small parts
This page opens the online ruler in millimeters. There is a tick for every millimeter, a longer tick at 5 mm, and a number every 10 mm. Use it for screws, beads, gemstones, watch straps, film and other small flat parts.
Put the part against the ruler with one end at zero. Pull a guide out of the ruler to the other end, and read the length, for example "92.1 mm". Hold Shift while you move a guide with the arrow keys to jump 10 mm.
A screen shows only whole pixels, so a millimeter is a few pixels wide. On a phone one millimeter is about 6 pixels, and on a standard monitor it is often 4 or fewer. Calibrate first, and use full screen for the cleanest result.
Millimeter to inch chart
The chart shows common millimeter sizes in decimal inches, with the nearest 1/16 inch. One inch is exactly 25.4 mm. To change mm to inches, divide by 25.4. To change inches to mm, multiply by 25.4.
| Millimeters | Decimal (in) | Nearest 1/16 in |
|---|---|---|
| 1 mm | 0.039 | 1/16 |
| 2 mm | 0.079 | 1/16 |
| 3 mm | 0.118 | 1/8 |
| 4 mm | 0.157 | 3/16 |
| 5 mm | 0.197 | 3/16 |
| 6 mm | 0.236 | 1/4 |
| 8 mm | 0.315 | 5/16 |
| 10 mm | 0.394 | 3/8 |
| 12 mm | 0.472 | 1/2 |
| 15 mm | 0.591 | 9/16 |
| 20 mm | 0.787 | 13/16 |
| 25 mm | 0.984 | 1 |
| 30 mm | 1.181 | 1 3/16 |
| 40 mm | 1.575 | 1 9/16 |
| 50 mm | 1.969 | 1 15/16 |
| 100 mm | 3.937 | 3 15/16 |
Things people measure in millimeters
- The width of a bead, a charm or a gemstone.
- The inside width of a ring, laid flat on the screen. A ring size chart then turns the diameter into a size.
- The width of a watch strap or a band.
- The diameter of a coin. A US dime is 17.91 mm and a US quarter is 24.26 mm, from the US Mint.
- Tape, ribbon and cord widths.
For round things, place the part so its edges touch two guides, and read the gap. For small parts, use the crosshair (press C) to see the exact spot under your finger or cursor.
How to read a millimeter ruler
Find the last number before the end of the object. The numbers are in millimeters here, and they appear every 10 mm. Then count the small marks after it. Each one is 1 mm. A mark at 37 means 37 mm, which is 3.7 cm.
The longer mark between two numbers is 5 mm. It helps your eye find the middle. If the object ends exactly on it, add 5 to the last number.
For half a millimeter, estimate by eye, or use the arrow keys on a guide. With snap on, a guide moves in whole millimeters. Turn snap off if you need finer steps.
Reference sizes in millimeters
You can check the scale with objects of known size. A credit card is 85.60 by 53.98 mm. A US cent is 19.05 mm, a nickel is 21.21 mm, a dime is 17.91 mm and a quarter is 24.26 mm. A euro 2 cent coin is 18.75 mm. A US Letter sheet is 215.9 mm wide, and an A4 sheet is 210 mm wide.
The coin sizes come from the US Mint and the European Commission. Both publish their sizes, so you can verify them.
Why a screen ruler has limits in mm
A screen is made of pixels, and a pixel has a size. On a typical laptop one millimeter is only four or five pixels. The ruler can only draw a mark on a pixel, so each mark may sit up to half a pixel from its ideal place. On a phone the pixels are smaller, which helps.
The match to the card also has a limit, because you line up two edges by eye. Expect small errors. For a part that must fit within a tenth of a millimeter, use calipers. A screen ruler is a quick check, not a replacement for a measuring tool.
Tips for the mm ruler
- Calibrate with a card, then check with a coin.
- Use the 0.1 mm buttons in the calibration step to refine the match.
- Press F for full screen.
- Open Options, then Snap guides to marks, so guides land on whole millimeters.
- Prefer a bigger scale? Open the cm ruler or the inch ruler.