Convert in to mm
At the browser default root font size of 16 px, 1in = 25.4mm. Change the base below if your project uses a different root size.
Formula
mm = in × 25.4
in = mm × 0.03937Both units are absolute in CSS, so this ratio never changes regardless of font size or user settings.
IN to MM conversion table
| in | mm |
|---|---|
| 8in | 203.2mm |
| 10in | 254mm |
| 12in | 304.8mm |
| 14in | 355.6mm |
| 16in | 406.4mm |
| 18in | 457.2mm |
| 20in | 508mm |
| 24in | 609.6mm |
| 28in | 711.2mm |
| 32in | 812.8mm |
| 36in | 914.4mm |
| 40in | 1016mm |
| 48in | 1219.2mm |
| 56in | 1422.4mm |
| 64in | 1625.6mm |
| 72in | 1828.8mm |
| 80in | 2032mm |
| 96in | 2438.4mm |
| 128in | 3251.2mm |
About in
CSS defines one inch as exactly 96 pixels. On screen this is a nominal measure, not a guarantee of physical size; in print stylesheets it maps to a real inch.
About mm
One millimetre is about 3.78 CSS pixels. Like centimetres it is a print unit that CSS accepts everywhere.
Which unit should you use?
| Property | Recommended unit | Why |
|---|---|---|
| Font size | rem | Scales with the user's browser font-size preference, which px ignores — an accessibility requirement. |
| Padding and margin | rem or em | rem for a consistent global rhythm; em when the spacing should track the component's own text size. |
| Borders and hairlines | px | You almost always want exactly one crisp pixel, not a value that scales. |
| Media query breakpoints | em | Historically the most reliable across browsers when the user zooms. |
| Print stylesheets | pt, cm, mm | These map to real physical measurements on paper. |
Frequently asked questions
What is 16in in mm?
16in equals 406.4mm at a 16 px root font size.
How do I convert in to mm?
Multiply the in value by 25.4. Going the other way, multiply the mm value by 0.03937.
Does browser zoom change the result?
No. Both units are absolute in CSS and keep the same ratio at any zoom level, though the rendered size on screen changes.
Why does CSS use 96 pixels per inch?
It is a historical convention from early desktop displays that became the fixed CSS reference. It has nothing to do with your monitor's real pixel density — the browser scales CSS pixels to physical ones for you.