MM to PX Converter
Millimetres to CSS pixels at the specified 3.779528 to the millimetre, plus what those pixels actually measure on a real screen.
210.0000mm = 793.7008px
210.0000mm in every absolute unit
px 793.7008 ← out
in 8.2677
cm 21.0000
mm 210.0000 ← in
pt 595.2756
pc 49.6063
The definitions these come from
1 inch 96px, 2.54cm, 25.4mm, 72pt, 6pc
1mm 3.779528px
1cm 37.795276px
1pt 1.333333px, which is why 12pt is 16px
Those 793.70 CSS pixels on a real screen
at 96 ppi 8.2677in, 21.0000cm
the CSS answer assumes 96 ppi
so the real length is 100.0% of the CSS one
on a 1080p 24-inch monitor 219.13mm
on the CSS reference pixel 210.00mm
on a 27-inch 4K monitor 123.68mm
on a 13-inch MacBook Pro 88.81mm
on an iPhone 15 43.83mm
The absolute units in CSS are defined against each other, not measured:
one inch is exactly 96 pixels, 2.54 centimetres, 25.4 millimetres, 72
points and 6 picas. So 210.0000mm is 793.7008px exactly, and no screen
is involved in that conversion.
A CSS pixel is not a device pixel and not a physical length. It is about
1/96 inch on a 96 ppi monitor and about a quarter of that on a phone,
where the browser draws one CSS pixel with several device pixels. That
is the whole reason `1cm` in a stylesheet does not measure a centimetre
on screen.
Physical units belong in a print stylesheet, where the browser knows the
paper size and the conversion means something. In a screen stylesheet
they are a slower way of writing pixels, and they break when a reader
zooms.
The inch has been exactly 2.54 centimetres since the international
agreement of 1959. Rounding it to 2.5 is an error of 1.6 percent, which
is four millimetres across a sheet of A4 and enough to lose a bleed.
12pt is 16px, and that is where the browser default font size comes
from. A point is 1/72 inch in publishing and CSS keeps that definition,
which is why type specified in points from a design tool converts to
pixels cleanly.
`rem` and `em` are the units to reach for in a stylesheet. They scale
with the reader's font size, which is a preference that people set
deliberately and that fixed physical units ignore.
For anything that will be printed, work in millimetres and set the
resolution separately. The pixel dimensions needed follow from the print
size and the resolution, and a file specified in pixels alone has no
physical size at all.
Output is valid and updates as you type.
Fix the highlighted fields to update the output.
One millimetre is 3.779528 CSS pixels. That is not a measurement of anything: it follows from the CSS specification defining one inch as exactly 96 pixels, and 25.4 millimetres to the inch.
Which means the conversion is exact and the result is not a millimetre. 210mm in a stylesheet
becomes 793.7 CSS pixels, and those pixels measure 210mm only on a display at exactly 96 pixels per
inch. On a 13-inch MacBook Pro they measure 89mm. On a phone, 44mm.
So millimetres are dependable in a print stylesheet, where the browser knows the paper, and a slower way of writing pixels everywhere else.
How to use
- Put in the length in millimetres.
- Read the pixel figure, and the rest of the units alongside it.
- Set a screen density if you want to know what those pixels actually measure.
Example
The width of an A4 page:
210.0000mm = 793.7008px
210.0000mm in every absolute unit
px 793.7008 ← out
in 8.2677
cm 21.0000
mm 210.0000 ← in
pt 595.2756
pc 49.6063
The definitions these come from
1 inch 96px, 2.54cm, 25.4mm, 72pt, 6pc
1mm 3.779528px
1cm 37.795276px
1pt 1.333333px, which is why 12pt is 16px
Those 793.70 CSS pixels on a real screen
at 96 ppi 8.2677in, 21.0000cm
on a 1080p 24-inch monitor 219.13mm
on a 27-inch 4K monitor 123.68mm
on a 13-inch MacBook Pro 88.81mm
on an iPhone 15 43.83mmA4 is 210mm on paper and somewhere between 44mm and 219mm on a screen, depending on the screen. That range is the reason to be careful with physical units.
Pitfalls
The conversion is exact, the physical result is not. 3.779528 px/mm is arithmetic from a definition. Whether those pixels occupy a millimetre depends on the display, and no modern display is 96 ppi.
Do not lay out a screen in millimetres. It is slower to read, it does not respond to a reader’s
zoom in the way rem does, and it implies a physical precision the browser cannot deliver. Use
rem for type and spacing, and percentages or grid for layout.
Millimetres are right for print stylesheets. Inside @media print the browser knows the paper
size, so @page { margin: 20mm } does what it says. That is the one place these units earn their
keep.
A 3mm bleed is 11.34px and that is not a useful number. For print work, stay in millimetres all the way through and set the resolution separately: the pixel dimensions follow from the print size and the PPI, which is a different calculation.
Rounding the conversion accumulates. 3.78 instead of 3.779528 is 0.05 percent, which is a tenth of a millimetre across A4. Fine for a margin, visible on a grid of 30 columns.
The mm in an SVG is a different question again. An SVG with a viewBox and a width in
millimetres scales its user units to that physical width, so the same drawing can be both, which is
exactly why SVG is the right format for something that has to print at a known size.
Compatibility
Arithmetic in the browser: nothing is uploaded and nothing is stored.
The relationships used are the CSS Values and Units definitions: 1in = 96px = 2.54cm = 25.4mm = 72pt = 6pc. They are exact and every browser implements them identically, because there is nothing to measure. The round trip is checked across every pair of units in the test suite.
The screen densities are real: about 92 ppi for a 1080p 24-inch monitor, 163 for a 27-inch 4K, 227 for a 13-inch MacBook Pro, 460 for a current iPhone. They are there to show how far the CSS reference pixel has drifted from any hardware.
mm works in every browser and has since the beginning. What varies is whether it means anything,
which depends on where you use it.
Frequently asked questions
How many pixels is 1mm?
Why is my millimetre not a millimetre on screen?
What should I use instead for layout?
rem for type and spacing, so it scales with the reader’s font size; percentages, fr units or
clamp() for layout. Physical units only for print.Does this work for print CSS?
@page margins in millimetres, size elements in millimetres,
and the browser’s print pipeline converts to the paper correctly.