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What's the conversion between InDesign's point measurement (as used in leading and text size etc) and millimetres? What's the pt : mm ratio?

I've seen sources suggesting that a point is 1/72 of an inch, and an inch is 25.4mm, so in theory a point would be 0.3528mm and a millimetre would be 2.8346 pts, but I'd like something authoritative as I might well have gone wrong somewhere there.

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    Not a significant thing to edit but it's actually "Vice Versa" – Ryan May 16 '14 at 14:05
  • 1 inch is exactly 25.4 mm – thebodzio May 16 '14 at 15:41
  • Picking nits a bit: it's not "InDesign points," it's "Postscript points," which are exactly 1/72nd inch. Traditional points are ever-so-slightly larger. InDesign gives you the option of working with either. – Alan Gilbertson May 16 '14 at 23:35
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1 pt is equal to

0.08333 picas

0.01389 inches

0.3528 mm (millimeters)

one nice feature in basically all of adobe's software is that it can do those conversions for you. You can simply type in any of the unit of measure and it will even do basic math for you too like +2mm

adding 2mm

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  • I'd add that 1 pica = 12 pt. – Mikhail V Mar 20 '16 at 22:04
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"Points" in typography e.g. in Indesign, Illustrator, Photoshop etc are "PostScript points" and, helpfully, they're a unit Google converts on searches like points to mm or mm to points.

Has to be points or postscript points, not pts or anything involving software names.

One point is 0.352777778 millimetres.

enter image description here

One millimetre is 2.83464567 PostScript points.

enter image description here

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In Adobe InDesign (don't know how it is in other adobe softwares) it's a bit more complicated actually BECAUSE OF the accuracy of InDesign's interpretation of decimals.

To test Evan's values I made a simple experiment:

When you set the baseline grid increment (edit > preferences > grids > baseline grid) to the value of 1 pt and then on the page you will create a rectangle snapped to the two nearest baseline grid lines... you will end up with a rectangle of a height of exactly 0.353 mm.

Why?

Because InDesign apparently rounds the values of 1 point (0.352777778 mm) to the three decimal places (therefore 0.353 mm).

So:

  • IN THEORY: 1 PostScript point = 0.352777778 mm
  • IN INDESIGN: 1 PostScript point = 0.353 mm
  • DIFFERENCE: 0.353 mm - 0.352777778 mm = 0.000222222 mm

It seems not important, but becomes a problem, when you will need to multiply the values for some reasons. Let's say you have 250 baseline grid lines incrementing by 1 pt, and you need to create a rectangle of exactly the height of those 250 lines.

  • IN THEORY: 250 * 0.352777778 mm = 88.1944445 mm
  • IN INDESIGN: 250 * 0.353 mm = 88.25 mm
  • DIFFERENCE: 88.25 mm - 88.1944445 mm = 0.0555555 mm

So the seemingly negligible INDESIGN/THEORY difference on the level of 1 point unit interpretation (0.353 mm - 0.352777778 mm = 0.000222222 mm) becomes "magically" more important after multiplication (88.25 mm - 88.1944445 mm = 0.0555555 mm)

Hope it helps! :)

PS: Or maybe We can just set the InDesign's default accuracy to higher values then three decimal places? If so all that I have written above is bullshit. But I don't know if such accuracy setting is somewhere available in InDesign. Maybe someone know?

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    It does not round it just the GUI just ever only shows 3 decimals. – joojaa Dec 4 '17 at 19:42
  • Apparently, You're right. Thank you for that answer!! I have just created baseline grid lines every 12 pt. And a square created between two nearest lines had a height of 4.233 mm. If my assumptions from the post above would be right then 12 pt * 0.353 should give me a square of a height of 4.236 mm not 4.233 mm. So yes - the GUI rounds values to the three decimal places. But the problem I have stumbled upon still exists - when you have to "trust" the GUI values and you have to treat them as a SOURCE of truth about the measurements needed for your calculations... then you're fuc*ed. – micz Dec 4 '17 at 23:50
  • Meaning - you have to remember that the values presented by Evan are the one used by InDesign to calculate stuff and every calculation You gonna make should be based ON THEM, not on the GUI info (since the GUI values are OUTCOMES of InDesign calculations, and they are already "contaminated" by GUI's tendency to present rounded values). – micz Dec 4 '17 at 23:50
  • why not change your preferences to be points then? Anyway i suspect the system is internally using points anyway so any mm measurement is still just rounded from points. Best not be too pedantic about things, no printes usually output things with 0.001 mm precision (thats 300dpi) or maybe 0.0001 at most – joojaa Dec 5 '17 at 5:58

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