In a Google Docs document, I would like to be able to specify the page margins (File → Page Setup) in millimeters instead of inches.
Is it possible to switch between imperial and metric units?
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Sign up to join this communityGo to Settings → Document Settings on the top right of the page and change the language to English (UK).
Rulers and margins will now both be in centimetres (cm).
You may need to log out and sign back in before you see the changes take effect.
Due to recent changes, the measuring system in Google Docs is now affected by your Google Account default language, not anymore by the document language. (Means, @Dominik's answer no longer works.)
More precisely it's the country sub-setting that influences the measuring system now. And Google artificially limits country options depending on the chosen language.
So:
Go to myaccount.google.com/language. You can also reach it via the gear icon on the Google Drive or Google Docs main pages.
Set the language to "English – United Kingdom". (If it's currently "English – United States", click the pen icon, then again "English", then "United Kingdom". Bit counterintuitive.)
Reload your Google Docs document.
Voila: rulers and the "File → Page Setup… → Margins" settings are in centimeters now.
This is not possible in the current version of Google Docs with "English (US)" regional settings. Until they change this you can convert millimeters to inches using this Google query:
<n>mm in inches
Or you could follow the tips in Dominiks answer, and change your language settings to "English (UK)".
To change the margins one can use File --> Page Layout --> Margins but doing so does not (for me at 2018-10-18) offer a change of the unit-of-measure of the margins.
Earlier answers show that this is influenced by the user's language. (1) It used to be the language of the doc (2) That was changed to the user's Google Account's language.
Well; I have set both to US English and have closed and opened the document but to no avail -- it stays at cm for me.
Here, though, is a low tech remedy: There are 2.54 cm to the inch. There is 0.39 inch to the cm. If the unit-of-measure stays stubbornly at the one you don't want, apply a conversion to get the desired margin. For example, I needed to have all four margins set to 1 inch, so set the value to 2.54 cm for each of the four.