InDesign Millimetre to Pixels Conversion Anomaly

[Scintilla_UK]

I annually produce Yearbooks for a client, printed at a finished size of 210 x 210mm.

I am now developing interactive digital versions to supplement these using InDesign and In5. When I convert 210 x 210mm with an online converter, I get 794 x 794px. When I convert the existing print document (through InDesign Preferences / Units) the new document size is 595 x 595px - what is going on?

Converting the metric to digital file sizes consistently is important as I think they will want both formats for a few years to come.

Thanks

[Eugene Tyson]

210mm in px is 794px

210mm in pts is 595pts

[Scintilla_UK]

Interesting, I will check now.

Thanks

[Scintilla_UK]

What am I doing wrong? Please see the attached screen recording of me converting the file ...

Thanks

[rob day · Accepted solution]

InDesign’s Pixel Ruler Unit is a static output dimension of 1/72".

So if you set your rulers to Inch units, your 210mm x 210mm doc will measure 8.2677" x 8.2677", or 595px x 595px — 8.2677*72= 595.2744.

Looks like your online converter was assuming a pixel has an output dimension 1/96"—8.2677*96 = 793.6992

[Scintilla_UK]

Thank you

[Eugene Tyson]

I might be wrong but might as well ask as I have always thought about it but never really got into it enough to care.

Indesign is outputting for the web an the px to pt as 1:1 for web where a postscript pt is 1/72 of an inch. 72pts is 1 inch.

But a pixel isn't necessarily 1/72 of an inch. And monitors typically display at 96ppi.

Which is erroneous too,  for example, a 32″ monitor with a 1080p resolution has 69ppi. At 4K, the same monitor is 138ppi.

But I assume the pixel conversion is assuming the 96 PPI.

[Scintilla_UK]

I created a box the size of the new document : 595 x 595px and pasted back in to the 210 x 210mm print document and it fits perfectly - so I will go with this as I want to seemlessly go from print to Digital and maybe back in the future.

Thanks all

[rob day]

Hi Eugene, If you are designing for web, the InDesign file will have to be exported. If it’s to an image format, an export to JPEG or PNG at 72ppi will export an image file with matching pixel dimensions—595px x 595px in this case.

If the exported image is placed in an HTML page all that matters is the pixel dimensions—HTML does not have a ppi property, all you can do is scale to fit the device.

HTML/CSS does define point size differently—1pt = 1/96"—so if the page is going to be recreated as HTML you have to consider that when defining the type size. I usually use pixels as the CSS unit to avoid the conversion.

[Eugene Tyson]

Thanks for the update and info - I don't do any web design work - so this is good to know going forward.