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I have an image in photo shop that looks like this...

enter image description here

I know that the length of the blue line has to be 20mm, how can I resize the whole image to scale?

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    do you mean resize image and in result blue line will be 20mm? – user2650501 Jun 20 at 2:02
  • your question is lacks of proper details :) please provide more information! – Design Phoenix Jun 20 at 3:06
  • Impossible to answer given provided information. If you need the line to be 30mm, scale 150%. For a 40mm line, you'd scale 200%. – Scott Jun 20 at 3:41
  • OP, please consider editing your question to add the output for the final result (screen, printer, etc.) – Emilie Jun 21 at 10:37
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In case you'd want to work in pixels, there's a script I made called Quick Resize that is a part of free scripts pack Scriptorator. It allows to resize an image based on a selected layer or a selection:

enter image description here

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Your question is hard (impossible?) to answer because we don't know why or where you need the image to be to scale and we don't know what happens with the image afterwards.

  • If your image is for web and you want it to be to scale on every screen it's displayed on
    Forget it. Different screens have different resolution (pixels per inch), so it's impossible to make an image which is automatically shown at a certain physical size.

  • If your image is for personal use and you just need it to be to scale on your screen

    • Find your screen resolution in px/cm:
      resolution [px/cm] = width of display [px] / width of display [cm]
    • Find the wanted width of the line
      wanted width [px] = resolution [px/cm] × wanted width [cm]
    • Find the scale factor
      scale factor [%] = wanted width [px] / actual width [px] × 100%
    • Scale the image to the found scale factor and view at 100% or simply set Photoshop's zoom level to the scale factor.
  • If your image is for printing on your own printer

    • Find the resolution needed to print to scale
      resolution [px/inch] = ( actual width [px] / wanted width [cm] ) × 2.54 [inch/cm]
    • Set the image resolution to this number without resampling. Now Photoshop knows what you mean by "100%" when printing.
  • If your image is for printing commercially
    I strongly recommend using a vector based layout application like InDesign/Illustrator/Inkscape etc. You'll have total control with the physical size of elements without having to do all the pixel math. Se my answer to how to scale an image so that part of the image gets a specific size.

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