I was an early user of Mac last century; I remember Jobs telling about how the Plus had the only screen with square pixels and how important it was for developers a consistent info on pixel size so that they could give to the user an exact scale. That if an image was being shown 100% scale in the screen, one centimeter in the ruler should be a real centimeter.

I think that this consistency was a key to win the field of graphic design.

So, I got a new 21.5 iMac some months ago and yesterday I discovered that the images I was sketching at 100% size were not the size I was seen in the screen. I enabled the ruler and measured it. Whoops, I need to set zoom at 150% to get a correct size, and even then it is a bit, sort if one hundredth part, inaccurate.

To my amazement, Preview does the 100% size right when showing pdfs, so the info on pixel size is available to developers... My guess is that I am missing some setting, is it?

EDIT: LibreOffice also does the right scale, so it is something with the adjusts of AppleWorks applications. Er... iWorks.

  • What is your scaling set to in System preferences -> Display?
    – Harcker
    Commented Jul 25, 2020 at 12:40
  • Default. If I set it to 768 lines then 20 cm at 100% measure 21cm, which is good enough, but I miss all the ability of seeing small typesizes.
    – arivero
    Commented Jul 25, 2020 at 13:29

1 Answer 1


If you select 'Actual Size' (Command 0) under the View > Zoom menu, then the page should display at ... actual size.

With this setting, an A4 page on the screen is the same size as an A4 piece of paper that I hold in front of the screen. This works for Retina displays and non-Retina displays.

Back in the old days, Mac screens had 72 pixels to the inch. So it was easy for 1 point on the page to equal 1 pixel on the screen. This ratio is what became 100% zoom.

Of course, screen pixel density has increased massively since then. Many pre-Retina displays were around 109 dpi. Retina screens are twice that, but scale everything up by 2, giving the same perceived resolution.

As a result, 72 pixels are less than an inch on such a display. So you have to use a higher zoom level to get Actual Size.

109 dpi divided by 72 dpi gives you 151%, which is the zoom factor you get when you select 'Actual Size' on a display with that pixel density.

enter image description here

In other words, the zoom % set for Actual Size will depend on the pixel density of your display.

Preview has a Preference to define 100% as either 1 point = 1 pixel, or 'size on screen equals size on printout', which presumably sets 100% to the Actual Size.

  • Oh, it works! I feel still a bit not intuitive, this 151, but it solves the issue. Now I only wished a way to go 4K without such small menu bars.
    – arivero
    Commented Jul 25, 2020 at 13:56
  • 2
    @arivero Ask another question about that: but the whole point of a 4K Retina display is that it 'doubles up' the pixels, so the image is NOT tiny, but the image is detailed.
    – benwiggy
    Commented Jul 25, 2020 at 13:59
  • An extra remark: Pages produces the 151%. Keynote seems to ignore it, because even if it allows to set centimeters in the ruler, it seems to consider that the basic unit is pixels. I missed the "Actual Size" because I was doing most of my testing with Keynote.
    – arivero
    Commented Jul 26, 2020 at 0:31

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