Easily calculate how many pixels you need for a good print

[Ecuadorian]

For magazines, posters and newspapers, you need this width in pixels:

Formula-English.png (19.37 KiB) Viewed 11890 times

For road billboards, you need this width in pixels:

Formula-Billboards.png (22.51 KiB) Viewed 11891 times

And that's it. It doesn't matter what units you use as long as it's the same units both up and down. This is based on a 20/20 Visual Acuity and a viewing height of 30º from the horizon for billboards. You should subtract the average Eye Height in your country from "Height to center of Billboard" if you want to be more accurate.

[2dhipoo]

Hi Ecuadorian -- I'm unclear what you mean by this formula.. can you please provide an example?

I have been doing renders of 3,000 pix (actual size is 3,000 by 1378), which, when I put into an A3 landscape template in InDesign (whose graphic box dimensions are 380mm long by 180mm high) I usually get an effective PPI of 200...

[Ecuadorian]

Hello.

Well, this formula is based exclusively on what looks good to the eye, with complete disregard for dpi.

Let's take your A3 print as an example...
Hold an A3 print at the distance you would normally look at it. If it's a render with lots of details, such as an illustration of assembly instructions, you're going to examine it closer, at about 30 cm distance from your eyes. But if it's just the front of a house, you're more likely to hold it at about 50 cm distance.

Then we have:
(38 cm / 50 cm) * 3438 = 2612.88
So the resolution you're using should perfectly fine, as long as there's good antialiasing.