Worked example
A 20 in wide image with 2,000 pixels across has 100 PPI and a 0.254 mm pixel pitch. At a perpendicular distance of 30 in, a central pixel spans about 1.146 arcminutes.
Measure the image area
Use the visible width, not a screen’s diagonal or the width including its bezel. For a printed image, use its final printed width. The pixel count must describe that same area after cropping and scaling.
A geometric comparison, not an eyesight test
The angle is measured across one pixel at the center of a flat surface viewed straight on. It describes size at the eye, not whether a particular viewer will see a pixel. Contrast, vision, sharpening, subpixels and printing all affect perceived detail.
Compare one change at a time
Double viewing distance while keeping size and pixel count fixed to make the angle approximately half as large. Increase physical width at the same pixel count to increase pixel pitch. This tool does not resample the image or choose a required resolution.
Explore the related calculation
For a different starting point, try the screen PPI calculator or return to the main calculator. Browse the complete tool directory for this site.
Reference and corrections
The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.
Found a problem? Email contact@pixelstoinches.org.