What it's for
This calculator solves both sides of the same problem: finding the largest size you can print a photo at with good quality from its pixel resolution, or finding how many megapixels your camera needs to print at a specific size — both based on your target DPI (dots per inch).
Formulas used
From image size to print size: width_in_inches = width_in_pixels / DPI (and the same for height), converted to centimeters by multiplying by 2.54. From print size to required pixels: required_pixels = (size_in_cm / 2.54) × DPI, and the megapixels are (width_px × height_px) / 1,000,000.
Why the standard DPI is 300
300 DPI is the graphic industry's standard for prints viewed up close (photos, magazines, books), ensuring the human eye can't make out individual print dots at normal viewing distance. For large prints viewed from farther away — banners, posters — lower DPIs (100-150) are common, since the viewing distance compensates for the lower resolution.
Worked example
A 6000×4000-pixel image (24 megapixels), at 300 DPI, allows a print up to 20×13.3 inches (about 50.8×33.9 cm) with no perceptible quality loss.
Limitations
The calculation assumes you'll use the image's full resolution with no cropping; if you need to crop the photo to fit the paper's aspect ratio, the effectively available resolution decreases. The ideal DPI also depends on print type and viewing distance — the 300 DPI used as the default here is a reference for photos viewed up close.
Frequently asked questions
Why use 300 DPI as the default?
Because it's the widely accepted graphic industry standard for photo prints viewed up close — at that density, the human eye can't distinguish individual print dots.
Can I print at a lower DPI than 300?
Yes, especially for large materials viewed from far away, like banners and posters, where 100 to 150 DPI is common and sufficient, since the viewing distance masks the lower resolution.
What happens if I print beyond the calculated maximum size?
The image will start losing noticeable sharpness, with visible pixelation or blurring, especially in prints viewed up close.