01Where 300 DPI came from, and why it is usually wrong
300dpi is roughly the finest detail a person with good eyesight can resolve at about 25 centimetres, which is reading distance. That makes it the correct target for something held in the hand: a book page, a small album, a postcard. It became the universal number because print shops needed one rule, not because it applies to every print.
Nobody views a metre-wide print from 25 centimetres. They stand back, because that is the only way to see the whole thing. And the further back they stand, the less resolution their eye can resolve, in direct proportion. At two metres, roughly 43dpi is genuinely all the eye can distinguish. This is why billboards are printed at 10 to 20dpi and look flawless from a car, and why a 24MP file makes an excellent two-metre gallery print despite "only" managing 20 by 13 inches at a strict 300dpi.
The Auto setting above uses this relationship instead of a fixed number. It is the real answer to "how big can I print", and it is usually far more generous than the 300dpi rule suggests.
02Cropping is more expensive than it looks
Cropping removes area, not just an edge, so the cost compounds. Take a native 3:2 frame from any standard camera. Crop it to 8x10 and you lose about 20% of the pixels. Crop it to a square and you lose a third. Crop to a 2:1 panoramic and you lose a quarter. Then remember that a crop for composition sits on top of the crop for aspect ratio, and a "slight tightening" of 20% on each axis costs another 36% of the file.
The practical response is at capture time, not in post. If you know a shot is going to be delivered square, or printed 8x10 for a client, frame it loose. Space around the subject is free while you are standing there and impossible to buy afterwards. The diagram above shows exactly what each ratio takes off your particular frame.
03Upscaling, and its limits
Machine-learning upscalers have genuinely changed this calculation. On a sharp, well-exposed, low-noise original, current tools produce believable results at two to four times the linear dimensions, which is four to sixteen times the area. That is real and worth using.
What they cannot do is recover information the sensor never recorded. A slightly soft frame upscales into a larger slightly soft frame, and noise upscales into larger, stranger noise. Upscaling multiplies whatever quality you started with; it does not add any. Treat the figures on this page as your reliable floor and any upscaling as headroom on top.
04Frequently asked questions
At a strict 300dpi, 28.8MP. At a realistic viewing distance for a print that size, about 200dpi is plenty, which needs roughly 12.8MP. A 24MP camera prints 16x20 comfortably.
Only for prints held in the hand. 300dpi is what the eye resolves at about 25cm. Larger prints are viewed from further away, and the requirement drops in proportion. Billboards run at 10 to 20dpi.
Modern upscalers add plausible detail and can buy two to four times the linear size on a sharp original. They cannot recover detail that was never captured, so a soft file becomes a larger soft file.
You lose area, not one edge. A 3:2 frame cropped to 8x10 loses about 20% of its pixels; cropped square it loses a third. If you know the delivery ratio, frame loose at capture.