Megapixels for Print Size Calculator

From megapixels to inches

Megapixels are the total pixel count in millions, and converting to a print size needs the aspect ratio as well.

For a 3:2 sensor, the pixel dimensions are found by taking the square root of the megapixel count divided by 6, then multiplying by 3 and 2. A 24 megapixel 3:2 sensor gives 6000 by 4000.

Divide by the required resolution for the print size. At 300 DPI that is 20 by 13.3 inches; at 200, 30 by 20. The megapixel figure alone is not enough because two cameras with the same count and different ratios print different shapes.

What megapixels do not tell you

Pixel count is one input to image quality and frequently not the limiting one. Sensor size, lens quality, focus accuracy, motion blur and noise all reduce the detail actually captured.

A 48 megapixel phone sensor and a 24 megapixel full frame camera do not produce equivalent detail, because the phone's individual photosites are far smaller and gather less light. In good conditions the gap narrows; in poor light it is enormous.

Which is why the answer to whether a file will print well is not answerable from the megapixel count. Viewing the image at 100% on screen shows the actual detail, and it frequently shows considerably less than the pixel count suggested.

The point of diminishing returns

For prints up to A3, roughly 12 to 16 megapixels is sufficient at 300 DPI. Above that the extra pixels only matter if the print is larger or the crop is heavy.

The practical benefit of higher counts is cropping latitude rather than print size. A 45 megapixel file cropped to half its area is still a 22 megapixel file, which prints large.

Which reframes the question for anyone choosing equipment. If prints are the output and they are A3 or smaller, resolution stopped being the constraint several generations ago, and lens quality and light gathering matter considerably more.

Sensor formats and their ratios

Most cameras use 3:2, inherited from 35mm film. Micro four thirds uses 4:3. Medium format is commonly 4:3 or 5:4. Phones often shoot 4:3 by default with a 16:9 option that crops.

The ratio determines which standard print sizes fit without cropping. 3:2 matches 4 by 6 exactly and crops for everything else. 4:3 matches nothing in the US standard set and is closer to 8 by 10.

For anyone printing regularly it is worth knowing which sizes a camera's native ratio fits, because composing with the eventual crop in mind is easier than fixing it afterwards.

Scanning, and choosing a resolution

Scanning works the same way in reverse: the scan resolution determines the pixel count, which determines the maximum print size.

Scanning a 6 by 4 inch photograph at 300 DPI gives 1800 by 1200 pixels, which reprints at the same size and no larger. Scanning at 1200 DPI gives 7200 by 4800, which prints at 24 by 16 inches.

For archival scanning the guidance is to capture more than currently needed, since rescanning is more work than storage. For film the resolutions are much higher because the original is small: 35mm negatives are commonly scanned at 3200 to 6400 DPI, producing files comparable to a modern digital camera.

There is one case where the megapixel figure genuinely constrains the print and it is worth naming: heavy cropping combined with a large output. A wildlife photograph cropped to a fifth of the frame and printed at A2 is asking a great deal of the sensor, and this is where the difference between a 24 and a 45 megapixel camera becomes real rather than theoretical. For anyone shooting subjects they cannot get close to, resolution is bought as cropping latitude rather than as print size.