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Arithmetic
Megapixel calculator
Megapixels are just width times height with the zeros hidden. The useful part is what that number lets you actually do.
Dimensions give megapixels. Megapixels plus the current shape give dimensions.
Result
Megapixels stopped mattering a while ago
Twelve megapixels prints beautifully at A4 and covers any screen you own. Phone makers moved to 48 and 200 not because the pictures needed it but because binning several small photosites into one output pixel improves low light performance. The headline number is a marketing artefact more than a quality one.
What does matter is whether you have enough pixels for the output you want. Roughly: 2 MP for a full screen, 6 MP for a 6 by 4 print, 9 MP for A4 at proper resolution, 18 MP for A3. Beyond that you are buying the ability to crop.
Common questions
Does more megapixels mean a better photo?
No. Lens quality, sensor size and light have far more effect. A 12 MP photo from a good sensor beats a 108 MP one from a tiny one in almost every situation.
How many megapixels do I need to print A3?
About 18 at 300 DPI. At 200 DPI, which is perfectly acceptable for something viewed from more than arm's length, around 8 is enough.
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