Image resolution explained
What megapixels actually buy you, how pixel dimensions drive file size, and how to work out the size an image should be.
In short
Resolution is the pixel dimensions of an image. File size scales with the pixel count, so halving both dimensions quarters the data. Work backwards from where the image will be displayed.
An image's resolution is simply how many pixels wide and tall it is. A 12-megapixel phone photo is about 4000 × 3000 — twelve million pixels, each with a colour.
Pixel count drives everything
Because the count is width × height, halving both dimensions quarters the pixels. Going from 4000 × 3000 to 2000 × 1500 removes 75% of the data. That squaring relationship is why resizing is such a powerful lever on file size, and why small reductions in dimensions feel disproportionately effective.
| Dimensions | Megapixels | Relative |
|---|---|---|
| 4000 × 3000 (phone photo) | 12.0 | 100% |
| 1920 × 1440 | 2.8 | 23% |
| 1200 × 900 | 1.1 | 9% |
| 800 × 600 | 0.5 | 4% |
More resolution is not more quality
Resolution sets the ceiling on detail; it does not create any. A soft, badly-lit 12-megapixel photo has no more usable detail than a sharp 4-megapixel one. And enlarging an image cannot add detail — the extra pixels are interpolated from their neighbours, which is why upscaled images look soft rather than sharp.
Working out what you need
Start from where the image will be seen. A card on a web page displayed at 400 pixels wide needs about 800 pixels to look crisp on a high-density screen — twice the display size is the usual rule. Anything beyond that is bytes nobody sees.
Mistakes worth avoiding
- Uploading full-resolution camera images to a web page. It is the most common cause of a slow-loading site.
- Upscaling to hit a "minimum resolution" requirement. It satisfies the number and not the intent.
- Cropping tightly from an already-small image and then wondering why it is soft.
Tools used in this guide
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