Three terms get thrown around constantly and mixed up almost as often: resolution, DPI, and file size. They're related, but they are not the same thing, and confusing them leads to genuinely bad decisions — like uploading a 12 MB photo to a website, or sending a printer something that comes out blurry.
Resolution: how many pixels
Resolution is simply the number of pixels in your image, written as width × height.
A photo that is 4,000 × 3,000 has twelve million pixels — twelve megapixels. That's all "12 MP" on a camera specification means.
Resolution has no physical size. A 4,000-pixel-wide image isn't "big" or "small" in centimetres; it's just 4,000 pixels. How large it appears depends entirely on what you display or print it on.
What resolution you actually need
| Use | Sensible width |
|---|---|
| Social media post | 1,080–1,600 px |
| Website image | 1,200–2,000 px |
| Email attachment | 1,600–2,000 px |
| Full-screen on a 4K monitor | 3,840 px |
| Printing at A4 | ~2,500 px |
Notice that almost nothing on this list needs your camera's full 4,000 pixels. That's the single most useful fact in this article.
DPI: only meaningful when printing
DPI means dots per inch — how many pixels get squeezed into each inch of printed paper. PPI (pixels per inch) means the same thing in practice.
Here is the part that confuses everyone: DPI is a printing instruction, not a property of the image. It is a number written in the file's metadata that tells a printer how large to print it. It does not change a single pixel.
The same 3,000 × 2,000 photo:
- At 300 DPI, prints at 10 × 6.7 inches
- At 150 DPI, prints at 20 × 13.3 inches
- At 72 DPI, prints at 41.7 × 27.8 inches
Identical file, identical pixels, identical file size. Only the printed dimensions change.
"Save it at 300 DPI" is meaningless for screens. A screen displays pixels; it ignores the DPI field entirely. If someone asks for a "300 DPI image" for a website, what they actually need is enough pixels. Ask them for the dimensions in pixels instead.
Why 300 DPI became the standard
At normal reading distance, most people can't distinguish detail finer than about 300 dots per inch, so it became the print industry's default for quality work. For a large poster viewed from several metres away, 150 DPI is perfectly adequate — and at billboard distance, far less.
Working out what a printer needs
Multiply the printed size in inches by the DPI:
- A4 (8.3 × 11.7 in) at 300 DPI → 2,490 × 3,510 pixels
- 6 × 4 in photo at 300 DPI → 1,800 × 1,200 pixels
- A3 poster at 150 DPI → 1,750 × 2,480 pixels
File size: how many bytes on disk
File size is what fills your storage and what upload limits measure. Three things control it:
1. Resolution
More pixels means more data. This is roughly proportional to the pixel count, so halving both width and height cuts the data to about a quarter — not a half. This is why resizing is far more effective than people expect.
2. Compression quality
The JPG quality setting decides how much detail is thrown away. Going from 100% to 80% often halves the file with no visible difference. Going below 50% starts producing obvious blocks.
3. Image content
This one surprises people. A photo of a plain blue sky compresses to a fraction of the size of a photo of a busy market, at identical resolution and quality — because compression works by finding repetition, and detailed scenes have less of it.
It's also why a photo taken against a plain wall can be half the size of the same subject in a cluttered room, which is genuinely useful when you're fighting a tight upload limit.
Putting it together
"I need to email these photos"
Resolution is what matters. Resize to about 1,800 px wide, quality 80%. DPI is irrelevant.
"The printer wants 300 DPI"
Ask what physical size they're printing. Multiply inches × 300 to get the pixels you need. If your photo has that many pixels, you're fine.
"The form says maximum 100 KB"
File size is the constraint. Reduce dimensions first, then quality. See our guide to online form uploads.
"My website is slow"
Almost always oversized images. Nobody needs a 4,000 px photo in a 600 px space. Resize to roughly double the display size, use WebP, and compress.
The thing you can't undo
You can always make an image smaller. You can never truly make it bigger.
Enlarging a 500-pixel image to 2,000 pixels doesn't recover detail that was never captured — software just invents plausible-looking pixels between the real ones, and the result looks soft or artificial. AI upscaling has improved this considerably, but it is still guessing.
So: keep your originals at full resolution, and always work on copies when you compress. Storage is cheap; a detail you threw away last year is gone for good.
Resize and compress your images →