Guide
Image dimensions and aspect ratio, explained
Almost every annoying thing an image does - the crop you did not ask for, the stretched face, the empty bars down the sides - comes from one mismatch: the shape you have is not the shape something wants. Knowing the three ways to resolve that is most of what there is to know.
Short answer
Aspect ratio is width against height, written as a pair like 16:9. When your shape does not match the target you have three choices: crop to fill (exact size, edges lost), fit inside (whole picture, shorter on one side), or stretch (exact size, distorted). Crop to fill is almost always the right one.
Dimensions and ratio are different things
Dimensions are the pixel counts themselves: 1920 x 1080. Aspect ratio is the relationship between them, simplified: 1920:1080 reduces to 16:9. Two images can share a ratio and have wildly different dimensions - 1280 x 720 and 3840 x 2160 are both 16:9.
This matters because requirements arrive in both forms. "At least 1,600 pixels on the longest side" is about dimensions. "Square" is about ratio. "Exactly 630 x 810" is both at once, and is the strictest kind there is.
The three ways to resolve a mismatch
Every resize tool does one of these. Most do not tell you which.
- Crop to fill. Scales until the target box is covered, then trims what hangs over. You get the exact dimensions and lose the edges; nothing is distorted. This is what you want almost every time.
- Fit inside. Scales until the whole picture fits. Nothing is lost and nothing is distorted, but the result comes out shorter than the target on one side - so it does not satisfy a rule asking for exact dimensions.
- Stretch. Forces both numbers. You get the exact dimensions and everything in the picture changes shape. Faces make it obvious immediately. It exists because some forms genuinely demand exact pixels and will take nothing else.
Common ratios and where they come from
16:9 - screens, video, most modern displays. 1280 x 720, 1920 x 1080, 3840 x 2160.
4:3 - older screens and projectors, and still what many cameras shoot. 1024 x 768, 800 x 600.
1:1 - profile pictures, product grids, anything that has to tile evenly.
4:5 - portrait social posts, and the shape a 600 x 750 document photo implies.
1.91:1 - link preview cards, which is roughly what 1200 x 630 is.
Common mistakes
Treating a maximum as a target. "No larger than 800 x 600" means fit inside. Cropping to exactly 800 x 600 also satisfies it, and throws away the edges of your picture for no reason.
Enlarging to reach a number. Scaling up interpolates between the pixels you have; it never recovers detail that was not captured. You can meet the requirement, but the result will be softer.
Letting the centre decide. An automatic crop takes the middle. If your subject is off-centre, crop it yourself first and resize afterwards.
Frequently asked questions
- What does 16:9 mean?
- For every 16 units of width there are 9 of height. It describes a shape rather than a size: 1280 x 720, 1920 x 1080 and 3840 x 2160 are all 16:9, differing only in how many pixels describe that same shape.
- How do I resize without stretching?
- Use crop to fill or fit inside - both keep the proportions. Stretch is the only one of the three that distorts, and it is never the default here. If a picture came back squashed, something forced both dimensions at once.
- Is it better to crop or to fit inside?
- Crop when the exact dimensions are the point - a form, a grid, a screen background - and the edges of the picture are expendable. Fit inside when the whole picture matters and the target is a ceiling rather than a requirement. If you are unsure which you are facing, read whether the requirement says "exactly" or "no larger than".
Last checked 2026-09. File sizes and platform limits change; where this guide states one, it is what we could verify at that date.