What is an aspect ratio?

The relationship between width and height, written as two numbers. 16:9 means sixteen units across for every nine down. What matters is that it describes shape and not size: 1600x900, 1280x720 and 800x450 are all 16:9, and all the same shape at different sizes. That is why a ratio is what platforms specify and pixels are what you supply.

My image says 959:540. What does that mean?

It means the dimensions do not divide neatly, which is normal for anything cropped by hand. 1918x1080 is two pixels narrower than 1920x1080, and reducing it exactly gives 959:540, which is correct and useless. This page shows both: the recognizable shape it is near, and the exact ratio underneath. If it says close to 16:9, treat it as 16:9, because two pixels is not a different shape.

Should I stretch or crop to change ratio?

Crop, almost always. Stretching changes the ratio by squashing the picture, which makes faces wider or narrower and is instantly visible even to people who could not say why. Cropping throws away part of the edge and leaves everything that remains correctly proportioned. The tool shows you the largest piece of your image that fits the shape you picked, which is what cropping to that ratio means in practice.

What size should I actually use?

Pick the ratio from what the picture is for, then choose a size large enough for the screen it lands on. For social, the common answers are on this page. As a rule, 1080 pixels on the short side is enough for almost any phone, and going much beyond 2000 mostly costs upload time rather than buying quality.

Why do platforms crop my photos?

Because your photo is a different shape from the space they have. Instagram's feed will not go taller than 4:5, so a 9:16 photo gets cut top and bottom, and a camera's 3:2 landscape gets cut at the sides in a square grid. Nothing has gone wrong, and the fix is to choose the ratio before you upload rather than let something choose it for you afterwards.

Is my image uploaded to work out the ratio?

No. The file is read by your own browser only far enough to learn its width and height, and then released. Nothing is uploaded, nothing is stored, and the calculator works with your connection off.

Aspect Ratio Calculator

This free aspect ratio calculator works out the ratio of any size, and the size for any ratio. Drop an image and it reads the dimensions for you, so you do not have to go and look them up first.

Drop an image to read its size

Or type the numbers below. Nothing is uploaded either way.

WidthHeight

16:9

Video, YouTube, most screens

1.7778 · landscape

Change the shape

Choose which side to hold, then pick a shape. The swatches are drawn at their real proportions.

What should I aim for?

The shape is usually decided by where the picture is going. These are the ones worth knowing.

Ratio	Where it is used	A good size
9:16	Instagram Stories, Reels, TikTok, YouTube Shorts	1080 x 1920
4:5	Instagram feed, portrait. The tallest it will not crop Instagram post resizer	1080 x 1350
1:1	Profile pictures, avatars, product grids Square cropper	1080 x 1080
2:3	Pinterest pins, and 4x6 photo prints	1000 x 1500
16:9	YouTube thumbnails, presentations, most screens	1280 x 720
3:2	What most cameras shoot, and 6x4 prints	1800 x 1200
Varies	Facebook covers, which have their own awkward shape Facebook cover resizer	820 x 312

Resizing several at once is what the social media image resizer is for, and the image resizer handles any size you like. If the shape is wrong rather than the size, crop it instead.

Runs entirely in your browser. Your images are never uploaded to a server.

How it works

Drop an image to read its width and height, or type them in yourself.
The ratio appears immediately, both as the shape people recognize and as the exact numbers.
To change shape, choose which side to keep, then pick a ratio. The other side follows.
If you are cropping rather than stretching, it tells you the largest piece of your image that fits.

Common use cases

Finding out what ratio a photo already is, before sending it somewhere with rules.
Working out the height for a 1600px wide banner that has to be 16:9.
Checking whether a photo will be cropped by Instagram before you post it.
Matching a set of images to one shape so a grid or a gallery looks deliberate.
Converting a camera's 3:2 photo to the 4:5 that fills the most space in a feed.

Frequently asked questions

What is an aspect ratio?: The relationship between width and height, written as two numbers. 16:9 means sixteen units across for every nine down. What matters is that it describes shape and not size: 1600x900, 1280x720 and 800x450 are all 16:9, and all the same shape at different sizes. That is why a ratio is what platforms specify and pixels are what you supply.
My image says 959:540. What does that mean?: It means the dimensions do not divide neatly, which is normal for anything cropped by hand. 1918x1080 is two pixels narrower than 1920x1080, and reducing it exactly gives 959:540, which is correct and useless. This page shows both: the recognizable shape it is near, and the exact ratio underneath. If it says close to 16:9, treat it as 16:9, because two pixels is not a different shape.
Should I stretch or crop to change ratio?: Crop, almost always. Stretching changes the ratio by squashing the picture, which makes faces wider or narrower and is instantly visible even to people who could not say why. Cropping throws away part of the edge and leaves everything that remains correctly proportioned. The tool shows you the largest piece of your image that fits the shape you picked, which is what cropping to that ratio means in practice.
What size should I actually use?: Pick the ratio from what the picture is for, then choose a size large enough for the screen it lands on. For social, the common answers are on this page. As a rule, 1080 pixels on the short side is enough for almost any phone, and going much beyond 2000 mostly costs upload time rather than buying quality.
Why do platforms crop my photos?: Because your photo is a different shape from the space they have. Instagram's feed will not go taller than 4:5, so a 9:16 photo gets cut top and bottom, and a camera's 3:2 landscape gets cut at the sides in a square grid. Nothing has gone wrong, and the fix is to choose the ratio before you upload rather than let something choose it for you afterwards.
Is my image uploaded to work out the ratio?: No. The file is read by your own browser only far enough to learn its width and height, and then released. Nothing is uploaded, nothing is stored, and the calculator works with your connection off.

Understand what this does

Plain explanations of what is happening to your file, and what it costs you.

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