Aspect Ratio Calculator
Solve for a missing width or height from any aspect ratio, or reduce two dimensions to their simplest ratio.
- Free, no account
- No watermark
- No usage limit
About the Aspect Ratio Calculator
Most aspect ratio calculators assume you already know the ratio, and that is the annoying part. You show up because you are about to post a Reel or a thumbnail and you just want the right size, and instead you get two empty boxes and a colon, so this one starts a step earlier. Tap Instagram Reel, or YouTube Thumbnail, or TikTok Video, and it fills in the ratio and the exact pixel size those platforms want, then draws the shape so you can see it before you commit, without the guesswork or digging through a size chart from last year.
You can still do the plain math, of course. Give it a ratio like 16:9 and one dimension and it returns the other the moment you type. Flip to Find ratio, drop in a width and height you already have, and it reduces them to the simplest whole-number ratio with the decimal shown next to it. The whole thing runs inside your browser, so nothing you type gets uploaded or saved, and it keeps working offline once the page has loaded.
How to use
- Start with a platform, or a ratio. Tap a social preset like Instagram Portrait or YouTube Video to load its ratio and recommended size in one move. Or skip the presets and set a ratio yourself with the 16:9, 4:3, 1:1, 9:16, and 21:9 buttons, or by typing any custom numbers.
- Enter the one dimension you know. Type a width to get the matching height, or a height to get the matching width. A preset already fills the width for you, so the height appears right away.
- Read the size and check the shape. The result panel shows the pixel size, and the preview box next to it draws that exact shape at scale, so a tall 9:16 looks tall and a square 1:1 looks square.
- Copy it. Hit Copy size to grab the width and height for your editor. Use Share to send someone a link that reopens the exact setup, or Embed to drop the live tool on your own page.
- Or work backward. Switch to Find ratio, enter a width and height, and read the simplified ratio plus its decimal.
Blank or zero fields show a dash instead of an error, so you never end up staring at a "NaN" wondering what you broke.
Social presets that hand you the size
Every platform publishes its own recommended sizes, and they change them just often enough that the chart you memorized is probably wrong now. A 4:5 Instagram portrait is 1080 by 1350. A YouTube thumbnail is 1280 by 720. A vertical Reel, Story, TikTok, or Short is 1080 by 1920. Miss one and the platform crops your work or pads it with bars, and you find out after you posted.
The presets carry all of that so you do not have to. Instagram gives you Square, Portrait, Reel, and Landscape. YouTube gives you Video, Thumbnail, and Shorts. TikTok, X, Facebook, and LinkedIn each drop in their common placements. One tap loads the ratio and the exact recommended width, the height falls out of the math, and the whole thing is scaled and previewed in front of you. If your team runs a slightly different house size, change the width and the height re-solves on its own, so the preset is a starting point rather than a rule.
You also see the shape. That sounds small until you have argued with a client about whether a post should be square or portrait, then realized you were both picturing different rectangles the whole time, which is what the preview box is there to settle. Tall, wide, square, ultrawide, it is drawn for you before you export a single frame.
Ratio is the shape, resolution is the size
This confuses more people than any other part of the topic, so it is worth being plain about. An aspect ratio is the relationship between width and height, like 16:9. It describes a proportion rather than a fixed size. A 16:9 frame looks identical whether it is 1280 by 720 or 3840 by 2160, because both hold the same width-to-height relationship, with one of them simply bigger and sharper than the other.
So 720p and 1080p are the same shape, with neither wider than the other. Pick the ratio for how the frame is framed, then pick the resolution for how crisp it needs to be. Those are two separate decisions, and mixing them up is how people end up wondering why their "16:9" export still got cropped. The presets set the shape for you and hand you a sensible resolution to go with it, and you are free to scale that resolution up or down while the shape holds.
When your content and the frame do not match
Sometimes the thing you have and the slot it goes into are different shapes. A widescreen clip headed for a vertical feed, say. You get three honest options and each one costs you something. Letterbox it, and black bars ride the top and bottom while every pixel stays visible. Pillarbox it, and the bars move to the sides instead. Or crop it, which fills the frame edge to edge and slices off whatever runs past the border.
If keeping the whole image matters, take the bars. If a full-frame look matters more and your subject sits safely in the middle, crop. The move that saves you is checking the crop on purpose instead of trusting the platform to auto-crop kindly, which it rarely does. Work out the size you can live with, resize to it, and you decide what gets cut instead of an algorithm.
Frequently asked questions
How does it find the missing dimension?
It scales the side you know by the ratio. Height comes from width times ratio height divided by ratio width, and width comes from height times ratio width divided by ratio height. A width of 1920 at 16:9 gives a height of 1920 times 9 divided by 16, which is 1080. Results round to a whole pixel, since you cannot export half of one.
Can I use my own ratio instead of a preset?
Yes. The presets and the ratio buttons are shortcuts, nothing more. Type any ratio width and height you want, including odd ones like 2.39:1 for anamorphic film or 3:2 for photography, and the tool treats it exactly the same. Editing a preset's width after you tap it works too, and the height re-solves on the spot.
Why does the preview matter if I already have the numbers?
Numbers are easy to misread when you are moving fast. 4:5 and 9:16 are both "portrait," but one is a lot taller than the other, and reading 1350 against 1920 does not make that obvious the way a drawn box does. The preview is a quick check that the shape is the one you meant before you build a whole layout around it.
Why doesn't my screen size reduce to a clean ratio?
Some sizes just do not. A 1366 by 768 laptop reduces to 683:384, which is correct and completely useless to say out loud. That is why the decimal sits right next to the simplified ratio in Find ratio mode. You read 1.78 and call it "basically 16:9" and move on with your day.
What does the decimal ratio mean?
It is width divided by height as a single number. 16:9 comes out to 1.78, and 4:3 to 1.33. Some design tools and spec sheets ask for that one value instead of a colon pair. It also makes comparing two shapes quick, because a bigger decimal is always a wider frame, no mental picturing required.
Do I need to keep text away from the edges of a Story?
For 9:16 vertical placements, yes. Platform buttons, captions, and profile bubbles crowd the top and bottom of a Story or Reel. The pixel size a preset gives you is the full canvas, but treat the outer margins as a danger zone and keep faces and important words nearer the center.
Is anything I enter sent anywhere?
No. The calculator runs entirely in your browser. Nothing you type is transmitted, stored, or logged, and every result is worked out on your own device. That also means it keeps calculating with the page open even if your connection drops.