Aspect Ratio Calculator

A ratio is a shape, not a size

An aspect ratio says how wide something is compared with how tall it is, and nothing about how big it is. 1280 by 720 and 3840 by 2160 are very different sizes with exactly the same shape, 16:9. That is why the ratio is the thing to match when a video, a photo or a design has to fit somewhere, and the size is what you choose afterwards.

Getting from a size to a ratio means dividing both sides by their greatest common divisor, the largest whole number that goes into both. 1920 and 1080 share 120, which leaves 16 and 9. The same shape can also be written as a single decimal, the width divided by the height, so 16:9 is 1.78:1. Cinema uses that form, which is where 1.85:1 and 2.39:1 come from.

The arithmetic is easy; the names are the trap. Plenty of real sizes are close to a famous ratio without being it. 1366 by 768 reduces to 683:384, a hair wider than 16:9. Ultrawide monitors sold as 21:9 are really 64:27, 43:18 or 12:5. 1440 by 900 reduces to 8:5, which is the same shape as 16:10 but does not look like it. This page gives the exact reduced ratio as the answer and puts the familiar name next to it, so you know both what the numbers are and what people will call them.

Which ratio for which job

• 16:9 for landscape video, TVs, most monitors, slide decks and video thumbnails.
• 9:16 for vertical video: Stories, Reels, TikTok and Shorts. It is 16:9 turned on its side.
• 1:1 for square posts, profile pictures, app icons and album cover art.
• 4:5 for portrait feed posts, which take more of a phone screen than a square, and for 8 by 10 inch prints.
• 3:2 for photos from most dedicated cameras and for 6 by 4 inch prints.
• 4:3 for many phone camera photos, older monitors and projectors, and a lot of tablet screens.
• 21:9 for ultrawide monitors and a cinematic letterbox look, keeping in mind that the actual panels are slightly wider than the name.

Social platforms publish their own recommended sizes and change them from time to time. The ratios above were the usual ones at the time of writing, but check a platform’s current upload guidance before a big campaign.

How to find the ratio of a size

1. Stay on the “Find the ratio” tab and type the width and the height. Pasting 1920x1080 into either box fills both.
2. Read the three figures: the reduced ratio, the decimal ratio and whether the shape is landscape, portrait or square. The hint under the ratio says when a common name such as 16:10 applies, or when the size is only close to one.
3. Open “How it was reduced” to see the greatest common divisor and the two divisions, which is handy when you have to show your working.
4. Open the size list to see other sizes with the same shape, and tap one to load it. Use the swap button to turn landscape into portrait.

How to fill in the missing width or height

1. Switch to “Resize to a ratio” and pick a ratio, or choose Custom and type your own, decimals included, such as 2.39 and 1.
2. Type the side you know into the width or the height box. The other one fills in as you type.
3. Choose how to round. Whole pixels rounds to the nearest pixel. Even numbers rounds to the nearest even value, which video encoders usually need.
4. Read the hint under the result. It gives the exact value before rounding and how far the rounded size drifts from the ratio, so you can decide whether a different width would divide cleanly.

The height is the width multiplied by the second number of the ratio and divided by the first. At 16:9 a 1000 pixel wide frame is 562.5 pixels tall, so it becomes 563, or 562 when you ask for even numbers.

When the shapes do not match: crop, fit or stretch

Knowing the target ratio is half the job. Putting a picture of one shape into a frame of another always costs something, and there are only three options:

• Crop: fill the frame and cut off whatever sticks out. Nothing is distorted, but part of the picture is gone.
• Fit: keep the whole picture and fill the gaps with bars or a background. Black bars on a film are this.
• Stretch: force the picture to the new shape. Faces get wider or thinner, and viewers notice almost at once.

The size this page gives you is the target for a crop or a fit. Stretching is almost never what you want, which is why photo and video editors keep a lock proportions switch on by default.

Copying the result into other software

• Copy ratio copies the reduced ratio with a colon, such as 16:9, which is what crop tools and export dialogs expect.
• Copy size copies the size with a plain letter x and no spaces, such as 1920x1080. That is the form command line tools such as ffmpeg accept, and it pastes cleanly into a file name.
• The multiplication sign on screen is for reading only. It never reaches the clipboard, because many programs do not accept it as a separator.
• On a phone, tap a size in the list to load it rather than retyping, then copy the result you need.

Aspect ratios in CSS, video tools and spreadsheets

On the web, the CSS aspect-ratio property keeps a box in shape while its width changes. It is written with a slash rather than a colon:

.video { width: 100%; aspect-ratio: 16 / 9; }

Giving an image its width and height attributes in HTML does a similar job: modern browsers work out the ratio from them and reserve the space before the image loads, so the page does not jump.

In video tools

In ffmpeg, the scale filter takes a width and a height, and a height of -2 means keep the ratio and round to an even number, so scale=1280:-2 is the even rounding on this page in one step. Editors such as Premiere Pro, DaVinci Resolve and CapCut set the ratio in the project or sequence settings, and every clip is then fitted or cropped to it.

In a spreadsheet

Excel and Google Sheets both have a GCD function, so with the width in A1 and the height in B1 the ratio is =A1/GCD(A1,B1)&":"&B1/GCD(A1,B1). That works for whole numbers only; scale decimals up first.

What this calculator assumes

• Square pixels. Some older and professional video formats store pixels that are wider or narrower than they are tall, so the stored size and the displayed shape differ. Work from the display size in that case.
• Rounding changes the ratio slightly. Whenever the exact value is not a whole number, the rounded size is a little off. The hint says by how much; below a tenth of a percent nobody will see it.
• Ratio says nothing about physical size. Two 16:9 screens can be any diagonal, and two 4:5 prints any size. For a physical size, pair the ratio with a length in the unit converter.
• Platform sizes change. The named uses in the size lists were accurate at the time of writing, but upload specifications are set by each platform.