What is an aspect ratio?
An aspect ratio is the shape of a rectangle written as width to height. A 1920 × 1080 screen is 16:9: for every 16 units across, it is 9 units tall. A square Instagram post is 1:1. A phone-screen video is 9:16, which is 16:9 turned on its side.
The Aspect Ratio Calculator does two jobs. Give it a width and a height and it reduces them to the simplest whole-number ratio, so 1920 × 1080 becomes 16:9. Give it a new width (or a new height) and it works out the other side so the picture keeps its shape, so 1920 × 1080 scaled to 1280 wide becomes 1280 × 720. It runs in your browser. Nothing is uploaded, and there's no account.
Two numbers is all it takes. You don't need an image file, only its size. That makes it just as useful for planning a print, a slide or a video frame as for resizing a photo you already have.
How to use the Aspect Ratio Calculator
- Type the width and height. The fields open with 1920 and 1080 filled in.
- Press Calculate Ratio. The simplified ratio appears under the button. The tool doesn't recalculate as you type, so press the button again after every change.
- To scale, fill in New Width or New Height. Leave the other one empty and press Calculate Ratio again. The missing side appears below, rounded to the nearest whole pixel.
Already know the ratio but not the pixels? Type the ratio itself into Width and Height. Width 16, Height 9, New Width 1200 gives a height of 675. That's how you answer "what is 1200 pixels wide at 16:9?" without any math.
Worked example. You have a 1920 × 1080 photo and need a 1280-pixel-wide thumbnail. Enter 1920 and 1080, then 1280 as the New Width, and press Calculate Ratio. The tool shows 16:9 and Height: 720px. Now clear New Width, type 500 as the New Height and press the button again. The answer is Width: 889px, because 500 × 1920 ÷ 1080 is 888.9, rounded up.
How aspect ratio is calculated
Finding the ratio means dividing both sides by the biggest number that goes into both of them evenly, the greatest common divisor. For 1920 and 1080 that number is 120:
- 1920 ÷ 120 = 16
- 1080 ÷ 120 = 9
So 1920 × 1080 is 16:9. The tool does this with Euclid's algorithm, the same method a greatest common factor calculator uses. It's also the same step as reducing a fraction: 1920/1080 simplifies to 16/9, which a fraction simplifier will show you too.
Scaling is one line of cross-multiplication. The new side keeps the same proportion as the old one:
- New height = new width × original height ÷ original width
- New width = new height × original width ÷ original height
Some people write ratios as a decimal against 1 instead. Divide width by height: 1920 ÷ 1080 = 1.78, so 16:9 is also "1.78:1". Cinema uses this form (1.85:1, 2.39:1). Screens and social apps use the whole-number form. They describe the same shape.
Common aspect ratios and their pixel sizes
Every size in this table was run through the aspect ratio calculator, and the middle column is what it returns.
| Pixel size | Calculator shows | Decimal | Where you see it |
|---|---|---|---|
| 1920 × 1080, 1280 × 720, 3840 × 2160 | 16:9 | 1.78:1 | HD and 4K TV, YouTube, most monitors |
| 1080 × 1920 | 9:16 | 0.56:1 | Phone video, Stories, Reels, Shorts |
| 1080 × 1080 | 1:1 | 1:1 | Square posts, profile pictures, album art |
| 1080 × 1350 | 4:5 | 0.8:1 | Portrait feed posts, 8 × 10 prints |
| 1024 × 768, 4032 × 3024 | 4:3 | 1.33:1 | Older TVs, iPads, most phone photos |
| 6000 × 4000, 720 × 480 | 3:2 | 1.5:1 | DSLR and mirrorless cameras, 4 × 6 prints |
| 1920 × 1200, 1440 × 900 | 8:5 | 1.6:1 | Laptop screens sold as "16:10" |
| 1500 × 500 | 3:1 | 3:1 | X (Twitter) header |
| 1584 × 396 | 4:1 | 4:1 | LinkedIn profile banner |
| 820 × 312 | 205:78 | 2.63:1 | Facebook page cover on desktop |
The 1:1 aspect ratio
1:1 means width and height are equal: a square. Any size works as long as both numbers match, so 1080 × 1080, 500 × 500 and 3000 × 3000 are all 1:1. To make a square from a wider photo, you have to cut something off. The Image Cropper has a 1:1 lock that keeps the crop box square while you drag it.
When the simplest ratio looks wrong
Plenty of real screens don't reduce to the neat number on the box. The Aspect Ratio Calculator shows the exact ratio, not the marketing one, and that surprises people.
| Pixel size | Sold as | Calculator shows | Why |
|---|---|---|---|
| 1366 × 768 | 16:9 | 683:384 | 1366 isn't a multiple of 16. It's 1.7786:1, a hair off 16:9 (1.7778:1). |
| 1920 × 1200 | 16:10 | 8:5 | 16:10 reduces to 8:5. Same shape, smaller numbers. |
| 2560 × 1080 | 21:9 | 64:27 | "21:9" is a label. The real shape is 2.37:1. |
| 3440 × 1440 | 21:9 | 43:18 | Also called 21:9, but it's 2.39:1, slightly wider than 2560 × 1080. |
| 4096 × 2160 | 4K | 256:135 | Cinema 4K is about 1.9:1. TV 4K (3840 × 2160) is a clean 16:9. |
| 1200 × 630 | "1.91:1" | 40:21 | The usual link-preview size. 1200 ÷ 630 is 1.905. |
None of these is an error. If you want to know whether a size is "close enough" to a standard, divide width by height and compare decimals. 683:384 and 16:9 differ by less than 0.1%, which no viewer will ever notice.
Limits and common mistakes
A few things trip people up with any aspect ratio calculator, this one included. Here's what to check first if a result looks off.
- Whole numbers only. The tool reads each field as a whole number and drops anything after a decimal point. Typing 2.39 and 1 gives 2:1, not 2.39:1. To work with a decimal ratio, multiply both sides by 100 first: Width 239, Height 100.
- No thousands separators. "1,920" is read as 1, so you'd get 1:1080. Type 1920.
- Press the button after every change. Results don't update as you type. If the answer looks stale, it probably is.
- Fill one scale field at a time. With both New Width and New Height filled in, only the height result is shown. Clear the one you don't need.
- Results are rounded to whole pixels. 1000 wide at 16:9 is really 562.5 tall. The tool shows 563. Video encoders often want even numbers, so you may want 562 instead.
- Units don't matter, as long as they're whole. The fields say px, but the math works for inches, centimeters or anything else. An 8 × 10 inch print returns 4:5, the same shape as a 1080 × 1350 post.
- It doesn't do diagonals. For screen size and sharpness from a diagonal, use the PPI calculator.
Changing ratio means cropping or adding bars. To turn a 1920 × 1080 (16:9) frame into 4:3 and keep the full height, enter 4 and 3 with 1080 as the New Height. You get 1440 wide, so 480 pixels have to go, 240 from each side. Keep the full width instead and 4:3 needs 1440 tall, which leaves 180-pixel black bars above and below.
Why the right ratio matters
Stretching. Force a 16:9 photo into a 4:3 box without cropping and everything gets squeezed sideways. Faces get narrower and circles turn into ovals. Scaling both sides by the same factor prevents it, and that's exactly what the scale fields do. To resize an actual file with its shape locked, use the Image Resizer.
Cropping you didn't choose. Upload a 3:2 camera photo to a slot that wants 4:5 and the app cuts off the sides for you, sometimes through someone's shoulder. Work out the target shape first and crop it yourself.
Black bars. A 4:3 video in a 16:9 player gets bars on the left and right (pillarboxing). A wide 2.39:1 film on a 16:9 TV gets bars on the top and bottom (letterboxing). Matching the ratio removes both.
Prints. A 4 × 6 print is 2:3 and an 8 × 10 is 4:5. A 6000 × 4000 camera file fits a 4 × 6 exactly, but an 8 × 10 from the same file loses about 17% of its width. Check before you order, not after.
Frequently asked questions
Is 1366 × 768 a 16:9 resolution?
Almost. It's sold as 16:9, but the exact ratio is 683:384, or 1.7786:1. True 16:9 is 1.7778:1. The difference is too small to see, so treat it as 16:9 for anything practical.
Is 21:9 really 21:9?
No. It's a marketing name. 2560 × 1080 reduces to 64:27 (2.37:1) and 3440 × 1440 to 43:18 (2.39:1). Real 21:9 would be 2.33:1.
What aspect ratio is 4K?
TV and monitor 4K (3840 × 2160) is exactly 16:9. Cinema 4K (4096 × 2160) is 256:135, about 1.9:1, a little wider.
Why is my result one pixel off from another calculator?
Rounding. Scaled sizes often land on a half pixel, and this tool rounds to the nearest whole one: 1000 wide at 16:9 is 562.5, shown as 563. Other tools may round down to 562. Both keep the shape to within a pixel.
Can I use inches or centimeters?
Yes, as long as the numbers are whole. A ratio has no units, so 8 × 10 inches gives 4:5. For 8.5 × 11, enter 85 and 110 and you'll get 17:22.
Does the calculator upload my image?
There's no image to upload. You only type two numbers, and the math runs in your browser.