What is an aspect ratio?
The aspect ratio is the shape of a rectangle: its width compared with its height, written as two numbers such as 16:9. It says nothing about size. A 1280 × 720 video, a 1920 × 1080 laptop screen and a 65-inch 4K TV are all 16:9, so a picture made for one fits the others without black bars or stretching. When the ratios differ, something has to give: the picture is cropped, squeezed, or shown with bars at the sides (pillarbox) or at the top and bottom (letterbox).
How to calculate an aspect ratio
Divide the width and the height by their greatest common divisor, the largest number that divides both. For 1920 × 1080 that number is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9. For 1280 × 1024 it is 256, giving 5:4. The resolution calculator above does this as you type, shows the ratio as a decimal (16:9 is 1.778:1), and draws the shape so you can see it.
Some screens don't divide neatly. 1366 × 768 works out to 683:384, about 1.779:1, which is so close to 16:9 that everyone calls it 16:9. The calculator gives the exact pixel ratio and the common name. The same is true of ultrawide monitors: 21:9 is a marketing name, while 3440 × 1440 is really 43:18 and 2560 × 1080 is 64:27.
How to resize without stretching
To keep the shape, the new height must be the new width × the original height ÷ the original width. A 16:9 image resized to 1280 pixels wide becomes 1280 × 9 ÷ 16 = 720 pixels high. Pick a ratio in step 2, or press Use ratio from step 1, then type the new width or the new height; the other side fills in. This image aspect ratio calculator works for photos, thumbnails and banners, and as a video aspect ratio calculator it flags odd results, because most video encoders need even widths and heights.
Units don't matter. As an aspect ratio calculator in inches it scales a 4 × 6 print to 8 × 12 or a 16:9 projector screen to its height, and as an aspect ratio calculator in pixels it gives exact pixel sizes, rounded to whole pixels with the exact value shown when they don't divide evenly.
Common aspect ratios and resolutions
| Ratio | Decimal | Common resolutions | Used for |
|---|---|---|---|
| 16:9 | 1.78 | 1280 × 720, 1920 × 1080, 2560 × 1440, 3840 × 2160 | TVs, most monitors and laptops, YouTube |
| 16:10 | 1.6 | 1280 × 800, 1920 × 1200, 2560 × 1600 | Laptops (MacBook), office monitors |
| 4:3 | 1.33 | 1024 × 768, 1600 × 1200, 2048 × 1536 | Old TVs and monitors, iPad, many cameras |
| 3:2 | 1.5 | 3000 × 2000, 6000 × 4000 | DSLR photos, Surface laptops |
| 21:9 (ultrawide) | 2.37 to 2.39 | 2560 × 1080, 3440 × 1440 | Ultrawide monitors |
| 1:1 | 1 | 1080 × 1080 | Square social posts |
| 4:5 | 0.8 | 1080 × 1350 | Portrait feed posts |
| 9:16 | 0.56 | 1080 × 1920 | Stories, Reels, Shorts and TikTok |
Screen aspect ratio and size
A screen aspect ratio calculator helps when you compare monitors. The diagonal alone hides the shape: a 34-inch ultrawide is about 31.4 inches wide but only 13.1 inches high, a little less tall than a 27-inch 16:9 monitor at 13.2 inches. Step 3 turns any diagonal and ratio into width and height in inches and centimeters. To see the resolution of the screen you are using now, press Use my screen, or open the screen resolution checker for the full details, including scaling.
Pick 16:9 in step 2 and the tool works as a 16:9 aspect ratio calculator for any width or height. If you work with 16:9 often, a quick rule as a 16:9 calculator: the height is the width × 0.5625, and the width is the height × 1.778.
How we tested this tool
We tested the calculator by script in Google Chrome on Windows with 25 width and height pairs and checked every answer: 1920 × 1080, 1280 × 720 and 3840 × 2160 (16:9), 1920 × 1200 and 2560 × 1600 (16:10, shown as 16:10 rather than 8:5), 1024 × 768 (4:3), 1280 × 1024 (5:4), 5120 × 1440 (32:9), 1080 × 1920 (9:16), 1080 × 1350 (4:5), 1080 × 1080 (1:1), 6000 × 4000 (3:2), 1366 × 768 (683:384, commonly 16:9), 3440 × 1440 and 2560 × 1080 (43:18 and 64:27, commonly 21:9), an iPhone at 1179 × 2556 (commonly 9:19.5), decimal ratios such as 2.39 × 1, and empty, zero, negative and text input, which show a prompt instead of a result. For resizing, 16:9 at 1280 wide gave 720 and at 1080 high gave 1920; 4:3 at 1001 wide gave 750.75, rounded to 751 with an even-number suggestion for video. For the diagonal, 27 inches at 16:9 gave 23.53 × 13.24 inches and 34 inches at 43:18 gave 31.36 × 13.13 inches. Use my screen was checked with a simulated 1536 × 864 screen at 125% scaling (it fills in 1920 × 1080) and with a browser that reports no screen size. We also measured the preview shape for 16:9, 9:16, 43:18, 1:1 and extreme 5000 × 1 input: it keeps the exact ratio and stays inside its box. The page was checked on desktop and at a 375 px phone width. Testing caught four problems we fixed: 1920 × 1200 was shown as 8:5, decimal ratios were shown as 239:100, phone screens were not recognized, and the preview shape was not drawn to scale.
Frequently asked questions
How do I calculate an aspect ratio?
Divide the width and the height by their greatest common divisor. For 1920 × 1080 the greatest common divisor is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9. The calculator does this for you as you type.
How do I resize an image without stretching it?
Keep the ratio the same: new height = new width × original height ÷ original width. A 1920 × 1080 image resized to 1280 pixels wide must be 720 pixels high. Enter the ratio and one new side in the resize section to get the other.
Why does 1366 × 768 not come out as exactly 16:9?
Because 1366 ÷ 768 is 1.779, a tiny bit wider than 16:9 (1.778). The exact pixel ratio is 683:384, but the screen is sold and described as 16:9. The calculator shows both.
Is 21:9 really 21:9?
Not exactly. 21:9 is a marketing name for ultrawide monitors. 2560 × 1080 is 64:27 (2.37:1) and 3440 × 1440 is 43:18 (2.39:1), while 21:9 itself would be 2.33:1.
Can I use inches or centimeters instead of pixels?
Yes. The ratio and the resize math work with any unit, so you can enter inches, centimeters or pixels. To get the width and height of a screen from its diagonal in inches, use the diagonal section.
Why should video dimensions be even numbers?
Most video encoders, including H.264 with the usual 4:2:0 color format, store color for 2 × 2 blocks of pixels, so width and height must be even. The calculator suggests the nearest even size when a result is odd.