An aspect ratio describes the shape of a rectangle — its width compared with its height — without saying anything about its size. A 1280 × 720 video and a 3840 × 2160 TV picture are both 16:9. This calculator works out the ratio from any pair of dimensions, or finds the width or height you need to keep a ratio when resizing a video, image, screen or canvas.
How to use the aspect ratio calculator
- Choose what to find: the aspect ratio, the height or the width.
- To find a ratio, enter the width and height. To find a missing side, pick a ratio from the list (or choose Custom and enter both parts) and enter the side you know.
- Keep Round to whole pixels on for screen and image work.
- Read the result on the tape. A scaled outline shows the shape, and a table lists other sizes with the same ratio.
Aspect ratio formulas
GCD is the greatest common divisor of the width and height, and rw:rh is the ratio. The decimal form, W ÷ H, is written as a number to 1 — 16:9 is 1.78:1 and cinema scope is 2.39:1.
Worked example
Finding a ratio. A monitor’s resolution is 1920 × 1080. The greatest common divisor of 1920 and 1080 is 120, and 1920 ÷ 120 = 16, 1080 ÷ 120 = 9, so the ratio is 16:9. As a decimal, 1920 ÷ 1080 ≈ 1.78:1.
Finding a missing side. You need a 16:9 thumbnail 1280 pixels wide. Height = 1280 × 9 ÷ 16 = 720 pixels. For a vertical 9:16 story 1920 pixels tall, width = 1920 × 9 ÷ 16 = 1080 pixels.
Common aspect ratios
| Ratio | Decimal | Typical resolutions and uses |
|---|---|---|
| 1:1 | 1.00 | 1080 × 1080 square posts |
| 4:3 | 1.33 | 1024 × 768, classic TV, iPad-style tablets |
| 3:2 | 1.50 | 6000 × 4000 camera photos, some laptops |
| 16:10 | 1.60 | 1920 × 1200, 2560 × 1600 laptops |
| 16:9 | 1.78 | 1280 × 720, 1920 × 1080, 3840 × 2160 |
| 21:9 (64:27) | 2.37 | 2560 × 1080 ultrawide monitors |
| 2.39:1 | 2.39 | Anamorphic widescreen films |
| 9:16 | 0.56 | 1080 × 1920 vertical video |
| 4:5 | 0.80 | 1080 × 1350 portrait social posts |
Practical tips
Match the ratio to avoid black bars
When the content’s ratio differs from the screen’s, players add bars: letterboxing (top and bottom) for wider content, pillarboxing (left and right) for narrower content. A 4:3 video on a 16:9 TV fills only 75% of the screen width.
Crop or pad, never stretch
If an image is the wrong shape, crop it or add a border. Stretching to fit changes the ratio and distorts faces and circles. The image resize calculator keeps the ratio locked while you change the size.
Use even numbers for video
Most video encoders require even pixel dimensions. If the calculated height is odd, round to the nearest even number; the distortion is far too small to see.
To turn a ratio and a diagonal into physical inches, use the screen size calculator, and for sharpness use the PPI calculator.
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 GCD is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9.
How do I find the height for a 16:9 width?
Multiply the width by 9 and divide by 16. A 1280-pixel-wide 16:9 video is 1280 × 9 ÷ 16 = 720 pixels tall.
Is 21:9 really 21:9?
Not exactly. 'Ultrawide 21:9' is a marketing name. Monitors at 2560 × 1080 or 3440 × 1440 are 64:27 (2.37:1) or 43:18 (2.39:1), slightly wider than a true 21:9 (2.33:1). The calculator uses 64:27 for its 21:9 preset.
Why is 1366 × 768 shown as 683:384?
That is its exact simplified ratio. It is sold as 16:9 because 1366 ÷ 768 ≈ 1.779 is within 0.1% of 16 ÷ 9 ≈ 1.778. The calculator shows the exact ratio and the closest common one.
What aspect ratio should I use for vertical video?
9:16, the portrait version of 16:9, such as 1080 × 1920. Short-form video apps and phone stories use it to fill a phone screen held upright.