What an aspect ratio means
The aspect ratio describes the shape of a rectangle, not its size. A 1280 × 720 video and a 3840 × 2160 video have different resolutions but the same 16:9 shape, so one can be scaled to the other without distortion. The first number is always the width, the second the height. A ratio above 1 is landscape, below 1 portrait, and 1:1 is square.
Ratios are written either as two whole numbers (16:9) or as a decimal relative to 1 (1.78:1). Cinema formats are usually written in decimal form, such as 1.85:1 and 2.39:1, while screens and cameras use whole numbers.
Try it here: Aspect Ratio Calculator
Open the full tool pageCommon ratios and where they are used
The table lists the ratios you meet most often, with typical pixel sizes. All sizes in a row have the same shape. The calculator has buttons for these ratios, so one click fills in the original size.
| Ratio | Decimal | Typical sizes | Used for |
|---|---|---|---|
| 16:9 | 1.78:1 | 1280 × 720, 1920 × 1080, 3840 × 2160 | HD and 4K video, TV, YouTube, most monitors |
| 9:16 | 0.56:1 | 1080 × 1920 | Vertical video: Reels, TikTok, Shorts, stories |
| 4:3 | 1.33:1 | 1024 × 768, 1600 × 1200 | Older screens, iPad, many compact cameras and phone photos |
| 3:2 | 1.5:1 | 6000 × 4000, 3000 × 2000 | DSLR and mirrorless photos, 4 × 6 prints |
| 16:10 | 1.6:1 | 1920 × 1200, 2560 × 1600 | Laptop and office monitors |
| 21:9 | about 2.37 to 2.39:1 | 2560 × 1080, 3440 × 1440 | Ultrawide monitors |
| 1:1 | 1:1 | 1080 × 1080 | Square posts, profile pictures, product photos |
| 4:5 | 0.8:1 | 1080 × 1350 | Portrait posts on Instagram |
| 2.39:1 | 2.39:1 | 4096 × 1716 | Widescreen cinema (scope) |
How to calculate the ratio of any size
Find the greatest common divisor (GCD) of width and height and divide both by it. For 1920 × 1080 the GCD is 120: 1920 / 120 = 16 and 1080 / 120 = 9, so the ratio is 16:9. For 3000 × 2000 the GCD is 1000, giving 3:2.
Some sizes do not reduce to a familiar pair. 1366 × 768, a common laptop resolution, reduces only to 683:384, but its decimal ratio of 1.779 is almost exactly 16:9 (1.778). Ultrawide 3440 × 1440 reduces to 43:18, about 2.39:1, which is sold as 21:9 even though 21:9 is exactly 2.33:1. That is why the calculator shows three things: the exact reduced ratio, the decimal ratio and the nearest common ratio, marked as exact or close.
Decimal ratios work too: enter 2.39 and 1 to get 239:100, or 1.85 and 1 for the flat cinema format.
Finding the missing width or height
To resize without stretching, keep the ratio constant. New height = new width × original height / original width. New width = new height × original width / original height. A 1920 × 1080 video scaled to 1280 wide: 1280 × 1080 / 1920 = 720. A 4000 × 3000 photo at 1200 high: 1200 × 4000 / 3000 = 1600.
In the calculator, enter the original size, then type either the new width or the new height; the other side is filled in. The Scale box does the same with a percentage, so 50 percent of 4000 × 3000 gives 2000 × 1500. The result also shows the new size in megapixels.
Rounding and even numbers for video
Pixels are whole numbers, so the exact result is often rounded. 1000 pixels wide at 16:9 is exactly 562.5 high, which becomes 563 with whole pixels. Most video encoders, such as H.264, require both sides to be even, so for video choose even pixels and the result becomes 562. Rounding changes the ratio by a tiny amount; the calculator tells you when the difference is noticeable.
When the result is only for layout, such as a CSS box or a print size in millimeters, choose decimals to keep the exact value.
Changing the ratio: crop or add borders
Scaling never changes the shape. To turn a 4:3 photo into a 16:9 image, you have two choices: crop away part of the top and bottom, or add bars or a background on the sides (letterboxing or pillarboxing). Stretching the image to the new shape is the third option, and almost always looks wrong because faces and circles become wider or narrower.
To get the crop size, keep the width and calculate the height at the new ratio: a 4000 × 3000 photo cropped to 16:9 at full width is 4000 × 2250. The image cropper on this site has the same ratio presets, and the image resizer scales images in your browser once you know the size you need. The calculator itself only works with numbers; nothing is uploaded.