Calculators
Aspect Ratio Calculator
Find the aspect ratio of any width and height (like 16:9), then resize to a new dimension while keeping the same ratio so images and videos never look stretched. Runs in your browser.
New size keeping the ratio: 1280 × 720
How to find a ratio and resize to match
- Enter the original width and height in pixels (or any other unit, as long as both use the same one) in the two boxes under Original size.
- Read the simplified ratio in the panel underneath, for example 16 : 9 for 1920 × 1080.
- Type either the width you want in the New width box or the height you want in the New height box. The other box fills in with the matching dimension and the line below reports the full new size, for example 1280 × 720.
- If the calculated dimension has decimals, round it to a whole pixel yourself before using it in an image editor or CSS.
Simplifying with the greatest common divisor
The tool divides both dimensions by their greatest common divisor, found with Euclid's algorithm. For 1920 × 1080 the divisor is 120, giving 16 : 9. A 12-megapixel phone photo of 4032 × 3024 has a divisor of 1008 and simplifies to 4 : 3. An Instagram portrait of 1080 × 1350 shares a divisor of 270 and comes out as 4 : 5.
Both inputs must be positive numbers. Decimals are handled by multiplying both figures by the same power of ten (up to six decimal places) so that the divisor search runs on whole numbers: 1.5 × 1 becomes 15 × 10 and simplifies to 3 : 2, and A4 paper entered as 29.7 × 21 centimetres is scaled to 297 × 210, which gives 99 : 70 exactly as the millimetre figures would.
When the result is not the ratio you expected
The calculator reports the exact mathematical ratio, not the nearest marketing name. Many real screen sizes are only approximately the ratio they are sold as, and the tool will show you the honest fraction.
| Size | Exact ratio shown | Usually called |
|---|---|---|
| 1366 × 768 | 683 : 384 | 16:9 (it is slightly wider) |
| 4096 × 2160 | 256 : 135 | 17:9 or DCI 4K |
| 2560 × 1080 | 64 : 27 | 21:9 ultrawide |
| 1125 × 2436 | 375 : 812 | 19.5:9 (iPhone X) |
| 1440 × 900 | 8 : 5 | 16:10 |
Common sizes whose exact ratio differs from the name usually attached to them.
How the resize is calculated
The new height is the new width divided by the original ratio (width ÷ height), rounded to two decimal places. Resizing 1920 × 1080 to a width of 1280 gives exactly 720. Resizing 1366 × 768 to 1280 gives 719.65, because the original is not a clean 16:9; use 720 if you are cropping to 16:9 or 719 if you must keep the exact proportion.
To solve for a target height instead, type it into the New height box and the width is worked out as the height multiplied by the original ratio. Whichever of the two resize boxes you edited most recently is the fixed dimension, and the other box displays the computed value. For 1920 × 1080 to a height of 500 the width comes back as 888.89, so use 889 pixels.
Standard sizes for the usual ratios
| Ratio | Typical sizes | Where it is used |
|---|---|---|
| 16:9 | 1280 × 720, 1920 × 1080, 3840 × 2160 | Video, TVs, YouTube, most monitors |
| 4:3 | 1024 × 768, 4032 × 3024 | Older displays, phone camera sensors |
| 3:2 | 6000 × 4000, 1500 × 1000 | DSLR and mirrorless cameras, 6 × 4 prints |
| 1:1 | 1080 × 1080, 500 × 500 | Square social posts, avatars |
| 4:5 | 1080 × 1350 | Instagram portrait posts |
| 9:16 | 1080 × 1920 | Stories, Reels, TikTok, phone wallpapers |
Reference sizes that simplify cleanly to the ratio shown.
Putting the numbers to work
- Responsive video embeds: a 16:9 container uses padding-bottom of 56.25%, which is 9 ÷ 16 × 100. Run your own ratio through the tool and divide height by width for other shapes.
- CSS aspect-ratio: the simplified ratio is exactly what the property wants, for example aspect-ratio: 4 / 5 for a 1080 × 1350 image slot.
- Thumbnails and hero images: resize to the column width you have, then hand both numbers to the Image Resizer to produce the file.
- Print layouts: enter dimensions in whole millimetres to check whether a photo will fit a frame without cropping.
Frequently asked questions
How do I find an aspect ratio?
Enter the width and height and the tool simplifies them to the smallest whole-number ratio — for example 1920 × 1080 becomes 16:9.
How do I resize without distortion?
Enter a new width and the calculator works out the matching height (or vice-versa) so the image keeps its original proportions.
What's a common aspect ratio?
16:9 is standard for video and most screens, 4:3 for older displays, and 1:1 for square social posts.
Is it free?
Yes — free and runs entirely in your browser, with no sign-up.
Why does 1366 × 768 show 683:384 instead of 16:9?
Because 1366 ÷ 768 is 1.7786, while 16 ÷ 9 is 1.7778. The two share no larger common divisor than 2, so the exact simplified ratio really is 683:384. Laptop makers call it 16:9 because the difference is less than a pixel row.
Can I resize to a target height instead of a width?
Yes. Type the height into the New height box and the New width box updates with the matching width. The box you typed in last is treated as the fixed dimension, so you can switch between the two freely.
What should I do with a height like 719.65?
Pixels must be whole numbers, so round it. The tool keeps two decimals so you can see how far off a whole pixel you are; 719.65 is within half a pixel of 720, which is the value to use.
Does the calculator work with units other than pixels?
Yes, as long as both numbers use the same unit. Decimals are fine too: A4 paper entered as 297 × 210 millimetres or as 29.7 × 21 centimetres simplifies to 99:70 either way, because the tool scales decimal inputs to whole numbers before reducing them.
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