Aspect Ratio Calculator
Simplify any width and height to a clean aspect ratio and calculate the missing dimension when you resize.
Original size
New size — fill in one value
Result
- Aspect ratio
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- Decimal ratio
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- New size
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- Scale
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How it works
Enter your original width and height and the aspect ratio is simplified instantly — 1920 ×
1080 becomes 16:9, 800 × 600 becomes 4:3. Then type either a new width or a new height and
the missing dimension is calculated so your image, video or div keeps exactly the same
proportions. When the numbers don't reduce to a tidy ratio (1366 × 768, for example) the
closest common ratio is shown with a ≈ sign, and the decimal ratio is always given so you
can drop it straight into CSS aspect-ratio.
Use the presets to work backwards: pick 16:9, 4:3, 1:1 or 21:9 and enter one side to get the other. Everything is calculated in your browser with plain JavaScript — no upload, no account, no ads — and the shape preview shows the proportions at a glance. Turn off "Round to whole pixels" if you need the exact fractional value.
Frequently asked questions
How do I calculate an aspect ratio?
Divide the width by the height and reduce the two numbers by their greatest common divisor: 1920 ÷ 1080 gives 1.778, and dividing both sides by 120 gives 16:9. This calculator does both as you type — you get the simplified ratio, the decimal value you can paste into the CSS aspect-ratio property, and the missing dimension for any new size.
What is the aspect ratio of 1920x1080?
1920 × 1080 is 16:9, the standard widescreen ratio for Full HD video, YouTube and most monitors, with a decimal ratio of 1.778:1. Other common results are 1280 × 720 (also 16:9), 800 × 600 (4:3), 1080 × 1080 (1:1) and 1080 × 1920 (9:16, used for Stories and Reels). Sizes that do not reduce neatly, like 1366 × 768, are shown as the closest common ratio with a ≈ sign.
How do I resize an image without losing the aspect ratio?
Change only one dimension and calculate the other from the original ratio. Enter your original width and height here, then type the new width (or the new height) and the matching value appears immediately, rounded to whole pixels so nothing gets stretched or squashed. Use that pair of numbers in your image editor or resizer and the picture keeps its exact proportions.