Aspect Ratio Calculator
Simplify any width and height to its lowest ratio, solve for the missing side, and resize a size to a new ratio. Works in pixels or inches, with 16:9, 4:3, 1:1, and cinema presets. Everything runs in your browser.
How to use
- Type a width and height in the Pixels or Inches tab. The simplified ratio and its decimal value update as you type; 1920 × 1080 reads 16:9.
- Pick a preset chip or type your own ratio, like 2.35:1 for cinema scope. A single number, such as 2.35, means 2.35:1.
- To solve for a missing side, choose whether you know the width or the height, type that value, and read the other side at the same ratio.
- To see resize math, read the two rows under Resize to the ratio: crop trims one side until the pair matches, expand grows a side instead. Nothing is uploaded; the math runs in your tab.
How the simplification works
A ratio is the pair of numbers left after you divide out everything the two sides share. The calculator runs the Euclidean algorithm, the classic greatest-common-divisor method described in Book VII of Euclid's Elements. Take 1920 × 1080: the greatest common divisor is 120, and 1920 ÷ 120 = 16 while 1080 ÷ 120 = 9, so the screen is 16:9. Whole numbers are never estimated; the arithmetic is exact.
Decimal sizes scale up to integers first. Both sides are multiplied by one million and rounded, which is exact for six decimal places, and then simplified. That is how 2.35:1 becomes the exact pair 47:20, and why a mixed entry like 1920 × 0.5 inches reads as 3840:1.
Worked example: from 1920 × 1080 to a 4:3 frame
- Simplify the size. The greatest common divisor of 1920 and 1080 is 120, so the screen is 16:9.
- Solve the missing side for 4:3 at width 1920: height = 1920 × 3 ÷ 4 = 1440. A 4:3 frame that wide is 1920 × 1440.
- Resize without guessing. Cropping keeps the height at 1080 and trims the width to 1440; expanding keeps the width at 1920 and grows the height to 1440. The first loses pixels, the second adds them.
Pixels and inches measure the same shape
A ratio carries no unit. A 1920 × 1080 monitor and a 19.2 × 10.8 inch print are the same 16:9 shape, because dividing one side by the other cancels the unit. That is why the tool has two tabs and one set of math: type whole pixels on one, decimal inches on the other, and the ratio comes out the same.
A diagonal alone is never enough. A 27 inch 16:9 screen measures about 23.5 × 13.2 inches, while a 27 inch 4:3 screen would measure about 21.6 × 16.2. You always need the ratio plus one side to pin a size down, and the calculator takes that side directly.
The inches mode is handy beyond screens. Chainmaille and jewelry makers compare a jump ring's inner diameter with its wire thickness using the same division.
Frequently asked questions
Is anything I type uploaded or stored?
No. Every calculation runs in your browser tab. Your numbers never leave the page, nothing is saved on our side, and the page sets no cookies. Once the page has loaded, it keeps working offline. Ad slots on the page are served by third-party networks, which follow their own privacy policies.
Why does 2.35:1 simplify to 47:20?
The calculator multiplies both sides by one million, rounds to whole numbers, and simplifies: 2.35:1 becomes 2350000:1000000, and the greatest common divisor of those two is 50000, which leaves 47:20. The result is exact rather than a rounded display. Cinema ratios are usually quoted in the decimal form because it reads faster.
What is the difference between crop and expand?
Crop keeps the side that lets the other shrink, so no dimension grows and you lose some of the image. Expand keeps the other side and grows one, so nothing is lost but the file gets bigger. Both results match the target ratio exactly; which one fits depends on whether you can afford to lose pixels or add them.
Can I enter decimal inches?
Yes. Decimals are exact to six decimal places. Beyond that, the calculator rounds before simplifying and says so, and a value that would round to zero at that precision is rejected rather than quietly flattened.
How do I find the ratio of a screen from its diagonal?
The diagonal alone is not enough. A 27 inch panel could be 16:9, 4:3, or 21:9 and the diagonal stays 27 inches. Read the width or height off the spec sheet, enter both, and the ratio comes out.
Why did my huge number get rejected?
Whole numbers stay exact only up to 9,007,199,254,099, the largest integer JavaScript can hold without gaps. Past that point the simplification would be silently wrong, so the calculator asks for a smaller value instead.