Aspect Ratio Calculator
Calculate a missing width or height from an aspect ratio, or get the ratio from two dimensions. Great for video, images and design. Free and in-browser.
Resize keeping ratio
Ratio from dimensions
How to use the Aspect Ratio Calculator
- 1Pick a direction: solve for a missing side from a known ratio, or reverse the process to derive a ratio from two measurements.
- 2To find a missing side, put the ratio into the width and height ratio fields (for example 16 and 9), then type the one real dimension you have into the pixel field.
- 3Watch the empty field fill in the matching dimension as you type; there is no calculate button, the result updates live.
- 4For a ratio from raw numbers, enter your actual width and height (say 1920 and 1080) and the tool reduces them by their greatest common divisor to 16:9.
- 5Use the copy button beside a result to send the value or the ratio string to your clipboard, ready to paste into your editor or design tool.
- 6Tap a preset chip such as 16:9, 4:3, 1:1, 3:2, or 9:16 to load a common ratio instead of typing the two numbers.
About the Aspect Ratio Calculator
The Aspect Ratio Calculator handles one specific chore: keeping width and height proportional when you change a dimension. Enter a ratio such as 16:9, 4:3, 1:1, or 2.39:1, type in the width or the height you know, and it fills in the matching side so the frame never stretches or squashes. Run it the other direction and it takes two real dimensions and reduces them to the smallest whole-number ratio using the greatest common divisor, turning 1920x1080 into a clean 16:9 instead of 1920:1080, or 2560x1600 into 16:10.
Everything is computed in your browser with JavaScript. Nothing you type is uploaded, logged, or sent anywhere, so the page keeps working with your connection off and is safe for unreleased dimensions or client work you would rather not paste into a cloud service.
Different people reach for it for concrete reasons. Video editors conform footage to 16:9 for YouTube, 9:16 for Reels and TikTok, or 2.39:1 for a cinematic letterbox without doing division in their head. Web and UI designers keep hero banners, thumbnail grids, and responsive image containers proportional as they scale across breakpoints. Photographers check whether a print size like 8x10 or 5x7 matches their sensor's native ratio before committing to a crop. Game and app developers size sprites, textures, and export canvases to a target display.
Two practical notes. Aspect ratios are unitless, so 16:9 is the same shape in pixels, inches, centimeters, or CSS points; you only need width and height to share a unit, since the tool preserves proportion rather than converting between units. And when a result lands on a fractional pixel, such as 853.33 wide, round to a nearby whole even number to avoid subpixel blur once the image is rendered.
Because the reduction is exact integer math, it also works as a quick shape check: reduce 1280x720 and 3840x2160 and both come back 16:9, confirming they belong on the same screen. Mismatched proportions are among the most visible and most avoidable quality problems in any visual medium, and this removes the arithmetic that causes them.
Frequently asked questions
How does it reduce 1920x1080 to 16:9?
It finds the greatest common divisor of the two numbers and divides both by it. The GCD of 1920 and 1080 is 120, so 1920/120 = 16 and 1080/120 = 9. This is exact integer arithmetic, so you get the smallest whole-number form every time rather than a rounded guess like 1.78:1.
The calculated side came out to 853.33 pixels. What do I use?
Not every ratio divides evenly into every target size. 1280 wide at 3:2 works out to 853.33 tall. The tool shows the exact figure; round it to a whole, ideally even, pixel, so 854. The difference is well under a pixel and rounding to even avoids subpixel blur when the image is rasterized.
Does it matter whether I enter pixels, inches, or centimeters?
No. A ratio is unitless, so 16:9 describes the same shape regardless of unit. Just keep the width and height in the same unit. The calculator preserves proportion only, it does not convert inches to pixels or apply any DPI.
Can I use it for vertical and square formats?
Yes. Enter the ratio in portrait order, such as 9:16 for Reels, TikTok, and Stories, or 4:5 for an Instagram portrait post, and 1:1 for square. Width and height are treated independently, so landscape, portrait, and square all behave the same.
Does any of my input leave my device?
No. Every calculation runs client-side in your browser with JavaScript. Nothing you enter is transmitted, stored, or logged on a server, and the page keeps functioning offline, which makes it safe for confidential or pre-release project dimensions.
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Frequently asked questions
What can it solve?+
Lock a ratio and find the missing dimension, or derive the ratio from width and height.