Aspect Ratio Calculator
Calculate matching width or height for 16:9, 4:3, 1:1, 21:9 or custom aspect ratios.
🔒 Runs entirely in your browser — nothing is uploadedAspect ratio formula
Aspect ratio describes the relationship between width and height. A 16:9 image is 16 units wide for every 9 units tall, whether those units are pixels, inches or centimeters. If you know a new width, the formula is new height = new width × ratio height ÷ ratio width. If you know a new height, reverse it: new width = new height × ratio width ÷ ratio height. The calculator applies whichever formula matches the selected known dimension.
Presets cover common formats. Use 16:9 for HD video and many thumbnails, 4:3 for older displays or presentations, 1:1 for square social images, and 21:9 for ultrawide layouts. Custom mode lets you type any width-to-height relationship, such as 3:2 for photos or 9:16 for vertical video.
Why preserving ratio matters
Resizing only one dimension without adjusting the other can stretch faces, distort product photos or create unexpected black bars in video. Keeping the aspect ratio means the shape stays consistent while the size changes. Designers use this when creating responsive images, video embeds, posters, icons, presentation slides and canvas layouts that need to fit a target space.
Practical dimension notes
The result may include decimals when the ratio does not divide evenly. For pixels, round to the nearest whole number or to the requirements of your editor, platform or codec. For print, keep enough precision for the final size. This page only calculates dimensions; it does not upload, inspect or resize media files. All entries stay in your browser.
If a platform requires even-numbered video dimensions, round one dimension and recalculate the other so the final file still stays as close as possible to the intended ratio.
Worked examples
To scale a 16:9 video to a width of 1280 pixels, the height is 1280 × 9 ÷ 16 = 720. Going the other way, a 4:5 portrait post that must be 1350 pixels tall needs a width of 1350 × 4 ÷ 5 = 1080 pixels. A 3:2 photo that is 6000 pixels wide scales to 4000 pixels tall, and a 21:9 banner slot 728 pixels wide needs a height of 728 × 9 ÷ 21 = 312 pixels.
Choose the known dimension based on your constraint. If a layout column fixes the width, solve for height; if a screen or video player fixes the height, solve for width. To reuse an existing size as a template, type its width and height directly into the ratio fields: 1920 and 1080 behave exactly like 16:9 because the formula only depends on the proportion between the two numbers.
How to use
- Choose or enter a ratioUse a preset such as 16:9 or type your own width-to-height ratio.
- Enter one new dimensionChoose whether you know the width or height, then enter that value.
- Copy the matching sizeRead the computed width and height that preserve the selected aspect ratio.