Aspect Ratio Calculator
Enter a width and height to reduce them to a simplified aspect ratio (like 16:9 or 4:3) and a decimal ratio, or switch to resize mode to work out the matching height for a new width without stretching or squashing the image.
How this calculation works
In ratio mode, the calculator finds the greatest common divisor (GCD) of the width and height and divides both by it, the same way you'd simplify a fraction. 1920 × 1080 shares a GCD of 120, so dividing both sides gives 16:9. The decimal ratio is simply width ÷ height, which is useful for comparing ratios that don't reduce to tidy whole numbers.
In resize mode, the calculator keeps the original width-to-height relationship fixed and solves for the missing side: new height = original height × (new width ÷ original width). Because the ratio between width and height never changes, the resized image or video keeps its original proportions and never looks stretched or squashed.
Both modes work in any consistent unit — pixels for screens and images, or centimetres and inches for print — as long as width and height use the same unit.
Worked example
Common aspect ratios
These are the ratios you'll run into most often for screens, photos, and video:
| Ratio | Decimal | Typical use |
|---|---|---|
| 1:1 | 1.0000 | Square — Instagram posts, album covers, profile photos |
| 4:3 | 1.3333 | Classic TV, older monitors, standard photo prints |
| 3:2 | 1.5000 | DSLR and mirrorless camera sensors, 4x6 prints |
| 16:10 | 1.6000 | Some laptop and desktop monitors |
| 16:9 | 1.7778 | HD/4K video, widescreen TV, most modern monitors |
| 21:9 | 2.3333 | Ultrawide monitors, cinematic/anamorphic video |
| 9:16 | 0.5625 | Vertical video — Stories, Reels, Shorts |
Why keep the same aspect ratio when resizing
If you change width and height independently, the image or video stretches or squashes — circles become ovals and faces look distorted. Keeping the width-to-height ratio constant while resizing (scaling both sides by the same factor) preserves the original proportions exactly, which is why design tools and video editors usually lock the aspect ratio by default and let you drag just one handle.
Letterboxing and pillarboxing
- Letterboxing: black bars added above and below content to fit a wider ratio onto a taller screen without cropping or stretching (e.g. a 21:9 film shown on a 16:9 TV).
- Pillarboxing: black bars added on the left and right to fit a taller ratio onto a wider screen (e.g. old 4:3 footage shown on a 16:9 display).
- Cropping: instead of adding bars, some players cut off the edges of the content to fill the screen — this avoids bars but loses part of the original frame.
- Stretching (not recommended): forcing content to fill the screen without preserving its ratio distorts every shape in the image.
Using ratios in CSS and design tools
Modern CSS supports an aspect-ratio property (e.g. aspect-ratio: 16 / 9) that keeps an element's proportions fixed as its container resizes, which is useful for responsive video embeds and image placeholders that shouldn't cause layout shift while loading. Design tools like Figma and Photoshop also let you lock a ratio when drawing or scaling a frame, using the same width-to-height relationship this calculator computes.