Resize anything without squashing it
This aspect ratio calculator solves the single proportion behind every clean resize: W₁ / H₁ = W₂ / H₂. Lock your original width and height, type a new width or a new height, and the tool computes the missing dimension so your image, video, or layout keeps its exact shape — no stretching, no letterboxing, no guesswork. It also reduces any width×height to its simplest whole-number ratio (like 16:9) and shows the decimal ratio. Everything runs in your browser instantly.
What an aspect ratio actually is
An aspect ratio is the proportional relationship between width and height, written as W:H. A 1920×1080 frame and a 1280×720 frame look identical in shape because both reduce to 16:9 — the same ratio at different sizes. The ratio is found by dividing both dimensions by their greatest common divisor (GCD):
1920 ÷ 120 : 1080 ÷ 120 = 16 : 9
decimal = 1920 ÷ 1080 = 1.778
Solving for a missing dimension
To resize while preserving the ratio, multiply the dimension you keep by the scale factor of the dimension you change:
new width = original width × (new height ÷ original height)
Example: scaling 1920×1080 down to 1280 wide → 1080 × (1280 / 1920) = 720, so the result is 1280×720. Scale the same source to 900 tall → 1920 × (900 / 1080) = 1600, giving 1600×900. Both stay perfectly 16:9.
Common aspect ratios cheat sheet
| Ratio | Decimal | Typical use | Example size |
|---|---|---|---|
| 16:9 | 1.778 | HD/4K video, YouTube, monitors | 1920×1080 |
| 9:16 | 0.563 | Reels, Shorts, TikTok, Stories | 1080×1920 |
| 4:3 | 1.333 | Legacy displays, some tablets | 1024×768 |
| 3:2 | 1.5 | 35mm photography, DSLR sensors | 3000×2000 |
| 1:1 | 1.0 | Square — Instagram, avatars | 1080×1080 |
| 21:9 | 2.333 | Ultrawide monitors, cinema | 2560×1080 |
| 2:1 | 2.0 | Open Graph / social share images | 1200×600 |
Who uses an aspect ratio calculator
- Front-end developers sizing responsive images, video embeds, and CSS
aspect-ratioboxes without layout shift (CLS). - Designers exporting assets at multiple resolutions while keeping the composition intact.
- Video editors converting between 16:9, 9:16, and 1:1 for different platforms.
- Photographers cropping to 3:2, 4:5, or 1:1 print and social formats.
- Marketers generating correctly-proportioned thumbnails, banners, and Open Graph images.
Aspect ratio and Core Web Vitals
Setting explicit width and height (or a CSS aspect-ratio) on media lets the browser reserve space before the asset loads, which prevents Cumulative Layout Shift — a Core Web Vitals metric. Use this calculator to compute the matching attribute values for every breakpoint, then pair it with the rest of the DevCalc toolkit, including the color contrast checker for accessible UI.