What Is an Aspect Ratio Calculator?
An aspect ratio calculator determines the proportional relationship between width and height from any dimensions. Enter a width and height, and this tool instantly calculates the simplified ratio, equivalent dimensions at different sizes, and the CSS aspect-ratio property value.
Common Aspect Ratios
16:9 — widescreen video, hero images, YouTube. 4:3 — traditional TV, presentations, blog images. 1:1 — Instagram posts, profile pictures, icons. 9:16 — Stories, TikTok, mobile video. 3:2 — DSLR photography, standard prints. 21:9 — ultrawide cinema, panoramic headers.
How to Use This Aspect Ratio Calculator
- Enter the original dimensions — Type the width and height of your image, video, or element in pixels.
- See the simplified ratio — The calculator reduces your dimensions to the simplest whole-number ratio (e.g., 1920×1080 → 16:9).
- Calculate missing dimensions — Enter a new width or height and the tool calculates the matching dimension that preserves the original ratio.
- Copy the result — Grab the ratio, the CSS aspect-ratio value, or the calculated dimensions for your project.
Worked examples: what size is that, really?
Many screen and image sizes are named after a ratio they do not quite have. Type a width and a height and the calculator reduces them by their greatest common divisor; when that leaves large numbers it also shows the nearest common ratio and how far off it is. These results were read from the calculator in Chromium on October 12, 2026:
| Size | What it is | Exact ratio | Decimal | Nearest common ratio |
|---|---|---|---|---|
| 1366 × 768 | Common laptop screen | 683:384 | 1.7786 | Close to 16:9 (0.05% wider) |
| 1440 × 900 | 16:10 laptop screen | 8:5 | 1.6000 | exactly 16:10 |
| 2560 × 1080 | Sold as a 21:9 ultrawide | 64:27 | 2.3704 | Close to 2.39:1 (0.82% narrower); 21:9 is 2.333 |
| 3440 × 1440 | Sold as a 21:9 ultrawide | 43:18 | 2.3889 | Close to 2.39:1 (0.05% narrower) |
| 1200 × 630 | Open Graph share image | 40:21 | 1.9048 | no common ratio within 2% |
| 1080 × 1350 | Instagram portrait post | 4:5 | 0.8000 | exactly 4:5 |
The two "21:9" monitors are not the same shape: 2560 × 1080 is 0.8% narrower than 3440 × 1440, and neither is exactly 21:9. That matters when you build a full-screen layout or a video for one of them. For CSS, use the exact pair (aspect-ratio: 64 / 27), or set the image's real width and height attributes and let the browser derive the ratio, which also reserves the space before the image loads.
Scaling to a new size. Switch on Lock ratio to keep the shape: editing either field then sets the other from the ratio captured when you locked, so 1920 × 1080 locked and typed down to a width of 1280 gives exactly 720, and a height of 2160 gives 3840. Clicking a ratio card keeps your current width and sets the matching height (a width of 1280 on 4:3 gives 960). With the lock off, the two fields are independent, which is what you want when you are measuring an existing size.
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools