Work out any aspect ratio in seconds
An aspect ratio describes the shape of a rectangle as width to height — 16:9 for most screens and videos, 4:5 for an Instagram portrait, 1:1 for a square. The Aspect Ratio Calculator reduces any width and height to its simplest ratio, shows it as a decimal, draws the shape, and tells you the closest standard ratio when the numbers do not divide neatly.
Switch to “Find a missing dimension” when you know the ratio and one side: resize a 16:9 video to a 1280px-wide player, work out the height of a 4:3 slide at 1920px, or scale a photo to 3:2 for print. The result is rounded to whole pixels by default, with the formula shown so you can check it.
How to use the Aspect Ratio Calculator
- Choose “Find the ratio” and type the width and height, or click a row in the common resolutions table.
- Read the simplified ratio, the decimal value and the orientation, and compare the shape in the preview.
- To calculate a size, choose “Find a missing dimension”, enter the ratio (for example 16 and 9), select whether you know the width or the height, and type that value.
- Click a preset such as 16:9, 9:16 or 2.39:1 to load the ratio instantly.
- Press “Copy CSS” to take the
aspect-ratiorule into your stylesheet.
Aspect Ratio Calculator features
Lowest-terms simplification
Uses the greatest common divisor, so 1920 × 1080 becomes 16:9 and 1080 × 1350 becomes 4:5. Decimal inputs are supported too.
Closest standard ratio
Odd sizes such as 1366 × 768 (683:384) are matched to the nearest well-known ratio — 16:9 in that case — with the percentage difference.
Missing dimension with formula
Finds the unknown width or height for any ratio and shows the multiplication behind it.
Visual preview and resolutions table
A scaled rectangle shows the shape, and a table of 720p, 1080p, 1440p, 4K, 8K, ultrawide and social sizes can be loaded with one click.
CSS snippet
Copy a modern aspect-ratio rule plus the padding-top percentage fallback for older browsers.
How to calculate an aspect ratio
Divide both the width and the height by their greatest common divisor (GCD). For 1920 × 1080 the GCD is 120, giving 16:9. For 1280 × 1024 the GCD is 256, giving 5:4. As a single number, the aspect ratio is simply width ÷ height: 16:9 = 1.778 and 4:3 = 1.333.
To find a missing side, cross-multiply. If the ratio is 16:9 and the width is 1280, the height is 1280 × 9 ÷ 16 = 720. If the height is 1080, the width is 1080 × 16 ÷ 9 = 1920. For general ratio problems — splitting quantities in a given ratio — use the Ratio Calculator.
Common aspect ratios and where they are used
| Ratio | Decimal | Typical use |
|---|---|---|
| 16:9 | 1.778 | HD/4K TV, YouTube, laptops, presentations |
| 4:3 | 1.333 | Older TVs and monitors, iPad screens, classic photos |
| 1:1 | 1.000 | Instagram square posts, profile pictures, app icons |
| 21:9 | 2.333 | Ultrawide monitors (actual panels are often 2.37 or 2.39) |
| 9:16 | 0.5625 | Vertical video: Reels, Shorts, TikTok, Stories |
| 4:5 | 0.800 | Instagram portrait posts |
| 3:2 | 1.500 | DSLR and mirrorless photos, 6 × 4 inch prints |
| 2.39:1 | 2.390 | Anamorphic widescreen cinema |
| 1:√2 (1.414) | 1.414 | ISO 216 paper: A4, A3, A5 — halving keeps the ratio |
Tips for images, video and CSS
- Resize with the ratio locked to avoid stretched faces and squashed logos; the Image Resizer keeps proportions automatically.
- If a source does not match the target ratio, crop rather than stretch. A 4:3 photo becomes 16:9 by trimming the top and bottom — try the Crop Image tool.
- In CSS,
aspect-ratio: 16 / 9reserves space before an image or video loads, which prevents layout shift. It is supported in all current browsers; the padding-top trick is only needed for very old ones. - Social platforms resize uploads, so export at the exact recommended pixels: 1080 × 1350 (4:5) for Instagram portrait, 1080 × 1920 (9:16) for Stories, 1280 × 720 for YouTube thumbnails.
- Sizes that cannot be reduced neatly — such as 1366 × 768 — are not “wrong”; manufacturers pick them for panel and memory reasons. Use the closest standard ratio when planning content.
Aspect Ratio Calculator FAQ
What is the aspect ratio of 1920 × 1080?
16:9. Both numbers divide by 120. The same ratio applies to 1280 × 720, 2560 × 1440, 3840 × 2160 (4K) and 7680 × 4320 (8K).
How do I calculate the height for a 16:9 image at a given width?
Multiply the width by 9 and divide by 16. A 1000px-wide image is 1000 × 9 ÷ 16 = 562.5px high — rounded to 563px. Choose “Find a missing dimension” and the aspect ratio calculator does it for you.
Why is 1366 × 768 shown as 683:384 instead of 16:9?
Because 1366 ÷ 768 reduces exactly to 683:384, which is 1.7786 — about 0.08% wider than true 16:9 (1.7778). The “Closest standard” row shows 16:9 so you can treat it as widescreen.
Is 21:9 the same as 2.39:1?
Not exactly. 21:9 is 2.333, while anamorphic cinema uses 2.39:1 (often called 2.40). Most “21:9” monitors are actually 2560 × 1080 or 3440 × 1440, both 2.37:1.
Which aspect ratio is best for Instagram?
Feed posts work from 1.91:1 landscape to 4:5 portrait; 4:5 (1080 × 1350) takes up the most screen. Stories and Reels are 9:16 (1080 × 1920).
What is the aspect ratio of A4 paper?
1:√2, about 1:1.414 (210 × 297 mm). Every ISO 216 size shares this ratio, so an A4 sheet folded in half is an A5 sheet with exactly the same proportions.