Pixel360Aspect ratio formula
An aspect ratio is the proportion between width and height. This aspect ratio calculator reduces any size to its simplest whole-number form and also shows the decimal ratio, which is easier for comparing shapes.
ratio = (width ÷ GCD) : (height ÷ GCD)
decimal = width ÷ height
missing height = width × ratio H ÷ ratio W
The GCD (greatest common divisor) is the largest whole number that divides both sides evenly. For 1920 and 1080 it is 120, which gives 16:9.
Worked examples
- 1920×1080: GCD 120 → 16:9 (1.778:1)
- 1280×1024: GCD 256 → 5:4 (1.25:1)
- 1366×768: GCD 2 → 683:384. That looks odd, but the decimal (1.779:1) is within 0.05% of 16:9, so it is sold as a 16:9 screen.
- Missing height at 4:3, 800px wide: 800 × 3 ÷ 4 = 600px
- Missing width at 9:16, 1920px tall: 1920 × 9 ÷ 16 = 1080px
Common aspect ratios and where they are used
| Ratio | Decimal | Typical resolution | Common uses |
|---|---|---|---|
| 1:1 | 1:1 | 1080×1080 | Instagram square posts, profile images, album art |
| 4:5 | 0.8:1 | 1080×1350 | Instagram portrait feed posts |
| 5:4 | 1.25:1 | 1280×1024 | Older desktop monitors, 8×10 in photo prints |
| 4:3 | 1.333:1 | 1024×768, 1600×1200 | Older monitors and TVs, many tablets, compact cameras |
| 3:2 | 1.5:1 | 6000×4000 | 35mm film, most DSLR and mirrorless sensors, 4×6 in prints |
| 16:10 | 1.6:1 | 1920×1200, 2560×1600 | Many laptops and productivity monitors |
| 16:9 | 1.778:1 | 1920×1080, 3840×2160 | HD and 4K TV, YouTube, most monitors, presentations |
| 21:9 | 2.333:1 | 2560×1080, 3440×1440 | Ultrawide monitors (actual ratio 64:27 or 43:18) |
| 9:16 | 0.563:1 | 1080×1920 | Stories, Reels, TikTok, YouTube Shorts |
Why the decimal ratio matters
Simplified ratios are exact, but they can hide how similar two shapes are. 683:384 and 16:9 look unrelated until you see both are about 1.78:1. The decimal form also lets you rank shapes: anything above 1 is landscape, exactly 1 is square, and below 1 is portrait. The calculator uses the decimal to name the nearest common ratio, so a 2560×1080 monitor shows as 64:27, close to 21:9.
Using aspect ratios in web design
On the web you rarely need to calculate heights by hand. The CSS aspect-ratio property lets a container keep its shape at any width, which prevents layout shift while images and embeds load. Copy the CSS from the first result and apply it to a video wrapper, a card thumbnail or a hero image.
When you need to scale a real image file to new pixel dimensions, use the image resize calculator. To check how large a photo will print at a given ratio and resolution, try the DPI and print size calculator.
Common mistakes
- Swapping width and height. 9:16 is portrait and 16:9 is landscape. The first number is always the width.
- Rounding too early. If you round the missing side, check that it is a whole pixel. 1000px wide at 16:9 is 562.5px tall, so pick 1008×567 or 1024×576 if you need an exact ratio.
- Assuming marketing names are exact. Ultrawide "21:9" panels and cinema "2.39:1" are close to each other but not identical. Use the real pixel size when it matters.
- Stretching instead of cropping. Forcing a 4:3 image into a 16:9 frame distorts it. Crop, or use
object-fit: coverin CSS.
Frequently asked questions
What aspect ratio is 1920×1080?
1920×1080 is 16:9. The greatest common divisor of 1920 and 1080 is 120, and 1920 ÷ 120 = 16, 1080 ÷ 120 = 9. The decimal ratio is 1.778:1.
Is 21:9 really 21:9?
Usually not exactly. 21:9 is a marketing name. A 2560×1080 monitor is 64:27 (2.370:1) and a 3440×1440 monitor is 43:18 (2.389:1), both slightly wider than a true 21:9 (2.333:1).
Is 16:10 the same as 8:5?
Yes. 16:10 simplifies to 8:5, and both equal 1.6:1. Displays are usually described as 16:10 so they are easy to compare with 16:9.
What aspect ratio should I use for Instagram?
Instagram feed posts commonly use 1:1 (1080×1080) or 4:5 portrait (1080×1350). Stories and Reels are 9:16 (1080×1920). Check Instagram’s help center before a campaign, because platform specs can change.
How do I keep an aspect ratio in CSS?
Use the aspect-ratio property, for example aspect-ratio: 16 / 9; on a video or card container. Set one dimension, usually width, and the browser calculates the other.