What is an aspect ratio?
An aspect ratio is the width of an image divided by its height, expressed as two numbers or as a decimal. It describes shape only, and carries no information about resolution, file size or physical dimensions.
At a glance
- Definition
- Width divided by height
- Written as
- 16:9, 16/9, or 1.78:1
- Says nothing about
- Resolution, file size, physical size or pixel density
- Simplified by
- Dividing both numbers by their greatest common divisor
- Common landscape ratios
- 4:3 (1.333), 3:2 (1.5), 16:9 (1.778), 1.91:1, 2.39:1
- Common portrait ratios
- 4:5 (0.8), 2:3 (0.667), 9:16 (0.5625)
In detail
An aspect ratio describes shape and nothing else. A 160 by 90 thumbnail and a 3840 by 2160 master are the same ratio, so they crop the same way, tile the same way and letterbox the same way, and they differ by a factor of over five hundred in pixel count. Every question about fitting is a ratio question; every question about sharpness or file size is a resolution question.
The conventional form is two whole numbers with the greatest common divisor removed, which is why 1920 by 1080 is quoted as 16:9 rather than as 1920:1080. Simplification is what makes two sizes comparable at a glance, and it is also where the surprises live: 1080 by 566 reduces only to 540:283, and 1200 by 627 only to 400:209, because the pixel dimensions were chosen for a rounded decimal rather than for a clean fraction.
The decimal form is the one to use when comparing. 1.91 against 1.78 immediately shows that a link preview card is wider than a widescreen frame, whereas 40:21 against 16:9 tells most people nothing. Quoting a ratio as a decimal against one is the usual convention for landscape, and quoting one against a decimal is clearer for portrait.
Ratio and resolution combine into the pixel count, and that is the number that governs memory and processing time. Doubling both dimensions keeps the ratio and quadruples the pixels, which is why a 4K frame takes four times as long to process as a 1080p one while looking like exactly the same picture.
Examples
function gcd(a, b) { return b === 0 ? a : gcd(b, a % b); }
const g = gcd(1920, 1080); // 120
1920 / g; // 16
1080 / g; // 9 160 x 90 0.014 MP
1280 x 720 0.92 MP
1920 x 1080 2.07 MP
3840 x 2160 8.29 MP
// identical shape, 576 times the pixels from first to lastThings that catch people out
- A ratio says nothing about quality. An image can be exactly the right shape and far too small to print.
- Not every ratio simplifies to something memorable. When it does not, quote the decimal or the pixel dimensions rather than inventing a rounded fraction.
- Two ratios within a few percent of each other still crop. 1.78 and 1.91 look interchangeable and are not.
Try it: Aspect Ratio Calculator
Enter a width and a height, or a ratio and one dimension, and it solves the missing one, simplifies the ratio, counts the megapixels and gives you a copyable CSS value.
Aspect Ratio Calculator
LOCAL · Browser-OnlyCalculate and convert aspect ratios for images, videos, and screens
1920 x 1080px · 16:9 · 1.7778:1
Common Aspect Ratios Reference
| Ratio | Decimal | Common Use |
|---|---|---|
| 16:9 | 1.778 | Widescreen TV, YouTube, monitors |
| 4:3 | 1.333 | Classic TV, iPad, presentations |
| 1:1 | 1.000 | Instagram square, profile pictures |
| 21:9 | 2.333 | Ultrawide monitors, cinema |
| 9:16 | 0.563 | Phone vertical, TikTok, Reels |
| 3:2 | 1.500 | 35mm photography, MacBook |
| 2:3 | 0.667 | Portrait photography, posters |
| 5:4 | 1.250 | Large format photography, older monitors |
| 32:9 | 3.556 | Super ultrawide monitors |
| 2.39:1 | 2.390 | Anamorphic cinema, widescreen films |
Related Image and canvas sizes
All image and canvas sizes →The CSS aspect-ratio property
How the aspect-ratio property works, what happens when a width and a height are also set, and how it removes layout shift when media loads.
Letterboxing versus cropping
What happens when an image does not match its frame: bars that keep everything, or a crop that discards the edges. How object-fit chooses between them.
16:9 aspect ratio
Where the 16:9 ratio came from, the pixel dimensions that hit it exactly, what it suits, and the places where a widescreen frame is the wrong shape.
DPI versus pixel dimensions
Why a raster file only stores pixels, what the DPI number in its metadata does and does not change, and how to work out a print size from a pixel count.
YouTube 1080p video size: 1920 by 1080
Why 1920 by 1080 is the standard full HD upload frame, what the player overlays cover, and how the thumbnail is a separate asset at the same shape.
YouTube 4K video size: 3840 by 2160
The 4K UHD frame at 3840 by 2160, why it is exactly four times the pixel count of 1080p, and what uploading above the watched resolution actually buys you.
Tools for this
- Aspect Ratio Calculator Free online aspect ratio calculator - calculate and convert aspect ratios for images, videos, and screens
- Image Resizer Free online image resizer - resize images to exact dimensions, percentage, or common presets