Free aspect ratio calculator
Find the aspect ratio of any width and height, or solve a missing dimension from a target ratio like 16:9, 4:3, or 9:16. The simplest ratio, decimal value, and resized dimensions are updated live, as you type.
On this page13 sections
Results are based on the dimensions you enter.
Results are estimates. Consult a professional.
What is an aspect ratio?
An aspect ratio is the proportion between an image's width and its height, written as two numbers with a colon between them, such as 16:9. It describes shape, not size. A 1920×1080 photo and a 1280×720 photo are both 16:9 because their width and height keep the same proportion, even though one holds far more pixels. This aspect ratio calculator does two jobs: it reduces any width and height to its simplest ratio, and it solves a missing dimension when you already know the ratio.
Aspect ratio vs. resolution — why they are not the same
Resolution counts pixels; aspect ratio describes shape. Two screens at different resolutions can share one ratio, and one ratio can appear at countless resolutions. That is why a 4K screen (3840×2160) and a 1080p screen (1920×1080) are both 16:9. When you scale an image up or down within its own ratio, nothing stretches; change the ratio and the image either crops or distorts.
How to calculate an aspect ratio from width and height
To turn a pixel size into a clean ratio, divide both sides by their greatest common divisor. The GCD is the largest whole number that divides both width and height with no remainder.
- Find the GCD of the width and the height. For 1920 and 1080 the GCD is 120.
- Divide both by that number. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9.
- Write the result as width:height. Here that is 16:9 — the reduced, simplest form.
How to resize an image and keep the same aspect ratio
The most common reason to use the calculator is resizing without distortion: you have a target ratio and one new dimension, and you need the other. Solve it by scaling the known side by the ratio.
Say you want a 16:9 banner that is 2560 px wide. The height is 2560 × (9 ÷ 16) = 1440 px. Enter the ratio and the one dimension you know and the calculator returns the other, live as you type.
Common aspect ratios and where they are used
Most screens, cameras, and social platforms settle on a handful of standard ratios. Use this table to pick the right one before you resize.
| Ratio | Decimal (w÷h) | Typical resolution | Where it is used |
|---|---|---|---|
| 16:9 | 1.78 | 1920×1080, 3840×2160 | TVs, monitors, YouTube, 4K video |
| 4:3 | 1.33 | 1024×768, 1600×1200 | Older TVs, iPads, presentations |
| 21:9 | 2.33 | 2560×1080, 3440×1440 | Ultrawide monitors, cinema |
| 3:2 | 1.50 | 6000×4000 | DSLR/mirrorless photography |
| 1:1 | 1.00 | 1080×1080 | Square posts, profile images |
| 9:16 | 0.56 | 1080×1920 | Vertical video — TikTok, Reels, Stories |
Sources: Wikipedia (16:9, 21:9 aspect ratio) and Display Ninja aspect-ratio guide. True 21:9 ultrawides are usually 64:27 (≈2.37); 21:9 is the marketing label.
Aspect ratio terms defined
A worked example using the aspect ratio calculator
You have a 1920×1080 video still and want a banner that keeps the same shape but is 2560 px wide. Two steps: confirm the ratio, then solve the new height.
Step 1 — Reduce the original to its ratio
Enter 1920 as width and 1080 as height. The GCD of 1920 and 1080 is 120, so both divide down to 16 and 9.
| Input | Value |
|---|---|
| Width | 1920 |
| Height | 1080 |
| GCD | 120 |
| Simplest ratio | 16:9 |
Step 1 result: 1920×1080 reduces to 16:9 (decimal 1.78).
Step 2 — Solve the new height for the 16:9 ratio
Switch to solve mode, keep the 16:9 ratio, and enter the new width of 2560. The height = 2560 × (9 ÷ 16).
Tips for choosing the right aspect ratio
- Match the destination. Use 16:9 for YouTube and monitors, 1:1 for feed posts, and 9:16 for Reels, TikTok, and Stories.
- Shoot wider than you need. A 3:2 or 16:9 original gives room to crop into a square or vertical later without losing detail.
- Never enter mismatched dimensions. Forcing a 4:3 image into a 16:9 box stretches faces and circles; crop instead, or accept letterbox bars.
- Keep one dimension fixed. When resizing, lock the side that matters (usually width for web) and let the calculator set the other.
For pixel density on a finished screen, pair this with the PPI calculator, which also reports the aspect ratio of any resolution.
Aspect ratio calculator — frequently asked questions
What is the most common aspect ratio?
16:9 is the most widespread today. It is the standard for TVs, computer monitors, YouTube, and 4K video, covering resolutions from 1280×720 up to 3840×2160.
How do I find the aspect ratio of an image?
Enter its pixel width and height. The calculator divides both by their greatest common divisor and returns the simplest ratio — for example, 1920×1080 becomes 16:9.
Will changing the aspect ratio distort my image?
Only if you stretch it to fit. Scaling within the same ratio keeps the image sharp and undistorted. Moving to a different ratio means you must crop, or accept black bars (letterboxing).
What is the difference between 16:9 and 21:9?
16:9 is standard widescreen. 21:9 is ultrawide — much wider relative to its height, used for cinema and ultrawide monitors. True 21:9 panels are usually 64:27 (about 2.37), with 21:9 as the rounded marketing name.
Wikipedia — 16:9 aspect ratio.Display Ninja — What Is Aspect Ratio? (16:9, 21:9, 4:3).Frequently asked questions about the free aspect ratio calculator
About this aspect ratio calculator
This aspect ratio calculator runs entirely in your browser — nothing you enter is sent anywhere. It reduces any width and height to its simplest whole-number ratio using the greatest common divisor, and solves a missing dimension from a chosen ratio so images and video resize without distortion, updating the instant you change a field.
Working with screen sizes? Pair it with the PPI calculator for pixel density. Browse the full everyday calculators shelf, or see every tool on the calculators index.