Aspect Ratio Calculator
Find the aspect ratio of any width × height (like 1920×1080 → 16:9), or lock a ratio and resize — enter one dimension and get the other so nothing gets stretched. Everything runs in your browser; nothing is uploaded.
Find the aspect ratio
Resize to keep a ratio
How the aspect ratio calculator works
This page has two calculators. The first finds the aspect ratio of a size: type a width and height and it reduces them to the smallest whole-number ratio by dividing both by their greatest common divisor (GCD). For 1920×1080 the GCD is 120, so it becomes 16:9. It also gives the decimal ratio (width ÷ height) and tells you the nearest standard ratio, so you can see at a glance whether a size is a clean 16:9, 4:3 or something in between.
The second calculator keeps a ratio while you resize. Choose a ratio — tap a preset like 16:9 or 4:3, or type your own — then enter a new width and it works out the height that keeps the proportions (or enter a height to get the width). This is exactly how you scale an image, video or design to a new size without squashing or stretching it: both dimensions stay on the same ratio.
Calculating a ratio by hand
To reduce a size to its aspect ratio manually, find the largest number that divides both the width and the height evenly (the greatest common divisor) and divide each by it. Take 1600×900: the GCD is 100, giving 16:9. For 1024×768 the GCD is 256, giving 4:3. If the two numbers share no common factor beyond 1, the ratio is already in its simplest form. When a size does not reduce to a familiar ratio — 1366×768 becomes 683:384 — it simply is not a perfectly standard shape, and this tool shows the true reduced ratio alongside the closest common one (≈16:9) rather than pretending it is exact.
Resizing without distortion
The golden rule for resizing is to change width and height together on the same ratio. If a 720-pixel-tall 16:9 image needs to be 1280 pixels wide, its height must become 720 to stay 16:9 — set anything else and it looks stretched. When the maths does not land on a whole pixel (for example 16:9 at a width of 1000 gives 562.5), the calculator rounds to the nearest pixel, which is what image software does too; the tiny rounding is invisible. If you need a genuinely different shape, don't stretch — crop to the new ratio instead, keeping the part of the picture you care about.
Common aspect ratios reference
| Ratio | Decimal | Typical use | Example sizes |
|---|---|---|---|
| 16:9 | 1.78:1 | HD & 4K video, most modern screens, YouTube | 1920×1080, 3840×2160 |
| 4:3 | 1.33:1 | Older TVs & monitors, many tablets, some cameras | 1024×768, 1600×1200 |
| 3:2 | 1.50:1 | 35mm & most DSLR/mirrorless photos | 3000×2000, 6000×4000 |
| 16:10 | 1.60:1 | Many laptops & productivity monitors | 1920×1200, 2560×1600 |
| 21:9 (64:27) | 2.37:1 | Ultrawide monitors, cinematic video | 2560×1080, 3440×1440 |
| 1:1 | 1.00:1 | Square — Instagram posts, profile pictures, avatars | 1080×1080, 512×512 |
| 9:16 | 0.56:1 | Vertical video — Stories, Reels, TikTok, Shorts | 1080×1920 |
| 4:5 | 0.80:1 | Portrait Instagram feed posts | 1080×1350 |
| 2:3 | 0.67:1 | Portrait photo prints (4×6), Pinterest | 1000×1500 |
| 2.35:1 | 2.35:1 | CinemaScope / anamorphic widescreen film | 2560×1090 |
Landscape, portrait and square
When the width is greater than the height the frame is landscape (ratio above 1, like 16:9); when the height is greater it is portrait (ratio below 1, like 9:16); and when they are equal it is square (1:1). Swapping the two numbers flips a ratio between landscape and portrait — 16:9 becomes 9:16 — which is why the same content needs a different crop for a widescreen video versus a phone-first Story or Reel.
Common uses
- Checking whether a screenshot or photo is 16:9, 4:3 or another ratio before uploading.
- Scaling a design or video to a new size without stretching it.
- Sizing thumbnails, banners and social posts to a platform's required ratio.
- Working out the height for a responsive image or video embed from its width.
- Choosing a print size that matches your photo's ratio to avoid unwanted cropping.
Privacy
The calculator is entirely client-side. The dimensions you enter are computed in your own browser, are never sent to a server, and are not stored or logged — so it is safe to use for any project, and it keeps working offline once the page has loaded.
Frequently asked questions
- What is an aspect ratio?
- An aspect ratio is the proportional relationship between an image or screen's width and its height, written as two numbers separated by a colon — for example 16:9 or 4:3. It describes shape, not size: a 16:9 photo has the same proportions whether it is 1280×720 or 3840×2160. The first number is the width and the second is the height, so 16:9 is wider than it is tall, while 9:16 is a tall (portrait) frame. Aspect ratio matters whenever you crop, resize, print or upload an image, because stretching content to the wrong ratio distorts it.
- How do I calculate the aspect ratio of an image or screen?
- Enter the width and height in the first calculator and it reduces them to the smallest whole-number ratio. Mathematically it divides both numbers by their greatest common divisor (GCD). For 1920×1080 the GCD is 120, so 1920÷120 = 16 and 1080÷120 = 9, giving 16:9. It also shows the decimal ratio (width ÷ height, e.g. 1.78:1) and the nearest standard ratio name, so you instantly know whether a size is a common format or a slightly off one.
- What is the aspect ratio of 1920×1080 (or 1280×720)?
- Both are 16:9 — the standard widescreen HD ratio. 1920×1080 (Full HD / 1080p) and 1280×720 (HD / 720p) reduce to exactly 16:9, as do 2560×1440 (1440p) and 3840×2160 (4K UHD). That is why they all display full-screen on a modern TV or monitor without black bars: they share the same shape and only differ in resolution (the number of pixels).
- How do I resize an image or video without stretching it?
- Use the second calculator. Set the ratio you want to keep (pick a preset like 16:9 or type your own), then enter the new width — it computes the height that preserves the ratio, and vice versa. To scale 16:9 down to 1280 pixels wide, it returns 720 for the height. Keeping both dimensions on the same ratio is what stops an image from looking squashed or stretched. If you actually need to change the shape, crop rather than stretch — our crop tool lets you lock a target ratio.
- What is the difference between 16:9, 4:3 and 21:9?
- 4:3 (1.33:1) is the older, boxier standard used by classic TVs and many tablets. 16:9 (1.78:1) is today's widescreen standard for video and most displays. 21:9 (geometrically 64:27, about 2.37:1) is ultrawide, used for cinematic monitors and letterboxed film. The larger the decimal ratio, the wider and shorter the frame. Portrait ratios like 9:16 and 4:5 are the same idea turned on its side, and are what social apps use for full-screen phone content.
- Why is my ratio not exactly 16:9, and is the tool private?
- Some real-world resolutions are not perfectly standard. 1366×768, a very common laptop resolution, reduces to 683:384 — close to but not exactly 16:9 — so the tool shows the true reduced ratio and notes the nearest standard one (≈16:9). That honesty helps you spot sizes that will show tiny black bars or be slightly cropped. Everything runs as plain JavaScript in your browser: the numbers you type are never uploaded, stored or logged, and the calculator works offline once the page has loaded.