📐 Aspect Ratio Calculator: Resize and Find 16:9, 4:3, 9:16
By ToolNimba Web Dev Team · Updated 2026-06-24
Enter the original size, then a new width OR a new height. Leave the other new box empty and it is computed to keep the same proportions.
An aspect ratio calculator keeps your images, videos and layout boxes in proportion when you scale them. Enter the original width and height, then a single new dimension, and it works out the matching width or height so nothing ends up stretched or squashed. It also reduces any pixel size to its simplest ratio, so you can see at a glance that 1920x1080 is just 16:9. No installs, no uploads, everything runs in your browser.
What is the Aspect Ratio Calculator?
The aspect ratio of a rectangle is the relationship between its width and its height, written as two numbers separated by a colon, for example 16:9 or 4:3. It does not describe how big something is, only its shape. A 16:9 frame has the same proportions whether it is 1280x720, 1920x1080 or 3840x2160, because each of those widths divided by its height gives the same value (about 1.778). That constant ratio is what your eye reads as the shape of a screen, photo or video, and it is the single number you protect whenever you scale content up or down.
To resize while keeping the ratio you scale both sides by the same factor. If the original is W x H and you choose a new width w, the matching height is h = w x H / W. Choose a new height instead and the width is w = h x W / H. Because both dimensions are multiplied by the same number, the proportions are preserved and the picture is not distorted. This is exactly what the "lock aspect ratio" or chain-link icon does in image editors and CSS layouts, and it is why a 1280 pixel wide 16:9 video must be 720 pixels tall rather than a rounded 700.
To express a size as a clean ratio you divide both numbers by their greatest common divisor (GCD), the largest whole number that divides both exactly. For 1920 and 1080 the GCD is 120, so 1920 divided by 120 = 16 and 1080 divided by 120 = 9, giving 16:9. Reducing by the GCD always lands on the smallest whole-number pair, which is why standard ratios like 16:9, 4:3, 3:2 and 1:1 are so compact. The decimal form (width divided by height) is useful too: 1.778 is 16:9, 1.333 is 4:3 and 1.0 is a perfect square. Some pixel sizes do not reduce to a famous name, so the decimal tells you which standard ratio they sit closest to.
Aspect ratio and resolution are easy to confuse but they answer different questions. Resolution is the actual pixel count, such as 1920x1080, and it controls sharpness and file size. Aspect ratio is only the shape, so 1280x720 and 3840x2160 are both 16:9 even though one has nine times the pixels of the other. When you brief a designer, a printer or a video platform you usually need to state both: the ratio so the frame is not letterboxed or cropped, and the resolution so the result is crisp enough for the screen it will play on.
Orientation matters as well. A ratio above 1.0 is landscape (wider than tall), a ratio below 1.0 is portrait (taller than wide), and exactly 1.0 is a perfect square. Flipping 16:9 gives 9:16, the full-screen vertical shape that powers TikTok, Instagram Reels and YouTube Shorts. Mixing the two is the most common social media mistake: upload a 16:9 clip to a 9:16 feed and the platform either adds black bars or crops the sides, so plan the orientation before you shoot or export.
In the browser, modern CSS has its own way to lock shape. The aspect-ratio property lets you write aspect-ratio: 16 / 9 on a container or image so it reserves the right height for its width automatically, which keeps responsive video embeds and image grids from jumping around as the page loads. This calculator gives you the exact numbers behind those rules, whether you are filling in a width and height attribute, setting a padding-top percentage for an older responsive embed, or sizing a hero banner that must match a fixed ratio across every breakpoint.
When to use it
- Resizing a photo or banner to a new width while letting the height follow so it is not stretched.
- Working out the height of a responsive video embed (such as 16:9) for a given container width.
- Checking whether an image is 16:9, 4:3, 3:2 or square before cropping or uploading it.
- Reducing an odd pixel size like 2560x1600 to its simplest ratio (16:10) to match a device.
- Planning thumbnail, hero and social-card dimensions that all share one consistent ratio.
- Converting a landscape 16:9 master into a 9:16 vertical or 1:1 square cut for social platforms.
How to use the Aspect Ratio Calculator
- Pick "Resize keeping ratio" to scale a size, or "Show simplified ratio" to reduce one.
- In resize mode, enter the original width and height in pixels.
- Type a new width OR a new height and leave the other box empty.
- Read the computed missing dimension and the ratio it preserves.
- In simplify mode, enter any width and height to see the lowest-term ratio and its decimal value.
- Round the result to whole pixels if your target platform rejects fractional dimensions.
Formula & method
Worked examples
You have a 1920x1080 image and want it 1280 pixels wide while keeping the ratio.
- Original width W = 1920, original height H = 1080.
- New width w = 1280, solve for the new height h.
- h = w x H / W = 1280 x 1080 / 1920
- h = 1,382,400 / 1920 = 720
Result: New size 1280 x 720 px, which is still 16:9.
Reduce a 2560x1600 laptop resolution to its simplest aspect ratio.
- Find gcd(2560, 1600). 2560 = 1600 x 1 + 960, 1600 = 960 x 1 + 640, 960 = 640 x 1 + 320, 640 = 320 x 2 + 0, so the GCD is 320.
- 2560 / 320 = 8
- 1600 / 320 = 5
- Decimal check: 2560 / 1600 = 1.6
Result: The ratio is 8 : 5 (also written 16:10), a decimal of 1.6 : 1.
Find the 9:16 vertical height for a 1080 pixel wide TikTok or Reels export.
- A 9:16 frame uses W:H of 9 to 16, so height is taller than width.
- New height h = w x 16 / 9 = 1080 x 16 / 9
- h = 17,280 / 9 = 1920
- Decimal check: 1080 / 1920 = 0.5625, the 9:16 value.
Result: Use 1080 x 1920 px, the standard full-screen vertical size for Reels, Shorts and TikTok.
Common aspect ratios and where they are used
| Ratio | Decimal (w / h) | Example size | Typical use |
|---|---|---|---|
| 16:9 | 1.778 | 1920 x 1080 | HD video, monitors, YouTube |
| 4:3 | 1.333 | 1024 x 768 | Older screens, some cameras |
| 3:2 | 1.5 | 1080 x 720 | DSLR photos, prints |
| 1:1 | 1.0 | 1080 x 1080 | Square social posts, avatars |
| 21:9 | 2.333 | 2560 x 1080 | Ultrawide monitors, cinema |
| 16:10 | 1.6 | 2560 x 1600 | Laptops, productivity monitors |
| 9:16 | 0.5625 | 1080 x 1920 | Phone video, Reels, Stories |
| 4:5 | 0.8 | 1080 x 1350 | Instagram and Facebook feed posts |
Recommended aspect ratios by social platform
| Platform and format | Aspect ratio | Pixel size |
|---|---|---|
| Instagram feed post | 4:5 | 1080 x 1350 |
| Instagram Reels and Stories | 9:16 | 1080 x 1920 |
| TikTok video | 9:16 | 1080 x 1920 |
| YouTube standard video | 16:9 | 1920 x 1080 |
| YouTube Shorts | 9:16 | 1080 x 1920 |
| Facebook in-feed video | 4:5 or 1:1 | 1080 x 1350 |
| X (Twitter) landscape video | 16:9 | 1920 x 1080 |
| LinkedIn square post | 1:1 | 1080 x 1080 |
16:9 resolutions at common heights
| Name | Width | Height |
|---|---|---|
| nHD | 640 | 360 |
| HD (720p) | 1280 | 720 |
| Full HD (1080p) | 1920 | 1080 |
| QHD (1440p) | 2560 | 1440 |
| 4K UHD | 3840 | 2160 |
Common mistakes to avoid
- Changing only one dimension by hand. Setting a new width but typing a rounded height (say 1280x700 instead of 1280x720) breaks the ratio and stretches the image. Let the calculator derive the matching side so both scale by the same factor.
- Confusing ratio with resolution. A ratio like 16:9 describes shape, not size. 1280x720 and 3840x2160 share the same 16:9 ratio but have very different pixel counts and sharpness, so check both ratio and resolution.
- Rounding before reducing. Rounding pixel values first can give a slightly wrong ratio. Reduce the exact original numbers by their GCD, then round only the final displayed size if you need whole pixels.
- Ignoring upscaling limits. The calculator will happily compute a larger size, but enlarging a small image adds no real detail. Scaling 640x360 up to 1920x1080 keeps the 16:9 ratio yet looks soft because the extra pixels are interpolated.
- Uploading the wrong orientation to social. Posting a 16:9 landscape clip into a 9:16 vertical feed leaves black bars or crops the sides. Decide on the target orientation first, then export to that exact ratio.
- Forgetting platform rounding rules. Some video encoders prefer dimensions divisible by 2 or 8. If your computed height is an odd number like 405, nudge it to the nearest even value and keep the ratio close rather than shipping a size the encoder rejects.
Glossary
- Aspect ratio
- The proportional relationship between width and height, written as width:height, such as 16:9.
- Resolution
- The actual pixel dimensions of an image or screen, for example 1920 x 1080.
- GCD
- Greatest common divisor, the largest whole number that divides two numbers exactly, used to reduce a ratio.
- Scale factor
- The single number you multiply both width and height by to resize without distortion.
- Portrait and landscape
- Landscape is wider than tall (ratio above 1), portrait is taller than wide (ratio below 1).
- Display aspect ratio (DAR)
- The shape of the picture as it appears on screen after any pixel stretching is applied.
- Pixel aspect ratio (PAR)
- The shape of an individual pixel; square pixels are 1:1, while some legacy video formats use non-square pixels.
- Letterboxing
- Black bars added above and below (or beside) content when its ratio does not match the display frame.
Frequently asked questions
How do I keep the aspect ratio when resizing an image?
Multiply both the width and the height by the same factor. In practice, fix one new dimension and let the other follow: new height = new width x original height / original width, or new width = new height x original width / original height. This calculator does that for you when you fill in one new box.
What is the aspect ratio of 1920x1080?
It is 16:9. The greatest common divisor of 1920 and 1080 is 120, so 1920 / 120 = 16 and 1080 / 120 = 9. As a decimal that is about 1.778 to 1, the standard widescreen ratio for HD video and most monitors.
How do I calculate height from width for a 16:9 video?
Multiply the width by 9 and divide by 16. For a 1280 pixel wide container the height is 1280 x 9 / 16 = 720. Use the resize mode here with an original size of 16 x 9 (or any 16:9 size) and enter your new width.
What does 16:9 mean exactly?
It means that for every 16 units of width there are 9 units of height. It describes the shape of the frame, not its size, so 1280x720, 1920x1080 and 3840x2160 are all 16:9 even though their resolutions differ.
What is the difference between aspect ratio and resolution?
Aspect ratio is the shape of the frame, expressed as a ratio like 16:9. Resolution is the actual pixel count, like 1920x1080, and it controls how sharp the image looks. Two images can share a ratio while having very different resolutions, so platforms usually ask for both.
Can I use decimal or non-whole dimensions?
Yes. The calculator accepts decimal widths and heights and will scale away the decimals before reducing to a whole-number ratio. The resize result is rounded to two decimal places so you can round to whole pixels yourself if needed.
Why is the simplified ratio not a familiar name like 16:9?
Not every size reduces to a well-known label. For example 1366x768 reduces to 683:384, which is close to but not exactly 16:9. The decimal value shown next to it tells you how near a standard ratio it really is.
What aspect ratio should I use for Instagram, TikTok and YouTube?
Use 9:16 (1080x1920) for TikTok, Instagram Reels and YouTube Shorts because it fills the whole phone screen. Use 4:5 (1080x1350) for Instagram and Facebook feed posts, and keep 16:9 (1920x1080) for standard long-form YouTube videos.
How do I convert a 16:9 video to 9:16 vertical?
You cannot just change the numbers without losing content, because the shapes are opposite. Either re-crop the footage to the 9:16 frame (keeping the subject centered) or place the 16:9 clip on a 1080x1920 background with bars. Use this tool to confirm the exact target pixels first.
What is the aspect-ratio CSS property?
It is a CSS rule like aspect-ratio: 16 / 9 that tells the browser to reserve the correct height for an element based on its width. It keeps responsive images and video embeds from shifting the layout while they load, and the ratio you set is the same one this calculator produces.