How to use this aspect ratio calculator
This calculator answers the question every designer, video editor, and developer eventually runs into: if I change the width, what does the height need to be?Pick a ratio, type the one dimension you already know, and read the other one off the result card. Nothing is uploaded and no image is required — this is pure arithmetic running in your browser.
- Choose your ratio. Tap a preset like 16:9 or 9:16, type a custom pair into the ratio fields, or click use original dimensions and paste the size of a file you already have. That last option is the fastest way to match an existing asset exactly.
- Enter the size you know. If you are constrained by a container width of 1280 pixels, type 1280 into the width box. The height fills in immediately.
- Read the result. The card shows the full pair, the simplified ratio, the decimal equivalent, and the total megapixels. Press Copy to put the dimensions on your clipboard as
1280 × 720.
Why aspect ratio matters more than resolution
Resolution tells you how many pixels an image contains. Aspect ratio tells you whatshape it is. You can scale a 16:9 video from 3840×2160 down to 640×360 and it still fills a widescreen frame perfectly, because the shape never changed. But push a 16:9 video into a 4:3 frame and something has to give: either the picture stretches, or the sides get cropped off, or you get black bars.
That is why matching ratios matters far more than matching exact pixel counts. Platforms crop aggressively to their own ratio. If you export a landscape image and upload it as an Instagram Story, the platform will crop it to 9:16 and it will decide which parts to discard — not you.
Common aspect ratios and where they are used
| Ratio | Example size | Typically used for |
|---|---|---|
| 16:9 | 1920 × 1080 | HD and 4K video, YouTube, most monitors and TVs |
| 9:16 | 1080 × 1920 | Instagram Reels, TikTok, YouTube Shorts, Stories |
| 1:1 | 1080 × 1080 | Square social posts, profile pictures, album art |
| 4:5 | 1080 × 1350 | Instagram portrait feed posts |
| 4:3 | 1024 × 768 | Older displays, iPad screens, many projectors |
| 3:2 | 6000 × 4000 | DSLR and mirrorless camera sensors, 4×6 prints |
| 21:9 | 3440 × 1440 | Ultrawide monitors (technically 43:18) |
| 2.39:1 | 2048 × 858 | Anamorphic widescreen cinema |
The math behind it
An aspect ratio is just a fraction. To find the ratio of a 1920×1080 image, divide both numbers by their greatest common divisor. The greatest common divisor of 1920 and 1080 is 120, and 1920 ÷ 120 = 16 while 1080 ÷ 120 = 9 — hence 16:9. To scale while preserving that shape, multiply both sides by the same factor.
Going the other direction is equally simple. If you know the ratio is 16:9 and you need a width of 1280, then the height is 1280 × (9 ÷ 16) = 720. Flip the fraction to solve for width instead: a height of 400 at 16:9 means a width of 400 × (16 ÷ 9) ≈ 711. This tool runs both formulas as you type, which is mostly about saving you from arithmetic slips on deadline.
A note on rounding
Pixels are whole numbers, but ratio math rarely divides evenly. A 16:9 width of 1000 gives a height of 562.5. Leave Round to whole pixels checked and you get 563 — off by half a pixel, which is invisible. If you are working in a vector tool or with CSS units where fractions are legal, uncheck it to see the exact value.