Aspect Ratio Calculator
Find the aspect ratio of any width and height, resize while keeping the ratio, and get exact sizes for 16:9, 9:16, 4:5, 1:1, A4 and more.
About Aspect Ratio Calculator
This aspect ratio calculator turns any width and height into its simplest ratio, such as 1920×1080 to 16:9, and shows the decimal form and how close it is to common ratios. It works for pixels, millimetres or inches.
To resize an image or video without stretching it, type a new width or height and the other side is calculated for you. Presets cover 16:9 for YouTube, 9:16 for Reels and Shorts, 4:5 and 1:1 for Instagram, 21:9 ultrawide, 3:2 photos and A4 paper (1:√2). Everything runs in your browser, so nothing is uploaded.
How to use Aspect Ratio Calculator
- Enter a width and height (pixels, mm, inches — any unit) to get the simplified ratio.
- Or tap a preset like 16:9, 9:16 or Instagram portrait.
- Type a new width or height under “Resize” and the other side is worked out to keep the same shape.
- Copy the size or check how close it is to the common ratios.
Frequently asked questions
How do I calculate an aspect ratio?
Divide the width and height by their greatest common divisor. 1920×1080: the GCD is 120, so the ratio is 16:9. The decimal form is 1920 ÷ 1080 ≈ 1.78.
How do I find the height for a new width?
New height = new width × original height ÷ original width. For 16:9 at 1280 wide: 1280 × 9 ÷ 16 = 720.
What size should a YouTube video and thumbnail be?
YouTube videos are 16:9 (for example 1920×1080 or 3840×2160), and thumbnails 1280×720 (also 16:9). Shorts and Reels are vertical 9:16, 1080×1920.
Why is A4 not a simple ratio?
A-series paper uses 1:√2 (about 1:1.414), so halving the sheet keeps the same shape. 210×297 mm is that ratio rounded to whole millimetres.
What is the aspect ratio of 1920×1080?
1920×1080 is 16:9. Both numbers divide by 120, giving 16 and 9; as a decimal it is about 1.78:1.
What size is a 9:16 video?
The common size is 1080×1920 pixels (Full HD, vertical), used for Instagram Reels, YouTube Shorts and Stories. 720×1280 is the same ratio at a lower resolution.
Is 4:5 or 1:1 better for Instagram posts?
Both work. 4:5 (1080×1350) is the tallest portrait size for a feed post and takes more screen space; 1:1 (1080×1080) is the classic square.
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