Aspect Ratio Calculator
Resize by 16:9, 4:3 or any ratio — width and height locked.
Lock an aspect ratio (16:9, 9:16, 4:3, 1:1 or a custom one) and resize proportionally: change the width and the height follows automatically, or vice-versa. See a live to-scale preview, the simplified ratio and the decimal ratio. Perfect for video, YouTube & Reels, photos, screens and design. Runs fully in your browser — nothing is uploaded.
How to use Aspect Ratio Calculator
Pick or read a ratio
Tap a preset such as 16:9, type your own values, or use From image to read the exact ratio from a picture.
Enter a dimension
Type the width you want in pixels — the height is calculated instantly to keep the ratio (or unlock to set both freely).
Scale or pick a size
Use the scale buttons (50%, 2×…) or a common-size preset like 1080p or IG Story to set the dimensions in one tap.
Fit inside a box
Enter a maximum width and height to get the largest version of your ratio that fits fully inside, ready to apply.
Copy the result
Copy the pixel dimensions or the CSS aspect-ratio value straight into your editor, and check the live preview.
Frequently asked questions
What is an aspect ratio?
An aspect ratio is the proportional relationship between width and height, written as W:H — for example 16:9. It describes the shape of an image, video or screen regardless of its actual size, so a 1280×720 and a 1920×1080 image share the same 16:9 ratio.
How do I resize an image without stretching it?
Keep the aspect ratio the same. Enter your original ratio (or read it from the image), then type the new width you need — this calculator works out the matching height so the image scales proportionally and never looks squashed.
What does 'fit inside a box' do?
It calculates the largest size of your chosen ratio that fits completely inside a maximum width and height — the same as CSS object-fit: contain. It's ideal for fitting a video or image into a fixed area with letterboxing, without cropping.
Why would I round to even numbers?
Many video codecs (like H.264) require even pixel dimensions. Turn on the 'even' option and the calculated width and height are rounded to the nearest even number so your exports don't fail or get silently adjusted.
How is the simplified ratio calculated?
The calculator divides your width and height by their greatest common divisor (GCD). For example 1920×1080 both divide by 120, giving the simplified ratio 16:9. The CSS output uses this simplified ratio.
Is it free and private?
Yes — completely free with no sign-up, and every calculation (including reading a ratio from an image) runs locally in your browser, so nothing you enter or upload is sent to a server.
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