Aspect Ratio Calculator
This free aspect ratio calculator finds the missing width or height when resizing an image, video, or design element, so the result keeps its original proportions instead of looking stretched or squashed. Nothing you enter is sent anywhere or stored.
How to Use It
- Select whether you know the new width or the new height.
- Enter the original width and height.
- Enter the new width or new height you already know.
- The matching dimension and simplified aspect ratio update instantly, no submit button.
How It Works
This calculator finds the ratio between the original height and width (or width and height, depending which one you're solving for), then applies that same ratio to the new dimension you already know. Multiplying by the exact same ratio the original had is what keeps the resized version proportional instead of distorted.
Formulas
scale by width: new height = new width × (original height ÷ original width)
scale by height: new width = new height × (original width ÷ original height)
A worked example: a 4000 by 3000 photo has a 4:3 aspect ratio. Resizing it to a new width of 1200 pixels needs a new height of 900 pixels (1200 × 3000 ÷ 4000) to keep that same 4:3 shape. A 1920 by 1080 video (16:9) resized down to a new height of 480 pixels needs a new width of about 853 pixels (480 × 1920 ÷ 1080) to avoid looking stretched.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What is this calculator actually for?
It finds the missing dimension when resizing an image, video, or design element so the result keeps the original proportions instead of looking stretched or squashed. If you know the original width and height and only a new width (or only a new height), this calculates the matching new height (or width) that preserves the same shape.
Why is the computed dimension rounded to a whole number?
Image and video dimensions are always a whole number of pixels, a fractional pixel is not meaningful. Rounding to the nearest whole pixel means the ratio is preserved almost exactly at normal sizes, with only a tiny, usually invisible difference at very small target sizes.
What if I already have a specific target width and height in mind?
If both dimensions are fixed by a requirement (a platform's exact upload size, for example) and they don't match your original aspect ratio, resizing to force both dimensions will distort the image. Cropping to the target ratio first, then resizing, avoids that distortion, this calculator is for finding a missing dimension, not for forcing two fixed, mismatched dimensions onto the same image.
How does the simplified aspect ratio relate to the resize calculation?
The aspect ratio (like 16:9 or 4:3) is the original width and height reduced to their simplest whole-number ratio, it describes the shape. The resize calculation uses the original width and height directly, not the simplified ratio, so the two numbers will always agree, the simplified ratio is just a more recognizable way to describe the same proportions.
Can you give a real-world example for both scale modes?
A 4000 by 3000 photo (a 4:3 ratio) resized to a new width of 1200 needs a new height of 900 to keep the same shape. A 1920 by 1080 video (16:9) resized down to a new height of 480 needs a new width of about 853 to avoid looking stretched.