Divide the width and height by their greatest common divisor to get the aspect ratio, then scale the other side by the same proportion. For 1920×1080 the divisor is 120, giving 16:9; a 1280-pixel-wide version therefore needs 1280 × 1080 ÷ 1920 = 720 pixels of height to keep the picture undistorted.
Editors, motion designers and anyone exporting clips for several platforms hit the same question: what height goes with this width so nothing stretches or letterboxes by accident? Enter the source frame in pixels and this calculator reduces it to a whole-number ratio such as 16:9, 9:16 or 256:135, shows the decimal form, names common formats, and works out the missing side for a target width or height. It also flags results that are fractional or odd, since H.264 and H.265 encoders expect even dimensions. One limit: it assumes square pixels, so anamorphic footage with a non-square pixel aspect ratio (for example DV NTSC at 720×480) shows its storage ratio, not its display ratio.
Type the width and height of the original video in pixels, for example 3840 and 2160.
Pick New width if the target width is set by the platform, or New height if the height is.
The matching dimension, the reduced ratio and the full resized frame appear as you type.
If the result is fractional or odd, round to the nearest even pixel before exporting.
Result Matching dimension (px): 720
Aspect ratio: 16:9
Decimal ratio: 1.778:1
Common name: HD / UHD widescreen
Resized frame: 1280 × 720 px
Full HD reduces to 16:9, so a 1280-pixel-wide export needs a 720-pixel height to avoid stretching.
Result Matching dimension (px): 1,080
Aspect ratio: 256:135
Decimal ratio: 1.896:1
Common name: DCI 4K cinema
Resized frame: 2048 × 1080 px
Cinema 4K is not 16:9; halving the width to 2048 gives the 2K DCI height of 1080.
Result Matching dimension (px): 720
Aspect ratio: 9:16
Decimal ratio: 0.563:1
Common name: Vertical (Reels, Shorts, TikTok)
Resized frame: 720 × 1280 px
A vertical 9:16 clip fitted to 1280 pixels tall needs a 720-pixel width.
ratio = (W ÷ g) : (H ÷ g), where g = GCD(W, H); new height = new width × H ÷ W; new width = new height × W ÷ H
W, HgEuclid's algorithm repeatedly replaces the larger number with the remainder of dividing it by the smaller until the remainder is zero; the last non-zero value is the divisor. Dividing both sides by it gives the smallest whole-number ratio. Resizing keeps the proportion W ÷ H constant, so the missing side is a single cross-multiplication. The decimal ratio is W ÷ H to three places, the form cinema uses (1.85:1, 2.39:1).
21:9 is a marketing label. 2560 ÷ 1080 is 2.370, and the exact reduced ratio is 64:27. True 21:9 would be 2.333, so monitors sold as 21:9 are close to it but not equal.
Consumer UHD at 3840×2160 is exactly 16:9. Cinema DCI 4K at 4096×2160 is 256:135, about 1.896:1, which is slightly wider than television 4K.
These platforms use vertical 9:16 video, typically 1080×1920. Enter those numbers and any target height to get the matching width for a smaller export.
Common codecs store colour at half resolution (4:2:0 chroma subsampling), so width and height must divide by two. An odd size is usually padded or cropped by one pixel by the encoder.
They describe the same shape. 16 ÷ 9 is 1.778, so the decimal form simply expresses the width relative to a height of one, as film formats like 1.85:1 and 2.39:1 do.
Yes, going from 16:9 to 9:16 or 1:1 means cropping, letterboxing or pillarboxing. This calculator handles proportional resizes; it keeps the original ratio and finds the other side.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 18). Video Aspect Ratio Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/video-audio/media-utilities/video-aspect-ratio-calculator/
Tools Directory Editorial Team. “Video Aspect Ratio Calculator.” Tools Directory, 18 Oct. 2026, https://tools.milunir.com/video-audio/media-utilities/video-aspect-ratio-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Video Aspect Ratio Calculator.” Tools Directory. Last modified October 18, 2026. Accessed October 27, 2026. https://tools.milunir.com/video-audio/media-utilities/video-aspect-ratio-calculator/.
<a href="https://tools.milunir.com/video-audio/media-utilities/video-aspect-ratio-calculator/">Video Aspect Ratio Calculator – Tools Directory</a>