Aspect Ratio Calculator
Given three of the four numbers, original width, original height, new width and new height, an aspect ratio calculator returns the fourth. Use it when you resize a YouTube thumbnail (16:9), prepare square or portrait Instagram assets, export an ultrawide banner (21:9) or keep an old 4:3 video from stretching. The maths is a single proportion, but running 1920 ÷ 1080 in your head is fiddly.
How to preserve an aspect ratio
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1
Pick a ratio
Choose a preset (16:9, 4:3, 1:1) or type a custom one like 2.35:1.
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2
Enter any one dimension
Usually the constrained side, for example a fixed width of 800 px.
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3
Read the matching side
The calculator returns the other dimension rounded to the nearest pixel.
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4
Optionally lock and resize
Change one side and watch the other recalculate so you never distort the image.
Common aspect ratios and where they show up
| Ratio | Decimal | Typical use |
|---|---|---|
| 16:9 | 1.778 | HD/UHD video, YouTube, modern TVs, most desktop monitors |
| 4:3 | 1.333 | Older video, iPad-style tablets, slides, security cameras |
| 1:1 | 1.000 | Square social posts, album covers, avatars |
| 4:5 | 0.800 | Portrait social posts and ads |
| 3:2 | 1.500 | DSLR photos, 6 x 4 in prints |
| 21:9 | 2.333 | Ultrawide monitors, cinematic web banners |
| 2.39:1 | 2.390 | DCI Scope / anamorphic cinema delivery |
| 9:16 | 0.563 | Stories, Reels, TikTok and Shorts vertical video |
The formula
new_height = new_width * (original_height / original_width)
Or using a known ratio W:H:
new_height = new_width * H / W
Integer pixels only: round to the nearest pixel, do not always round down, to avoid one-pixel gaps.
Cropping vs letterboxing
If the destination ratio does not match the source, you have two honest choices:
- Crop (fill) - Keep the shorter side and trim the longer. Losing pixels is fine for decorative images, but bad for screenshots or text.
- Fit (letterbox/pillarbox) - Keep everything and add padding on the side that is short. Common for films shown on 16:9 TVs.
“Stretch to fill” is the third option, and it is always wrong.
Pixel density and DPI
Ratios are unitless. A 1920x1080 image and a 3840x2160 image are both 16:9, just at different pixel densities. Keep the ratio right first; decide on absolute pixels, DPI or PPI second.
Frequently Asked Questions
Divide the larger by the smaller, or compute GCD(w, h) and simplify. 1440 x 1080 simplifies to 4:3 because GCD is 360. 1920 x 1080 simplifies to 16:9.
Pixel dimensions have to be integers. Round to 1081 (nearest), or use 1080 and accept a hairline offset. For hero images and video, prefer exact pairs like 1920x1080 or 1280x720.
For standard YouTube video, export 16:9 such as 1920x1080 or 2560x1440. YouTube adapts the player to other ratios, so avoid baking black bars into the file. Use 9:16 for Shorts and other vertical-first cuts.
Close but not identical. 21:9 is 2.333:1 and is common for ultrawide monitors. DCI Scope cinema delivery is 2.39:1, so a scope film can still show slim bars on a 21:9 display.
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