Aspect Ratio Calculator
Given three of the four numbers — original width, original height, new width, new height — an aspect ratio calculator returns the fourth. You use it every time you resize a thumbnail for YouTube (16:9), crop a square for Instagram, export a widescreen banner (21:9) or keep an old 4:3 video from stretching. The maths is a single proportion but running it in your head for 1920 ÷ 1080 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 monitors |
| 4:3 | 1.333 | Old TVs, iPad, presentation slides, security cameras |
| 1:1 | 1.000 | Instagram feed, album covers, avatars |
| 3:2 | 1.500 | DSLR photos, print 6x4 postcards |
| 21:9 | 2.333 | Ultrawide monitors, cinematic videos |
| 2.39:1 | 2.390 | Anamorphic cinema scope |
| 9:16 | 0.563 | Stories, Reels, TikTok 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, do not floor, to avoid off-by-one 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, bad for screenshots or text.
- Letterbox (fit) — Keep everything, add bars on the short side. 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 size in absolute pixels 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 drop to 1080 and accept a hairline offset. For hero images and video, prefer exact ratios like 1920x1080 or 1280x720.
YouTube renders 16:9 (1920x1080 or 2560x1440) as its native format. Uploads in other ratios get letterboxed with black bars. Shorts are 9:16 vertical.
Close but not identical. 21:9 is exactly 2.333:1 and is the common monitor/TV ultrawide ratio. Cinema scope is 2.39:1. A film shot in scope will letterbox slightly even on a 21:9 display.
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