📐 Aspect Ratio Calculator
Calculate aspect ratios and scale image and video dimensions proportionally without distortion. Supports 16:9, 9:16 (TikTok/Reels), 4:3, and 1:1 presets.
1. Original Image / Video Dimensions
2. Scale to New Resized Dimensions
What Aspect Ratio Calculator Does
An aspect ratio is a pair of pixel dimensions reduced to its simplest form, and reducing it is exactly the greatest common divisor problem you met at school. 1920 by 1080 shares a divisor of 120, which turns it into 16:9. 3840 by 2160 shares 240 and reduces to the same 16:9, which is why 4K and Full HD are interchangeable shapes at different sizes.
The practical use is the other direction: you know the shape and one dimension, and you need the other. A 16:9 banner that must be 1200 pixels wide is 675 tall, and getting that wrong by a few pixels is what produces the letterboxing and the awkward crops that show up after something is already published.
Not every pair reduces to something recognizable, and that is informative rather than a failure. An iPhone screenshot at 1290 by 2796 reduces to 215:466, which is nobody’s idea of a standard ratio — modern phone displays simply are not built to the classical shapes.
Where a ratio genuinely does not fit the target, the honest answer is to crop rather than to stretch. Scaling to a different shape distorts everything in the frame, and it is always visible on faces.
How to Use Aspect Ratio Calculator
- Enter original width and height in pixels (or pick a standard preset)
- Enter your new target width or height in the scaling section
- Instantly see the simplified aspect ratio and scaled dimensions without distortion
Formula Used by Aspect Ratio Calculator
Reducing to a ratio, and solving for the missing side
ratio = width ÷ gcd : height ÷ gcd; height = width × (ratioH ÷ ratioW)
- gcd
- the greatest common divisor, found by repeatedly taking remainders
Worked example
1920 × 1080, then a 16:9 image that must be 1200 wide.
- gcd(1920, 1080) = 120
- 1920 ÷ 120 = 16, 1080 ÷ 120 = 9 → 16:9
- Solving: 1200 × (9 ÷ 16) = 675
Result: 16:9, and the 1200-wide version is 1200 × 675.
Common ratios and where they come from
| Ratio | Decimal | Typical use |
|---|---|---|
| 16:9 | 1.778 | Video, YouTube, most displays — 1920×1080, 3840×2160 |
| 4:3 | 1.333 | Older screens, many projectors, some cameras |
| 3:2 | 1.500 | 35 mm film, most DSLRs, 6×4 inch prints |
| 1:1 | 1.000 | Profile pictures, Instagram squares |
| 4:5 | 0.800 | Instagram portrait — 1080×1350 |
| 9:16 | 0.563 | Stories, Reels, TikTok — 1080×1920 |
| 21:9 (often 64:27) | 2.370 | Ultrawide monitors — 2560×1080 reduces to 64:27, not 21:9 |
How to Read Your Result
An odd ratio usually means a phone
If a screenshot reduces to something like 215:466, nothing is wrong — phone panels are designed to physical constraints rather than to classical ratios. Treat the decimal as the useful number in that case, and match it rather than trying to name it.
Ultrawide is marketing, not arithmetic
2560×1080 is sold as 21:9 and actually reduces to 64:27, which is 2.370 rather than 2.333. The label is a convenient approximation. If you are computing exact dimensions, use the real reduction.
Crop to change shape, resize to change size
Resizing a 4:3 photo into a 16:9 slot squashes it. The correct sequence is to crop to the target ratio first — deciding what to lose — and only then scale to the pixel size you need.
Limitations & Accuracy Notes
- Whole pixels only, so a computed dimension is rounded and the result can be off by one from an exact ratio.
- It reduces the numbers you supply; it does not know which ratio a platform expects.
- Non-square pixels — an occasional issue in video formats — are not modeled.
- It does not crop or resize anything; use the cropper for that.
Frequently Asked Questions
What is an Aspect Ratio?
What aspect ratio is used for TikTok and Instagram Reels?
How does the automatic ratio simplification work?
What does an aspect ratio actually describe?
What happens if I resize without preserving the ratio?
Which ratios are common and where?
What is the difference between cropping and scaling?
Why are my pixel dimensions not whole numbers?
Is my data stored?
References & Further Reading
- MDN — aspect-ratio — How ratios are expressed in CSS and how the browser resolves them