Aspect Ratio Calculator
100% private — runs on your device, never uploaded. Works offline once loaded.
Enter your original width and height, then a new width or a new height — the tool fills in the missing dimension so the proportions stay identical, and shows the simplified ratio.
What an aspect ratio is
An aspect ratio describes the relationship between the width and height of a rectangle, written as two numbers such as 16:9, 4:3 or 1:1. It does not tell you the actual size in pixels — only the shape. That is why a tiny 160×90 thumbnail and a huge 3840×2160 display can share the same 16:9 ratio: both have widths that are 16/9 times their heights.
This calculator takes your original width and height, reduces them to their simplest whole-number form using the greatest common divisor, and shows that ratio. So if you enter 1920 by 1080 it reports 16:9, and if you enter 1200 by 800 it reports 3:2. Knowing the simplified ratio makes it easy to spot standard formats and to recognise when a new size would distort the image.
Finding the missing dimension
The most common task is resizing without stretching. Suppose a photo is 4000×3000 and you need it to be 1200 pixels wide for a web page. Enter the original size, type 1200 into the new width box, and the calculator returns a new height of 900 — the only height that preserves the 4:3 shape at that width.
It works the same way in reverse. If you have a fixed height to fill, such as a 250-pixel banner slot, type that into the new height box and the required width is calculated for you. Because the ratio is fixed by your original dimensions, you only ever supply one new value; supplying the other is what would risk squashing or stretching the result.
- New height = new width ÷ (original width ÷ original height)
- New width = new height × (original width ÷ original height)
- The simplified ratio never changes when you resize proportionally
When to use it
Aspect ratio maths comes up constantly in design and media work. Video editors match footage to 16:9 or 9:16 for social platforms, photographers export to 3:2 or 4:3 prints, and web developers keep image containers proportional so layouts do not jump. Getting the second dimension exactly right avoids letterboxing, cropping surprises and blurry upscales.
It is also handy for planning before you touch an editor. Because the result updates as you type, you can quickly test several target widths and read off the heights, or confirm that a requested size actually matches the original ratio. If the numbers do not divide cleanly, the calculator still gives you the precise decimal so you can round sensibly for your final export.
Frequently asked questions
How is the missing dimension calculated?
The tool divides your original width by the original height to get the ratio, then multiplies or divides your new value by it. For example 4000×3000 at 1200 wide gives 1200 ÷ (4000/3000) = 900 tall.
Why does the simplified ratio show odd numbers sometimes?
Because it reduces to the smallest whole numbers that describe the shape. 1366×768, for instance, simplifies to 683:384 rather than a neat 16:9, which tells you it is slightly non-standard.
Can I use it for video and screen sizes?
Yes. It works for any rectangle — photos, video frames, monitors, print sizes or web containers — as long as you want to keep the proportions unchanged.
What if the result is not a whole number?
The calculator shows the exact decimal so you can round to the nearest pixel yourself. Most editors accept fractional inputs and round on export.
Does entering both new width and height work?
Enter only one. The tool prioritises the field you last edited and calculates the other so the ratio is preserved rather than distorted.
Is my data private?
Yes. Every calculation happens in your browser with basic arithmetic, so your numbers are never uploaded or stored.
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