Simplify a ratio, then resize with confidence.
Enter an original width and height to reduce them to the simplest ratio using the greatest common divisor. Add a new width or height and the missing side is solved for you, so images and videos scale without stretching or black bars.
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Common aspect ratios
| Ratio | Decimal | Where you see it |
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How aspect ratio math works
Reducing to the simplest ratio
An aspect ratio is just the relationship between width and height. To simplify it you find the greatest common divisor (GCD) of the two numbers, then divide both by it. For 1920 by 1080 the GCD is 120, so the ratio becomes 16 to 9. The decimal value, width divided by height, is another useful form: 16:9 equals about 1.778.
The GCD is found with the Euclidean algorithm: repeatedly replace the larger number with the remainder of dividing it by the smaller, until the remainder is zero. The last non-zero value is the GCD. It is fast, exact for whole numbers, and never needs guessing.
Solving for a missing side
Once you know the original width and height, keeping the same shape at a new size is a single proportion. Given a new width, the new height is new width times original height divided by original width. Given a new height instead, the new width is new height times original width divided by original height. Matching the ratio this way keeps content sharp and avoids stretching.
Why it matters for images and video
When a source and a display have different aspect ratios you get letterboxing (bars on top and bottom), pillarboxing (bars on the sides), or a stretched picture. Resizing along the original ratio removes all three problems. It is the same idea whether you are exporting a thumbnail, fitting a video into a player, or planning a print at a fixed proportion.
Common questions
What is an aspect ratio?
It is the proportional relationship between an image or screen's width and its height, written as width to height, for example 16:9. It describes shape, not size, so a small and a large screen can share the same ratio.
How do you simplify a ratio like 1920 by 1080?
Find the greatest common divisor of both numbers, then divide each by it. The GCD of 1920 and 1080 is 120, so 1920:1080 reduces to 16:9. This calculator does that automatically as you type.
How do I resize an image without distortion?
Keep the same aspect ratio. Enter your original width and height, then a new width, and the new height is computed as new width times original height divided by original width. The picture stays in proportion.
What is the difference between 16:9 and 4:3?
16:9 is a wide format with a decimal of about 1.778, used by HDTV, YouTube and most monitors. 4:3 is nearly square at about 1.333, used by older televisions, many tablets and classic photography. Content made for one leaves bars when shown on the other.
Which aspect ratio is best?
There is no single best ratio. 16:9 suits most video and displays, 21:9 fits cinematic and ultrawide work, 4:3 and 1:1 suit certain photos and social posts. Choose the one that matches your target screen or platform.
Does aspect ratio affect image quality?
Changing the ratio itself does not lower quality, but forcing content into the wrong ratio does, because it stretches or crops the picture. Resizing along the original ratio preserves the look while changing only the pixel count.
Results are rounded for display. Aspect ratio simplification assumes whole-number pixel dimensions; fractional inputs are rounded before finding the greatest common divisor.