Free Aspect Ratio Calculator
Simplify image and video aspect ratios and calculate scaled dimensions with exact and rounded pixel results. Use advanced options and copy or download your results. Browser-based and no signup required.
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How to use the aspect ratio calculator
Simplify image and video aspect ratios and calculate scaled dimensions with exact and rounded pixel results. No signup required.
- Enter original whole-pixel width and height, or choose a landscape, portrait or square example.
- Choose whether to set a target width or a target height and enter that new dimension.
- Compare the simplified ratio, exact scaled dimensions and nearest whole-pixel dimensions with the shape preview.
- Adjust decimal precision as needed and copy or download the dimension report.
Who uses an aspect ratio calculator
Designers, video editors and website builders resize visuals all the time. Knowing the matching height for a new width prevents stretched faces and squashed logos. It also helps when a platform asks for exact pixel sizes, such as a 1080 by 1350 portrait post or a 1280 pixel wide video thumbnail.
Developers use it to set width and height attributes on images so the page does not jump while it loads. Photographers use it to check whether a crop will keep the shape of the original frame.
Video work is another common case. If you export a 1080p clip for a site that wants a 720 pixel tall player, the calculator gives the matching width before you open your editing software. It works the same way for print layouts, presentation slides and game screenshots, as long as you think in pixels or any other consistent unit.
Worked example with real numbers
Enter an original size of 1920 by 1080, choose Set a new width and type 1000. The simplified ratio is 16:9 and the decimal width divided by height is 1.7778. The scaled height is 1000 x 1080 / 1920 = 562.5 pixels. Because screens cannot show half a pixel, the nearest whole-pixel size is 1000 by 563.
That rounding changes the shape slightly: 1000 / 563 = 1.7762 instead of 1.7778. The difference is tiny, but it explains why the exact and rounded rows do not match. A cleaner choice here is a width of 1280, which gives exactly 720.
Now try a 4:5 portrait. Enter 1080 by 1350, choose Set a new height and type 1920. The scaled width is 1920 x 1080 / 1350 = 1536 pixels, with no rounding needed.
How the ratio and scaling are calculated
The simplified ratio divides both sides by their greatest common divisor. For 1920 and 1080 that divisor is 120, which gives 16:9. Some common screen sizes do not reduce to a familiar pair. A 1366 by 768 display reduces to 683:384, which is close to 16:9 but not exact, as its decimal value of 1.7786 shows.
Scaling keeps the decimal ratio constant. When you set a width, the new height equals the new width multiplied by the original height and divided by the original width. When you set a height, the formula is reversed. The original sides must be whole pixels from 1 to 1,000,000, while the target value can include decimals.
Common mistakes to avoid
Do not mix up the dimension selector. If you type a new height while Set a new width is selected, the result describes a different size. Also check whether a platform wants an exact ratio or simply a maximum size before rounding.
Avoid scaling a small image up and expecting more detail. The calculator tells you the matching size, but enlarging a 640 pixel photo to 1920 pixels in an editor will still look soft.
Useful tips and limitations
- Whole-pixel rounding can introduce a small ratio difference.
- The tool calculates dimensions only; it does not modify an image or video file.
- Pick target widths that are multiples of the simplified ratio's first number to get whole-pixel heights with no rounding.
- Use the decimal ratio to compare two sizes quickly, since equal decimals mean the shapes match.
Common questions
Can this aspect ratio calculator resize my actual image?
It calculates matching dimensions and previews the shape. It does not upload, crop or resize image files. Use the image editing tools to modify a file.
Why do the rounded aspect dimensions differ from the exact result?
The exact scale can require fractional pixels. Rounding to whole pixels changes the ratio slightly; use the exact dimensions when precision is more important.
Can I calculate portrait and social-media aspect ratios?
Yes. Landscape 16:9, portrait 9:16, square 1:1 and 4:5 examples are included, and custom positive dimensions are supported.
Why is 1366 by 768 not shown as 16:9?
The calculator reduces the exact numbers, and 1366 by 768 reduces to 683:384, which is only approximately 16:9.
Can the target dimension include decimals?
Yes. The original width and height must be whole pixels, but the new width or height can be a decimal value.
Which result should I use for HTML image attributes?
Use the nearest whole-pixel width and height, since HTML width and height attributes take whole numbers.
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