Aspect Ratio Calculator

Scale width and height while keeping the same aspect ratio — perfect for images and video. Shows the simplified ratio such as 16:9.

Everything is calculated in your browser — nothing you type is uploaded anywhere.

New Dimensions

Simplified Aspect Ratio

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Scaled to of the original size.

Enter an original width and height greater than zero to see a result.

Now enter the new dimension you know.

What is an aspect ratio?

An aspect ratio is the relationship between the width and the height of a rectangle, written as two numbers separated by a colon — 16:9, 4:3, 1:1. It describes shape, not size: a 1920 × 1080 video frame, a 1280 × 720 thumbnail, and a 640 × 360 preview are all 16:9, because dividing width by height gives roughly 1.778 in every case.

That distinction is why resizing by eye causes trouble. If you change a width but pick the wrong height, the image no longer matches its original ratio, and whatever displays it must either stretch the picture (faces get wide or narrow), letterbox it with bars, or crop part of the frame away. Keeping the ratio fixed while changing the size is called proportional scaling, and it is exactly what this calculator does.

How to use this calculator

  1. Enter your original width and original height. You can also pick a common ratio from the dropdown to fill both fields, then edit them.
  2. Choose whether you know the new width or the new height — whichever one your target space dictates.
  3. Type that one value. The other dimension is solved immediately and shown in the read-only field and in the result panel.

The panel also reports the simplified ratio of your original size (reduced to its smallest whole numbers), the same ratio as a decimal in x:1 form, and the scale factor as a percentage so you can see at a glance whether you are shrinking or enlarging. The Swap width and height button flips the original dimensions, which is handy when you need the portrait version of a landscape ratio.

The formula

Proportional scaling is a single cross-multiplication. If the original size is W × H, the new dimensions must satisfy:

  • new height = new width × H ÷ W
  • new width = new height × W ÷ H

To express the ratio in whole numbers, divide both the width and the height by their greatest common divisor (GCD) — the largest number that divides both exactly. For 1920 and 1080 the GCD is 120, so 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, giving 16:9. This calculator multiplies your inputs by 1000 before reducing, so decimal entries such as 2.35 still reduce to a clean whole-number ratio.

Worked example

Say you shot a photo at 4032 × 3024 and you need it to fit a content column that is 1200 px wide. Entering those values gives:

  • new height = 1200 × 3024 ÷ 4032 = 900, so the resized image is 1200 × 900;
  • the simplified ratio is 4:3, because the GCD of 4032 and 3024 is 1008;
  • the scale factor is 1200 ÷ 4032 ≈ 29.8% of the original size.

Had you guessed 1200 × 800 instead, you would have been working at 3:2 — the image would appear vertically squashed by about 12%, or lose a strip top and bottom if cropped to fit.

Rounding and whole pixels

Pixels are whole numbers, but ratios often are not. Scaling a 1920 × 1080 frame to 1000 px wide calls for a height of 562.5 px, which no image file can store. The calculator shows the rounded value as the answer and tells you the exact figure underneath whenever rounding occurred, so you can decide how to absorb the difference. In practice a half-pixel error is invisible; where it matters — video encoders, for instance, often require dimensions divisible by 2 or even 16 — pick a nearby width that divides cleanly, such as 960 × 540 rather than 1000 × 562.5.

Common aspect ratios

  • 16:9 — the standard for HD and 4K video, and for most monitors and TVs (1920 × 1080, 3840 × 2160).
  • 9:16 — 16:9 turned on its side, used by vertical phone video and short-form platforms.
  • 4:3 — analogue television and early computer displays; still common for presentation slides and some camera sensors.
  • 3:2 — the 35 mm film frame, inherited by most DSLR and mirrorless cameras.
  • 1:1 — square, widely used for avatars, product thumbnails and social posts.
  • 64:27 — ultrawide monitors, usually marketed as 21:9 even though the exact ratio is about 2.37:1.
  • 1.85:1 and 2.39:1 — the two common theatrical projection formats, called flat and scope.

If you are laying out a page rather than resizing a file, CSS can hold a ratio for you: the aspect-ratio property accepts values like 16 / 9 and lets the browser compute the second dimension automatically. Use this calculator when you need concrete numbers — export presets, canvas sizes, thumbnail dimensions — and the CSS property when the container should stay flexible.

References

✏️ Edit this page on GitHub