Aspect Ratio Calculator

1. Enter the original width and height in any unit, as long as both use the same one.
2. Read the aspect ratio, its decimal form and the format name, which all update as you type.
3. To resize, type a new width and the matching height that preserves the ratio appears instantly.

Each output answers a different question:

An aspect ratio describes the shape of a rectangle by comparing its width to its height, independent of its actual size. A 16:9 frame has the same proportions whether it is a phone thumbnail or a cinema screen, which is why the ratio, not the pixel count, decides whether an image looks correct.

The ratio is written as two numbers separated by a colon, so 16:9 means sixteen units of width for every nine of height. Because it is a proportion, you can scale both numbers by the same factor and the shape never changes, and that single fact is what lets designers move one layout across dozens of screen sizes.

The tool divides the width by the height, then reduces that fraction to its smallest whole-number form using the greatest common divisor of the two values.

In symbols the reduced ratio is w/h = (w / d) : (h / d) where d is the greatest common divisor of the width and height. Dividing both sides by d cannot change the proportion, so the reduced pair describes exactly the same shape in the smallest numbers.

1. Take the width and height as whole numbers.
2. Find their greatest common divisor.
3. Divide each by that divisor to get the simplest ratio.

To resize instead, it multiplies the new width by the original height and divides by the original width, which is the proportion h2 = (w2 * h1) / w1.

Take a 1920 by 1080 image. The greatest common divisor of 1920 and 1080 is 120, and 1920/120 : 1080/120 = 16:9, the widescreen standard. The decimal form is 1920 divided by 1080, or about 1.778 to 1.

Now resize it to a 1280-pixel-wide thumbnail. Keeping 16:9 means the height must be (1280 * 1080) / 1920 = 720, giving 1280 by 720. The shape is untouched, so nothing looks stretched.

By contrast, forcing that same image into a 1280 by 800 box changes the ratio to 8:5, and the difference is what produces the subtle squashing you sometimes see in badly resized banners.

A handful of ratios cover almost everything you will meet, and knowing them saves time when a platform asks for a specific shape.

If a number here surprises you, the percentage calculator is handy for working out how far one shape sits from another as a percentage.

The ratio is fixed by the two dimensions alone, but a few habits quietly change it without you noticing.

Cropping

Trimming pixels from one edge alters width or height on its own, so cropping is the most common way a ratio shifts from, say, 3:2 to 16:9.

Non-Proportional Resizing

Dragging only one handle, or typing a width and height that are not linked, breaks the proportion and stretches the content.

Padding and Borders

Adding a frame or letterbox changes the overall dimensions of the canvas even when the picture inside keeps its shape.

Rounding to Whole Pixels

A device cannot show a fractional pixel, so a resized value is rounded, which nudges the true ratio by a tiny amount.

Preparing Images for a Platform

Social networks and video sites require exact shapes, and the calculator gives the resize height in one step so nothing is rejected or auto-cropped.

Responsive Web Design

Reserving space for a video or hero image needs the decimal ratio, which slots directly into CSS aspect-ratio rules to stop layout shift.

Printing and Framing

Matching a photo to a standard frame means checking whether 3:2 or 4:3 fits before you order, saving a wasted print.

1. Mixing Up Width and Height

16:9 and 9:16 are different shapes. The first number is always the width.

2. Resizing Without Locking the Ratio

Changing width and height independently distorts the image. Change one and let the other follow.

3. Comparing Ratios by Subtraction

Ratios are proportions, so compare them by division or by their decimal form, not by subtracting the numbers.

4. Assuming Pixels Equal Ratio

Two images with very different pixel counts can share the same ratio; it is the proportion, not the size, that matters.

5. Ignoring Rounding on Resize

A resized dimension is often not a whole pixel, so expect a value rounded to the nearest one.

The reduction is exact for whole-number inputs, and the resize is mathematically precise before pixel rounding, but the tool describes shape only.

It is the proportion of width to height, written as two numbers such as 16:9. It describes shape independent of size, so any image with that proportion looks the same whether large or small.

Divide the width by the height, then reduce the two numbers by their greatest common divisor. For 1920 by 1080 the GCD is 120, giving 16:9. This calculator does both steps for you.

16:9 is the standard for video, laptops and televisions. For photography 3:2 and 4:3 are common, and 1:1 and 9:16 dominate social media.

Change one dimension and let the other follow the same ratio. Enter a new width here and the matching height is calculated so the proportions stay identical.

Sixteen units of width for every nine of height. A 1920 by 1080 screen is 16:9 because both numbers reduce by 120 to 16 and 9.

No. Resolution is the pixel count, such as 1920 by 1080. Aspect ratio is the shape those pixels form. Different resolutions can share the same ratio.

Divide the first number by the second. So 16:9 is 16 divided by 9, about 1.78. The decimal is useful in CSS and code.

The bars, called letterboxing, appear when content of one ratio is placed in a frame of a different ratio. Match the ratios, or expect padding to fill the gap.

Yes. When the height is greater than the width the ratio is a portrait one, such as 9:16 for vertical video. The decimal is then less than one.

No. The calculation runs entirely in your browser, and nothing you type is sent anywhere unless you choose to Save a result, which stays on this device only.

• Aspect ratio (image) (Wikipedia).
• Ratios explained (Maths Is Fun).
• Greatest common factor (Maths Is Fun).
• 16:9 aspect ratio (Wikipedia).