Aspect Ratio Calculator: Free Online Resizer

An aspect ratio calculator finds the missing width or height when you resize an image or video, and it reduces any width and height to a simple ratio such as 16:9. Designers, video editors, and site owners use it to scale proportionally, avoid stretched visuals, and get the new size and the simplified ratio in one click.

What Is Aspect Ratio and How Is It Calculated?

Aspect ratio is the proportional relationship between an object's width and height, written as two numbers separated by a colon. To calculate it, divide both numbers by their greatest common divisor. A 1920 by 1080 frame shares a divisor of 120, so it simplifies to 16:9.

The decimal equivalent is simply width divided by height. A 16:9 frame is about 1.78, a 4:3 frame is about 1.33, and a square 1:1 frame is exactly 1. Anything above 1 is landscape, and anything below 1 is portrait.

To resize without distortion, keep that relationship fixed. New height equals new width times original height divided by original width. New width equals new height times original width divided by original height. The calculator does this arithmetic for you.

Because a ratio compares two numbers, the unit does not change it. A 12 by 8 inch print and a 1200 by 800 pixel image are both 3:2, even though the fields are labeled in pixels.

How to Use the Aspect Ratio Calculator

The tool has three tabs, and each one answers a different question. Follow these steps to get a result in a few seconds.

1. Choose a tab: Scale by Width if you know the new width, Scale by Height if you know the new height, or Find Ratio if you only want the ratio of a size.
2. Type the Original Width (px) and Original Height (px) of your image, video, or screen. On the Find Ratio tab, use the Width (px) and Height (px) fields instead.
3. On Scale by Width, enter the New Width (px). On Scale by Height, enter the New Height (px).
4. Click Calculate. On the Find Ratio tab, click Find Ratio.
5. Read the result panel: the missing dimension (shown to two decimals), the simplified aspect ratio, and a plain-language label when the ratio is a well-known one.
6. Click Clear to reset the tab, or click one of the buttons under Common ratios (16:9, 4:3, 1:1, 9:16, 21:9, 3:2, 2:3, 4:5) to explore a standard format.

All fields need positive numbers. If one is empty or zero, the tool asks you to fill in all of them.

Worked Examples: Scaling and Finding a Ratio

Here are three realistic cases that show what you type and what comes back.

Example 1: Scale by Width

You have a 1920 by 1080 video frame and need a 1280 pixel wide version for a blog post. Enter 1920, 1080, and a new width of 1280. The calculation is 1280 x 1080 / 1920 = 720. The tool shows a new height of 720.00px, a ratio of 16:9, and the label Widescreen HD. Your target size is 1280 by 720.

Example 2: Scale by Height

Your 1200 by 800 photo must fit a slot that is exactly 500 pixels tall. Enter 1200, 800, and a new height of 500. The width is 500 x 1200 / 800 = 750. The ratio is 3:2, labeled as the DSLR and 35mm format.

Example 3: Find Ratio

A design file is 1080 by 1350. Enter those numbers on the Find Ratio tab. The greatest common divisor is 270, so the result is 4:5, which matches the Instagram portrait format, with a decimal equivalent of 0.8.

When the answer is not a whole number, such as scaling 1200 by 800 to a width of 500 (333.33 high), round to the nearest whole pixel and accept a tiny difference. A one pixel rounding error is invisible in practice.

How Do You Read and Act on the Results?

Each result gives you a size you can use right away and a ratio that tells you whether your target format matches your source. Compare the ratio to the format you need before you export anything.

• New height or width: Use this as your export size, resize setting, or the width and height attributes on an image tag.
• Simplified ratio: If it matches your destination format, you can scale freely. If not, you must crop or add padding, because scaling alone will stretch the image.
• Decimal equivalent: A quick way to compare two sizes. If 1.78 and 1.33 are far apart, the frames will not fit without cropping.

This table lists the formats the tool labels or offers as presets, with common uses.

If the ratio you get has no label, such as 5:3 or 37:20, the tool still shows the simplified numbers. That usually means the source was cropped by hand or comes from an uncommon device.

Who Needs an Aspect Ratio Calculator?

Anyone who changes the size of visual content without wanting to distort it benefits from this tool. These are the most common situations.

• Web developers and SEOs: Setting width and height on images lets the browser reserve space before the file loads, which reduces layout shift. Google's guidance on Core Web Vitals names unsized images as a common cause of Cumulative Layout Shift. The same numbers feed the CSS property aspect-ratio: 16 / 9 for responsive embeds.
• Video editors and YouTubers: Work out a 720p or 1080p export from a larger master, or confirm a clip is 16:9 before uploading.
• Social media managers: Check that a graphic suits a 1:1, 4:5, or 9:16 placement before you design the full set.
• Photographers and print buyers: Learn whether a 3:2 camera file will fit a 4:5 frame, or how much will be cropped.
• Advertisers: Build one creative in several sizes, then compare their results with the ad metrics calculator.

If you are planning a batch of images and want a single common size, the average calculator can give you the mean width and height of the set before you pick a target.

Best Practices, Common Mistakes, and Troubleshooting

Most bad results come from a wrong input, not a wrong formula. Check these points when a number looks off.

Enter the original size, not a guess

The tool can only preserve the proportions you give it. Read the true pixel dimensions from your file's properties, not from how large it appears on screen.

Do not fill the wrong tab

If you know the new width but use the Scale by Height tab, you will get a different answer than you expect. Match the tab to the dimension you already know.

Use whole numbers for the ratio

The ratio is simplified using whole numbers, so decimal inputs such as 8.5 by 11 can give a rounded, misleading ratio. Multiply both values by 10 first (85 by 110) and you get the exact 17:22.

Cropping is not scaling

If your source is 3:2 and your target is 16:9, no scaling will match them. Scale to the width you need, then crop the extra height, or place the image on a background of the right shape.

Round carefully

Video encoders often require even numbers. If the result is 333.33, choose 334 rather than 333. Always round to a whole pixel in your final export.

Store the numbers with the image

Add the final width and height to your image tags so the page layout stays stable. For a wider look at related utilities, browse the online calculators and utilities category.

What This Calculator Does Not Cover

This tool works with width and height only. Many searches for aspect ratio mean something else, so here is how those differ.

• Tires: A tire's aspect ratio is the sidewall height as a percentage of the tread width. A 205/55R16 tire has a sidewall of 55% of 205 mm, or about 113 mm. This is a percentage, not a width to height ratio.
• Wings: Wing aspect ratio is the span squared divided by the wing area, which is a different formula.
• Diagonal sizes: The tool does not take a diagonal measurement. Enter the width and height of the screen, not the inch size marketed on the box.

It also does not resize, crop, or upload files. It gives you the numbers, and you apply them in your editor, CMS, or code.

Enter your original size above, pick the tab that matches what you know, and you will have a distortion-free size and a clear ratio in seconds. Try the aspect ratio calculator on this page with your next image or video.