Aspect Ratio Calculator

Calculate aspect ratios, resize dimensions proportionally, and explore common presets like 16:9 and 4:3. Free online tool.

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Lock Ratio Set both dimensions before locking. The tool keeps their current width ÷ height ratio, then rounds the other dimension to a whole number when you change one side.

Presets Each preset multiplies its ratio numbers by 120. The result is simplified: 21:9 sets 2520 × 1080 and displays 7:3. A preset clears Resize fields and updates an active ratio lock.

Width Enter positive whole numbers for Width and Height. The ratio and the width ÷ height decimal update as you type; the decimal has four places. Ctrl/⌘+L inside the tool clears dimensions, results, the ratio lock and the selected preset.

Height

— Aspect Ratio

— Decimal

Resize (maintains ratio) New Width and New Height use the current original dimensions. Enter either new dimension to calculate the other, rounded to a whole number. The original size stays unchanged, and Lock Ratio is not required.

New Width

New Height

Preview The preview shows the ratio as a box within 300 × 200 pixels, with each side at least 20 pixels. Very narrow or wide ratios are only approximate. Image files stay unchanged.

Read the full guide How to Find Aspect Ratio: Formula, Worked Examples and Resolution Table

Examples, details and FAQ Worked examples, how it compares with other tools, and answers to common questions.

Examples

The ratio is width and height divided by their greatest common divisor; the decimal is width ÷ height to four places. These are the tool’s results for common screen and image sizes:

Width × Height	Ratio	Decimal	Where you meet it
1920 × 1080	`16:9`	`1.7778`	Full HD video and monitors
2560 × 1080	`64:27`	`2.3704`	Monitors sold as “21:9”
3440 × 1440	`43:18`	`2.3889`	Monitors also sold as “21:9”
1366 × 768	`683:384`	`1.7786`	Older laptops, close to but not 16:9
1080 × 1350	`4:5`	`0.8000`	Portrait feed images
1170 × 2532	`195:422`	`0.4621`	iPhone 13 and 14 screen in pixels

”21:9” is a marketing name: neither ultrawide resolution reduces to 21:9, and the two differ from each other. When a ratio comes out with large numbers, as 683:384 does, compare the decimal with a preset instead (1.7786 against 1.7778 for 16:9).

Resizing. With 1920 × 1080 as the original, a new width of 1280 gives 720, and a new height of 2160 gives 3840. A new width of 1000 gives 563: the exact value 562.5 is rounded to the nearest whole pixel, so the result is off the original ratio by half a pixel. Pick widths that divide evenly (multiples of 16 for 16:9) when the ratio must be exact.

How Lock Ratio and Presets Work

Lock Ratio stores width ÷ height at the moment you turn it on. Every later change to one side recalculates the other from that stored ratio and rounds it to a whole number, so typing 1000 and then 1920 as the width returns to 1080.
A preset sets the size to 120 × the ratio numbers: 16:9 becomes 1920 × 1080, 4:3 becomes 480 × 360 and 21:9 becomes 2520 × 1080. The ratio is shown in lowest terms, so the 21:9 preset displays 7:3.
Decimal sizes are accepted. The ratio is then computed from the rounded values, so 1.5 × 1 shows 1.5:1.

In CSS, the aspect-ratio property takes the same pair as a fraction, such as aspect-ratio: 16 / 9 (MDN). The calculator only works out numbers; it does not crop or resize image files.

Common Aspect Ratios

Ratio	Use Case
16:9	Widescreen monitors, YouTube, HDTV
4:3	Classic TV, presentations
21:9	Ultrawide monitors, cinema
1:1	Instagram square posts
9:16	Mobile stories, TikTok, Reels
3:2	DSLR photography, Microsoft Surface laptops

Aspect Ratio in Web Design

Understanding aspect ratios is crucial for responsive design, video embeds, image cropping, and CSS aspect-ratio property. This calculator helps designers and developers quickly find the right dimensions for any target ratio.

Limits

Width and Height must be positive. An empty field, 0 or a negative number shows — for the ratio and the decimal, without an error message.
The ratio is simplified with whole numbers. With decimal sizes the result keeps the decimals: 33.867 × 19.05 shows 33.867:19.05, not 16:9, although the decimal is 1.7778.
Resize and Lock Ratio round the calculated side to the nearest whole pixel, so a result can be up to half a pixel off the exact ratio.
The preview is a shape within 300 × 200 pixels with each side at least 20 pixels; very narrow or very wide ratios are approximate.
The calculator works only with numbers. It does not open, crop or resize image or video files.

FAQ

What is an aspect ratio?

An aspect ratio is the proportional relationship between width and height. For example, 16:9 means for every 16 units of width, there are 9 units of height.

How does the resize feature work?

Enter the original Width and Height, then type a New Width or New Height under Resize. The other value is calculated from the original ratio and rounded to a whole pixel; the original size does not change. Lock Ratio does the same on the original fields.

What are common aspect ratios?

16:9 (widescreen monitors, YouTube), 4:3 (classic TV, presentations), 21:9 (ultrawide monitors), 1:1 (Instagram square), 9:16 (mobile stories), 3:2 (DSLR photos).

Why does 1366 × 768 show 683:384 instead of 16:9?

The ratio is width and height divided by their greatest common divisor, and for 1366 and 768 that divisor is 2, so 683:384 is already in lowest terms. The size is close to 16:9 but not equal: compare the decimals, 1.7786 against 1.7778.

Is my data sent anywhere?

The sizes you type are not sent: the calculation runs in your browser tab, and the tool does not save them in the browser's storage. The page analytics records only the tool name and the action (calculate) when you finish editing Width or Height or choose a preset, without the numbers.