Aspect Ratio Calculator

What is an Aspect Ratio Calculator?

An aspect ratio calculator keeps an image, video or screen in proportion while you resize it. You enter the original width and height, then either a new width or a new height, and the tool solves the missing dimension so the shape stays identical. It also shows the simplified ratio of the original, like 1920:1080 reducing to 16:9. The result is a clean, distortion-free resize every time.

What does this tool do?

It answers two related questions at once. First, “if I change one dimension, what is the other?” — type a new width and it returns the matching height, or type a new height and it returns the matching width. Second, “what ratio is this?” — it reduces your original width and height to their simplest whole-number form, such as 16:9, 4:3 or 1:1. That makes it ideal for resizing photos, exporting video, or labelling a screen resolution without stretching anything.

How does it work?

The tool uses a single proportion: the new dimensions must form the same ratio as the original. Because width divided by height stays constant, you can solve whichever side is missing with one multiplication.

If you provide a new width, the formula is:

New height = New width × (Original height ÷ Original width)

If you provide a new height, it flips:

New width = New height × (Original width ÷ Original height)

Here every symbol is a length in pixels (or any consistent unit). Original width and original height describe the shape you want to keep; the new value is the one dimension you already know; and the tool returns the other.

For the simplified ratio, the tool divides the original width and height by their greatest common divisor (GCD) — the largest whole number that divides both evenly. For 1920 and 1080, the GCD is 120, so both sides divide down to 16:9. This reduction is shown for whole-number inputs, giving you the familiar shorthand designers and TVs use.

Examples

Each example follows the exact logic above: one multiplication to solve the missing side, and a GCD reduction for the ratio.

Example 1 — shrink 1920×1080 by setting a new width.

• Original is 1920 × 1080, which simplifies to 16:9 (GCD 120).
• You set the new width to 1280.
• New height = 1280 × 1080 ÷ 1920 = 720.
• Result: 1280 × 720, still 16:9.

Example 2 — the same resize from the other direction.

• Original is again 1920 × 1080.
• This time you set the new height to 720.
• New width = 720 × 1920 ÷ 1080 = 1280.
• Result: 1280 × 720 — the calculation is reversible, so either input lands on the same pair.

Example 3 — a square image (1:1).

• Original is 1080 × 1080, which simplifies to 1:1 (GCD 1080).
• You set the new width to 400.
• New height = 400 × 1080 ÷ 1080 = 400.
• Result: 400 × 400, a perfect square, because in a 1:1 ratio the two dimensions always match.

Example 4 — fitting 1920×1080 into a narrower width.

• Original is 1920 × 1080 (16:9).
• You set the new width to 800.
• New height = 800 × 1080 ÷ 1920 = 450.
• Result: 800 × 450, the correct 16:9 height for an 800-pixel-wide banner.

Common resolutions and their ratios

Every ratio below is produced by the same GCD reduction the tool uses, so you can treat this as a quick lookup. Each row divides the width and height by their greatest common divisor.

Resolution (W×H)	GCD	Simplified ratio	Common name
1920 × 1080	120	16:9	Full HD / 1080p
1280 × 720	80	16:9	HD / 720p
3840 × 2160	240	16:9	4K UHD
2560 × 1440	160	16:9	QHD / 1440p
1024 × 768	256	4:3	Classic display
800 × 600	200	4:3	SVGA
1280 × 1024	256	5:4	SXGA
1080 × 1080	1080	1:1	Square (social)
1080 × 1920	120	9:16	Vertical / stories
2560 × 1080	40	64:27	Ultrawide (sold as 21:9)

Notice that 1080p, 720p, 4K and 1440p all reduce to 16:9 despite very different pixel counts — that is exactly why a 720p video fills a 1080p screen with no black bars: same ratio, different size.

Common uses

Keeping proportions correct matters across many roles. Typical situations include:

• Front-end developers sizing a responsive img or thumbnail to an exact pixel width while computing the height that avoids layout shift.
• Video editors exporting a clip to a smaller delivery size — dropping 1920×1080 to 1280×720 — without squashing faces.
• UI and brand designers resizing a hero image to fit a column and reading off the matching height instantly.
• Photographers scaling a full-resolution export down for the web while staying true to the original frame.
• Students and beginners learning how a resolution like 1920×1080 maps to the shorthand 16:9 using the greatest common divisor.

Tips and common mistakes

A few details keep your resizes accurate:

• Resizing is not the same as cropping. This tool scales the whole frame to a new size while preserving the ratio; it never trims edges. To change the ratio itself (say 16:9 to 1:1) you must crop, which removes pixels.
• Enter only one new dimension. The point of the tool is to solve the missing side. If you type both a new width and a new height that do not match the original ratio, you would be describing a stretched image, not a proportional one.
• Keep units consistent. Compare pixels to pixels. The math works for any unit, but mixing pixels with centimetres in the same calculation gives a meaningless result.
• Rounding can cost a pixel. Some target widths produce a fractional height (for example an odd width on a 16:9 frame). Rounding to the nearest whole pixel is normal and shifts the ratio by a negligible amount.
• Not every resolution reduces to a tidy ratio. 1366×768 is marketed as “16:9” but actually reduces to 683:384, because 1366 and 768 share only a GCD of 2. The tool reports the true simplified ratio, not the marketing label.

Limitations and notes

This calculator handles two-dimensional ratios — width against height — and solves for one missing dimension at a time from a single known value. The simplified-ratio output uses whole-number GCD reduction, so it is most meaningful when your original width and height are whole pixels; decimal inputs still resize correctly but may not collapse to a neat shorthand like 16:9. It performs scaling, not cropping, so it cannot convert one ratio into a different one without you choosing which pixels to keep. There is no rounding policy baked into the ratio itself: if a solved dimension lands on a fraction, round it to the nearest whole pixel for real exports. Everything runs locally — the resize and the GCD reduction are plain JavaScript executing in your browser, so the dimensions you enter are never uploaded, logged or stored, and the tool keeps working offline once the page has loaded.

For more developer utilities, pair this with the hex to RGB converter, the base64 encode and decode tool and the UUID generator on the dev tools page.