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Aspect Ratio Calculator
Resize an image or video keeping its aspect ratio — enter one new dimension and get the other with no distortion — or reduce any width×height to its simplest ratio (1920×1080 → 16:9) via the greatest common divisor. Includes common-ratio presets (16:9, 4:3, 1:1, 9:16, 4:5, 21:9), the decimal ratio, and a fit-vs-crop helper. Computed entirely in your browser. Accurate, instant and free — for United States.
100% in-browser — nothing is uploaded
Resized to 16:9
1280 × 720
Edit either dimension — the other updates to keep 16:9.
gcd reduction, and fit vs crop
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
Ratios are reduced with the greatest common divisor and resizes solve new height = new width × (ratio height ÷ ratio width). Pixel dimensions are whole numbers, so a computed dimension is rounded to the nearest pixel; the exact (possibly fractional) value is noted when rounding occurs. Fit and crop describe how a source maps into a differently-shaped box.
Wikipedia · Aspect ratio
Working with other measurements? Convert between metric and imperial with the Unit Converter, or turn numbers into Roman numerals with the Roman Numeral Converter.
Width to height, reduced to lowest terms
An aspect ratiois the proportion of a rectangle's width to its height, written W:H. To name it, you reduce the two dimensions to their simplest whole-number form by dividing both by their greatest common divisor (gcd). So 1920×1080is not “1920:1080” — its gcd is 120, giving 16:9. The same ratio describes 1280×720 and 3840×2160, because they all reduce to 16:9.
Reduce (gcd)
W×H → simplest ratio
- 1920×1080 → 16:9 (gcd 120)
- 3000×2000 → 3:2 (gcd 1000)
- 1080×1920 → 9:16 (portrait)
Resize (keep ratio)
Solve the missing side
- new H = new W × (H ÷ W)
- new W = new H × (W ÷ H)
- 1280 wide @16:9 → 720 tall
Fit vs crop
Same ratio, different fill
- Fit = largest that fits (letterbox)
- Crop = smallest that fills (trim)
- Both keep the ratio
Scale without distortion
To resize an image or video without stretching it, you keep the aspect ratio fixed and let one dimension follow the other. Pick a ratio (type it or choose a preset), enter the new width and the calculator returns the matching height as new W × (ratioH ÷ ratioW) — or enter the new height to get the width. Because both sides scale by the same factor, the shape is preserved and nothing is distorted.
Whole pixels vs fractions
Common ratios, and fit vs crop
Common presets
One click sets the ratio
16:9 video & widescreen, 4:3 classic displays, 1:1 square posts, 3:2 photography, 21:9 ultrawide/cinema, 9:16 Reels/TikTok/Stories, 4:5 Instagram portrait.
Fit vs crop
Into a target box
Fit (contain) scales down until the whole image is visible — the largest size that fits, possibly with bars. Crop (cover) scales up until the box is filled — the smallest size that covers it, trimming the overflow.
100% in your browser
Frequently asked questions
16:9. The aspect ratio is the width and height reduced to their simplest whole-number form by dividing both by their greatest common divisor (gcd). For 1920×1080 the gcd is 120, so 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9 — giving 16:9, the standard widescreen/Full-HD ratio. The same ratio describes 1280×720, 2560×1440 and 3840×2160, because they all reduce to 16:9.
Lock the ratio and solve for the missing dimension: new height = new width × (original height ÷ original width), or new width = new height × (original width ÷ original height). Enter one new dimension in the calculator and it returns the other, so the image scales without stretching. For example a 1920×1080 image resized to 1280 wide becomes 1280×720 — both are 16:9, so it scales down cleanly with no distortion.
Any width×height that reduces to 16:9 — the width is 16 units for every 9 units of height. Common 16:9 sizes are 1280×720 (HD), 1920×1080 (Full HD), 2560×1440 (QHD) and 3840×2160 (4K UHD). To find the 16:9 height for any width, multiply the width by 9 ÷ 16 (0.5625); to find the width for a height, multiply by 16 ÷ 9 (≈1.7778). The calculator does this for you from the 16:9 preset.
Both scale an image into a target box while keeping its aspect ratio, but they resolve the leftover space differently. Fit (letterbox / "contain") scales the image down until the whole thing is visible inside the box — the largest size that fits — which can leave empty bars on two sides. Crop (fill / "cover") scales up until the box is completely filled — the smallest size that covers it — so the overflow on the long side is trimmed off. Fit shows everything; crop fills everything.
Because a ratio is only meaningful in its simplest form. 1920:1080 and 16:9 describe the exact same shape, but 16:9 is the canonical, comparable name — you can instantly see it matches any other 16:9 screen or video. Dividing both numbers by their greatest common divisor (gcd) is what produces that lowest-terms form: it is the largest number that divides both dimensions evenly, so the result has no common factor left. That is why 3000×2000 reduces to 3:2 (gcd 1000) rather than 3000:2000.
No. This is a pure arithmetic tool — it works only with the width and height numbers you type, computing ratios and resized dimensions entirely in your browser with JavaScript. No image file is ever uploaded, read or stored; nothing is sent to a server, and it works offline once the page has loaded. You are only ever sharing dimensions, never the picture itself.
Method & references
Method: a width×height is reduced to its simplest ratio by dividing both by their greatest common divisor (Euclidean algorithm), so 1920×1080 becomes 16:9. A resize keeps the ratio by solving new H = new W × (ratioH ÷ ratioW) (or the mirror for width). Fit scales to the largest size inside a box; crop scales to the smallest size that fills it. Displayed pixels are rounded to whole numbers. Everything runs in your browser — no image is uploaded.
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed 5 July 2026. Figures follow the methods and sources set out in our editorial standards.
This tool computes aspect ratios and resized dimensions from the width and height you enter. Displayed pixel dimensions are rounded to whole numbers, so a proportional resize may differ by up to one pixel from the exact value. It works only with numbers — no image file is ever uploaded.
Keep going
Same-category tools follow this colour; a cross-category link keeps its own.