Aspect Ratio Calculator: Find Width, Height, or Ratio
Lock the ratio, width, or height and the other two compute in real time, with a to-scale preview. Eleven presets for YouTube 16:9, TikTok 9:16, Instagram 4:5, Open Graph 1.91:1, and cinema. Auto-reduces to lowest terms. Free and 100% in-browser.
Click any lock icon to fix that field. Edits to the other two will recompute around it.
Simplified ratio
16:9
Decimal ratio
1.7778
width ÷ height
Dimensions
1920 × 1080
Orientation
Landscape
1.78× wider
Common ratios
Three-Way Calculation
Lock any one of {ratio, width, height} and we compute the other two. Edit any field to switch the lock — no separate modes or tabs.
Eleven Built-In Presets
YouTube 16:9, TikTok 9:16, Instagram 1:1 and 4:5, Open Graph 1.91:1, Cinemascope 2.35:1, photography 3:2, ultrawide 21:9, and more.
Lowest-Terms Reduction
GCD-based reduction simplifies any width/height pair to lowest integer terms — 1920×1080 becomes 16:9, 4032×3024 becomes 4:3.
100% Client-Side
No data leaves your browser — no signup, no upload. The math runs instantly via JavaScript and works offline once loaded.
Aspect Ratio Calculator: Lock One Value, Compute the Rest
An aspect ratio calculator works out the missing dimension when you know any two of ratio, width, and height. Lock the value you want fixed, type the others, and the tool solves the rest in real time — height = width × (ratio_h ÷ ratio_w). It reduces any width×height pair to lowest terms with the GCD (1920×1080 → 16:9), shows the decimal ratio, and draws a to-scale preview. Free and 100% in your browser.
How to use the aspect ratio calculator
- Decide what stays fixed and click its lock icon — usually the Ratio field (16:9 by default).
- Type a known value into either unlocked field, e.g. a Width of 1500 px.
- Read the computed value instantly in the other field, plus the Simplified ratio, Decimal ratio, and Orientation stats.
- Need a standard size? Click a preset chip — 16:9, 9:16, 4:5, 1.91:1, or A4 — to set the ratio and rescale.
- Press the copy icon on Simplified ratio or Dimensions to grab the value as
16:9or1920x1080.
What is an aspect ratio and how does reduction work?
Aspect ratio is the proportion of width to height, written W:H — 16:9 means 16 units wide for every 9 tall. It describes shape independent of size, so 1920×1080 and 1280×720 are both 16:9. To put any pair in lowest terms, this tool runs the Euclidean algorithm for the greatest common divisor, then divides both sides by it. The widescreen 16:9 standard itself was no accident: SMPTE engineer Kerns H. Powers proposed it in 1984 as the geometric-mean compromise between the 1.33:1 TV frame and the 2.39:1 cinema frame, so any common format crops or letterboxes symmetrically.
"The height-to-width ratio of all ISO 216 pages is the square root of two (1:1.4142). When a sheet is cut in half parallel to its shorter side, the two halves keep the same ratio."— ISO 216, the international paper-size standard, as summarized on Wikipedia: ISO 216
That √2 property is why A4 enlarges to A3 with an exact 141% (√2) scale and no re-cropping — the idea dates to a 1786 letter by Georg Christoph Lichtenberg. The calculator stores A4 as the 210:297 preset, which reduces to 70:99 (√2 has no exact integer ratio, so it never simplifies to small numbers).
Worked examples: input → output
Lock 16:9 · width 1500 → height
1500 × (9 ÷ 16) = 843.75 px → round up to 844
Lock 9:16 · height 1920 → width
1920 × (9 ÷ 16) = 1080 px (a clean 1080×1920 Stories asset)
Reduce 3024×4032 (iPhone photo)
GCD(3024, 4032) = 1008 → 3:4 portrait (the same shape as 4:3 turned vertical)
Edge case · subpixel height
A locked ratio often yields a fractional dimension — 1500 px at 16:9 gives 843.75 px, not a whole number. Image software needs integers, so round up (ceil) to 844; rounding down leaves a one-pixel row uncovered. The tool keeps two decimals so you can see the exact value before you round.
Aspect ratios by platform (2026)
Required ratios and recommended pixel dimensions for the platforms people target most. Each platform updates its specs periodically — check the current developer docs before mass-producing assets.
| Platform | Ratio | Recommended dimensions |
|---|---|---|
| YouTube video | 16:9 | 1920×1080 (HD), 3840×2160 (4K) |
| YouTube thumbnail | 16:9 | 1280×720 minimum |
| YouTube Shorts | 9:16 | 1080×1920 |
| Instagram feed (square) | 1:1 | 1080×1080 |
| Instagram feed (portrait) | 4:5 | 1080×1350 |
| Instagram Stories / Reels | 9:16 | 1080×1920 |
| TikTok video | 9:16 | 1080×1920 |
| X / Twitter post image | 16:9 | 1200×675 |
| LinkedIn cover | 4:1 | 1584×396 (personal); 1128×191 (company) |
| Facebook cover | 2.7:1 | 820×312 (desktop) |
| Open Graph image | 1.91:1 | 1200×630 |
| Pinterest pin | 2:3 | 1000×1500 |
| Cinema (Cinemascope) | 2.39:1 | Theatrical wide release |
Why decimal presets like 1.91:1 do not reduce to small numbers
The GCD reducer rounds to integers first, so it can only simplify whole-number pairs. Decimal ratios are therefore stored as the smallest integer pair that hits the value: 1.91:1 is held as 191:100 and 2.35:1 as 47:20. Those share no common factor, so the "Simplified ratio" stays 191:100 rather than collapsing. That is correct — 191:100 is genuinely the lowest integer form of 1.91. If you want a tidy whole-number ratio, start from pixel dimensions (1200×630) instead of a decimal.
One more behavior worth knowing: the visual preview floors every side at 40 px and caps the longest side at 320 px. So a punishing ratio like 100:1 still renders as a readable bar rather than a one-pixel sliver — the preview is a proportion sanity-check, not a pixel-accurate mockup.
Common aspect ratio mistakes
Swapping W and H
9:16 is portrait, 16:9 is landscape. Reverse them and a TikTok clip plays sideways on YouTube, or vice versa.
Ignoring safe zones
A 9:16 Stories video that puts text or faces in the top or bottom 15% gets covered by the platform UI. Keep critical content centered.
Rounding subpixels down
843.75 px must round to 844, not 843. Rounding down leaves a one-pixel row the asset never covers.
Letterbox vs crop
Letterboxing adds bars to keep content; cropping cuts content to fit. Produce assets at the platform-native ratio to avoid both.
Runs 100% in your browser
Your dimensions never leave your device. Every calculation runs locally in JavaScript and copying uses your browser's native clipboard — no uploads, nothing leaves your device. I tested all eleven presets, locked each of the three fields in turn, and pushed extreme inputs (4K 3840×2160, a 1:1 square, the 100:1 stress case, and decimal ratios like 1.91:1). Output stays instant and the preview never collapses, because each side is floored at 40 px.
Frequently asked questions
Is this aspect ratio calculator free?
Yes — 100% free with no signup and no usage cap. Every preset, the GCD reduction, decimal output, and visual preview are unlocked for everyone.
Does my data get uploaded anywhere?
No. The math runs in your browser with JavaScript and nothing is sent to a server, so the tool works offline once the page loads.
How do I find the height for a width at a set ratio?
Lock the Ratio field, type the width, and read the height. The formula is height = width × (ratio_h ÷ ratio_w), so 1500 px at 16:9 gives 843.75 px — round up to 844.
Why does 1.91:1 not simplify further?
Because 191:100 is already the lowest integer form of 1.91 — the two numbers share no common factor. Decimal ratios rarely reduce to small whole numbers; start from pixels (1200×630) if you want a tidy ratio.
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Last updated: June 2, 2026 · Runs 100% in your browser — no uploads, nothing leaves your device.
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