Aspect Ratio Calculator for Width and Height
Calculate a missing width or height while keeping an aspect ratio. Compare common ratios or reduce a pair of dimensions to a ratio.
Aspect Ratio Calculator workspace
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
Calculate a missing width or height while keeping an aspect ratio. Compare common ratios or reduce a pair of dimensions to a ratio. An aspect ratio describes shape, not resolution or file quality. Two images can share a ratio while containing very different amounts of detail. Check the destination's current size requirements and whether it crops, fits, or stretches the supplied asset.
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.
Related design & calculator tools
Square 1:1 icons for every browser size
Meta Tag PreviewerCheck your 1.91:1 og:image renders right
Open Graph GeneratorBuild the 1200×630 OG tags
CSS Filter GeneratorVisual treatment for assets at any ratio
Image CompressorShrink the assets you size here
Color PickerSample brand colors from references
Percentage CalculatorWork out scale and resize percentages
Unit ConverterConvert px, cm, and inches for print
All ToolsBrowse the full Toolk toolbox
Last updated: September 15, 2026 · Runs 100% in your browser — no uploads, tool input is not sent to Toolk.
Frequently asked questions
How does the three-way lock work in this aspect ratio calculator?
You fix exactly one of the three values — ratio, width, or height — by clicking its lock icon. The tool then solves the other two live: height = width × (ratio_h ÷ ratio_w), width = height × (ratio_w ÷ ratio_h), or a fresh ratio from any dimension pair you type.
Why doesn't a decimal ratio like 1.91:1 simplify further?
The GCD reduction rounds to whole numbers first, so it only simplifies integer pairs. Decimal presets are stored at their smallest integer size — 1.91:1 is held as 191:100 and 2.35:1 as 47:20 — and those pairs share no common factor, so they are already as low as they go.
Do my dimensions get uploaded anywhere?
No. Every calculation runs locally in your browser, and Toolk's page analytics do not receive the dimensions or ratios you enter. Once the page has loaded, the calculator keeps working offline.
What should I do when the computed height lands on a fraction?
A locked ratio often yields subpixel values — 1500 px at 16:9 is 843.75 px. Read the two-decimal figure, then round up to 844 for image assets: rounding down leaves a one-pixel row uncovered.
Which Toolk tool pairs well after sizing an Open Graph image?
Take the 1200×630 canvas you sized here to the Meta Tag Previewer at /tools/meta-tag-previewer to confirm the finished og:image renders correctly inside a social share card before shipping.