This aspect ratio calculator does the two jobs every designer, editor and developer needs weekly: reduce a resolution to its simplest ratio (1920 × 1080 → 16:9), and find the matching dimension when you resize one side.
If both “new” fields are filled, the new height wins for the resize computation. Non-integer results round to the nearest pixel — for broadcast and codec work, prefer even numbers (chroma subsampling demands them).
Ratios Are Fractions Wearing a Colon
Simplifying a ratio is dividing both sides by their greatest common divisor — 1920 and 1080 share 120, leaving 16:9 (the same GCD machinery as our GCF Calculator, pointed at screens). The decimal form (1.7778:1) is what CSS aspect-ratio, video metadata and projector specs trade in. Physical screens measured from their diagonal live at our TV Size Calculator. Part of our Other Calculators collection.
Worked Examples (Verified)
- 1920 × 1080 → ÷120 → 16:9 · resize to width 1280 → height = 1080 × (1280 ÷ 1920) = 720 — the 720p you already knew, derived
- 4000 × 3000 photo → 4:3 · printed at 10 inches wide → 7.5 inches tall
- 1080 × 1350 social portrait → 4:5 — the feed-optimal portrait ratio, and 2560 × 1080 → 21:9 ultrawide (gcd 240: 64:27 exactly, conventionally labeled 21:9)
- Odd but real: 1366 × 768 laptop panels are 683:384 — “16:9-ish” marketing over honest arithmetic; this calculator never rounds a ratio into a prettier lie
The Ratios You’ll Meet
| Ratio | Decimal | Where it lives |
|---|---|---|
| 16:9 | 1.778 | HD/4K video, TVs, most monitors |
| 21:9 | 2.333 | Ultrawide monitors, cinema scope (2.39:1 is the film-true figure) |
| 4:3 | 1.333 | Classic TV, iPad, many camera sensors |
| 3:2 | 1.500 | 35mm film, DSLRs, Surface devices |
| 1:1 | 1.000 | Square social posts, medium format |
| 4:5 / 9:16 | 0.800 / 0.563 | Portrait feeds / vertical video (Reels, Shorts, TikTok) |
| 1.85:1, 2.39:1 | — | Theatrical flat and scope standards |
Related Tools
- TV Size Calculator — diagonals to inches and feet
- PPI Calculator — density for the resized result
- Other Calculators — the full collection
Frequently Asked Questions
How do you calculate an aspect ratio?
Divide width and height by their greatest common divisor: 1920:1080 ÷ 120 = 16:9. For the decimal form, divide width by height (1920 ÷ 1080 = 1.778). Any units work as long as both sides share them.
How do I resize without distorting?
Keep width ÷ height constant: new height = old height × (new width ÷ old width). 1920×1080 shrunk to 1280 wide → 720 tall. This calculator does it either direction — give it the new width or the new height.
What aspect ratio is 4K?
3840 × 2160 simplifies to 16:9 — “4K” names the approximate horizontal pixel count, not a new shape. (Cinema DCI 4K is 4096 × 2160, a slightly wider 1.896:1.)
Why is my “21:9” monitor actually 64:27?
Marketing rounding: 2560 × 1080 reduces exactly to 64:27 (2.370), close to but not exactly 21:9 (2.333). True 21:9 resolutions exist (3440 × 1440 is 43:18, also rounded). The exact fraction matters for pixel-perfect scaling; the label is the nickname.
What ratio should vertical video be?
9:16 (1080 × 1920) for full-screen vertical platforms. Safe areas for text sit in the central ~4:5-to-1:1 band depending on platform UI overlays — shoot 9:16, compose defensively.
Is this aspect ratio calculator free?
Yes — free, instant, no sign-up.