Aspect ratio calculator
Resize while keeping proportions and find the simplified ratio (like 16:9) — in your browser.
Runs 100% in your browserAspect ratio: —
How to calculate aspect ratio
- Enter the original size. Type the original width and height to lock the ratio.
- Change one dimension. Enter a new width or height; the other is calculated.
- Use the result. Read the matching dimension and the simplified ratio.
Ratio, resizing and why it's reduced
An aspect ratio is the proportional relationship between width and height — what stays fixed while the actual
pixel dimensions change. Resize while holding it constant and the image or video keeps its shape; break it and
you get the tell-tale stretched faces or squashed logos. Solving for the missing side is one cross-multiplication:
to fit a 1920×1080 source into a 1280-wide slot, 1280 × (1080 ÷ 1920) = 720. The tool also shows the
ratio in lowest terms by dividing both numbers by their greatest common divisor, which is how
1920:1080 collapses to the familiar 16:9 — handy for confirming at a glance that two
differently-sized assets share the same proportions.
The ratios you'll actually meet
A handful of standards cover most work. 16:9 is the default for video, monitors and YouTube; 9:16 is its vertical twin for phone-first formats like Reels, Shorts and TikTok. 4:3 survives in older displays, many cameras and slide decks; 1:1 (square) is the safe feed format; 21:9 is ultrawide and cinematic. Print and photo sizes follow their own — 3:2 for classic 35mm-style frames, 2:3, 5:4. Matching the target's ratio before you export saves you from the platform's automatic crop deciding which third of your composition to throw away.
Letterbox, crop, or the CSS property
When a source ratio doesn't match its destination you have two honest choices: letterbox (fit
the whole frame and accept bars on two sides) or crop (fill the frame and lose some edges) —
stretching is the one option that's always wrong. On the web you rarely need to compute a height at all: the CSS
aspect-ratio: 16 / 9 property reserves the right space from the width alone, which also prevents the
layout from jumping as images load. Use this calculator to pin down the exact pixel pairs for exports and
thumbnails, and reach for the CSS property for fluid, responsive layouts.
Frequently asked questions
- Enter your original width and height to set the ratio, then change either the new width or new height — the tool fills in the other dimension so the proportions stay identical.
- It reduces your width:height to lowest terms using the greatest common divisor — for example 1920×1080 simplifies to 16:9.
- 16:9 (HD video and most screens), 4:3 (legacy displays), 1:1 (square / social posts), 21:9 (ultrawide), 3:2 (photography) and 9:16 (vertical / stories).
- Yes — dimensions can be fractional, and results are rounded to two decimals where needed.