Aspect Ratio Calculator
Aspect ratio is width divided by height, held constant while scaling. A 1920×1080 image is 16:9, so at 1280 wide the height must be 720. The formula is new height = new width × original height ÷ original width — the same arithmetic that stops an image looking stretched.
Solve for a missing width or height at a target ratio, or check whether two sizes share the same shape. Everything runs instantly in your browser — nothing is sent anywhere.
Paper sizes (US)
US customary sizes. Most of the world uses the metric ISO 216 series (A4, A3, A5) instead — see the Spanish, Italian, or Greek version of this page for those.
The same shape, at any size
An aspect ratio is a proportion, so resizing while keeping it
fixed is just cross-multiplication: otherDim = knownDim × (ratioOther / ratioKnown).
Comparing two sizes is the reverse operation — reduce each pair
of numbers to its simplest form by dividing both by their greatest common divisor (GCD), then check if
the reduced ratios match.
Worked example
Ratio mode: at 16:9 with a known width of 1920px, the height is 1920 × (9 / 16) = 1080. Compare mode: is 1920×1080 the same shape as 1280×720? GCD(1920, 1080) = 120, so 1920×1080
reduces to 16:9; GCD(1280, 720) = 80, so 1280×720 also reduces
to 16:9 — same ratio, confirmed by the math rather than eyeballing
it.
Frequently asked questions
What does a ratio like 16:9 actually mean?
It's a proportion, not a size — 16:9 means the width is 16 units for every 9 units of height, at any scale. A 1920×1080 screen, a 1280×720 screen, and a 320×180 thumbnail are all 16:9, because 1920:1080, 1280:720, and 320:180 all reduce to the same 16:9 once you divide out their greatest common divisor. The ratio is unitless — it works the same for pixels, inches, or millimeters.
How is the missing dimension calculated?
otherDimension = knownDimension × (ratioOfTheOtherSide / ratioOfTheKnownSide). If you know the width is 1920px and you want 16:9, the height is 1920 × (9/16) = 1080. Flip it the other way for a known height: a 1080px-tall image at 16:9 needs a width of 1080 × (16/9) = 1920.
Why do 1920×1080 and 1280×720 count as the same aspect ratio?
Because both reduce to 16:9 once you divide by their greatest common divisor (GCD). GCD(1920, 1080) = 120, and 1920÷120 = 16, 1080÷120 = 9. Separately, GCD(1280, 720) = 80, and 1280÷80 = 16, 720÷80 = 9 — same 16:9. Different pixel counts, identical shape.
Can I use this for paper sizes too?
Yes — a paper size is just another ratio, so the Letter, Legal, and Tabloid presets above drop straight into the ratio fields the same way 16:9 does. One curiosity if you ever work with the international A-series instead (A4, A3, A5 — see the Spanish, Italian, or Greek version of this page): those sizes are built from a width:height ratio of 1:√2 ≈ 1:1.41421, which is what lets an A-size sheet fold exactly in half into the next size down with the same proportions.