Aspect Ratio / Crop Calculator
Enter pixel or print dimensions to simplify a ratio, scale one side to match a target ratio, or size a crop (and letterbox frame) so an image or video fits a destination format.
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Result
The formula
Aspect ratio is width ÷ height, usually written in lowest terms as W:H (for example 1920×1080 → 16:9). To find a missing side, multiply the known side by the matching ratio part. To crop inside a frame, keep the full shorter relative dimension and trim the longer one so the result matches the target ratio; to pad (letterbox), expand the shorter side instead.
Worked example
- Given 1920 × 1080, divide both by gcd(1920, 1080) = 120.
- Simplified ratio = 16:9 (decimal 1.777…).
- Cropping a 4000 × 3000 still to 16:9 → crop box 4000 × 2250.
Result: 16:9 · crop 4000 × 2250
How aspect ratio and crop boxes are calculated
The tool treats width and height as positive lengths in the same unit (pixels, millimetres, or inches). It simplifies ratios with the Euclidean gcd, scales missing sides with cross-multiplication, and builds crop or pad rectangles by comparing source and target aspect values.
Simplifying W:H
Integer dimensions are reduced by their greatest common divisor. Non-integers are rounded to whole units before gcd so pixel and print sizes still land on familiar pairs such as 16:9 or 4:3 when they are exact multiples.
The decimal form W/H is shown as well, which helps compare near-matches (for example 1.78 vs 1.777… for “almost 16:9”).
Finding a missing side
Given a target ratio Rw:Rh and a known width, height = width × Rh / Rw. With a known height, width = height × Rw / Rh. Keep the same unit on every length.
Crop inside vs pad (letterbox)
Crop (fit inside) returns the largest rectangle of the target ratio that still fits inside the source. If the source is relatively wider than the target, height stays and width is reduced; if it is relatively taller, width stays and height is reduced.
Pad (fit outside) returns the smallest rectangle of the target ratio that can contain the source — the classic letterbox or pillarbox frame when you add bars instead of trimming pixels.
What we do not model
No lens distortion, safe-area overlays, DPI conversion, or encoder crop macros. Orientation labels (landscape / portrait / square) follow W vs H only; mirrored or rotated metadata is ignored.
Interesting facts
16:9 became the HDTV default
ITU-R BT.709 and consumer HDTV settled on 16:9 as the common landscape format for 720p and 1080p, which is why 1920×1080 and 1280×720 both reduce to 16:9.
Cinema often uses wider than 16:9
Digital cinema and many streaming titles use ~2.39:1 (often called “Scope”), while 2.35:1 remains a common marketing label — both are wider than 16:9 and need side crop or pillarboxing when shown on a TV.
A4 paper is √2, not a video ratio
ISO 216 A-series sheets keep a 1:√2 ≈ 1:1.414 aspect so folding in half preserves the ratio. That is unrelated to 4:3 or 16:9 video frames.
Square 1:1 is still a social staple
Instagram’s early feed popularized 1:1 crops; many platforms still offer square presets alongside 4:5 portrait and 9:16 Stories/Reels vertical frames.
Frequently asked questions
Divide both dimensions by their greatest common divisor and write the result as W:H. Example: 1920÷120 and 1080÷120 → 16:9. This calculator does that automatically and also shows the decimal W/H.
Use height = width × 9 / 16. For a 1920-wide frame, height = 1920 × 9 / 16 = 1080. Choose “Find missing side”, enter 16 and 9, and set the known side to width.
Pick “Crop / pad to ratio”, enter the source width and height, set the ratio to 16:9, and choose crop. You get the largest 16:9 rectangle that fits inside the source — typically a centered crop in editors.
Crop trims the source to the target ratio (you lose edges). Letterbox/pad keeps the whole source and adds empty bars so the outer frame matches the target ratio.
Yes. 1920 and 1080 share a gcd of 120, so they simplify exactly to 16:9. Other 16:9 sizes include 1280×720, 2560×1440, and 3840×2160.
Yes, as long as width and height use the same unit. The ratio math is unitless; only mixed units (for example mm × in) give a wrong ratio.
Sensors and export presets sometimes use near-16:9 sizes (for example 1920×1082). The tool still simplifies what you enter; compare the decimal to 1.777… to see how close you are.
9:16 is simply 16:9 rotated. Enter ratio parts 9 and 16, or use height as the longer side when solving a missing dimension for Stories, Reels, and Shorts.
References
- Recommendation ITU-R BT.709 — Parameter values for HDTV Defines the 16:9 HDTV picture format used by common HD pixel sizes.
- ISO 216 — Writing paper and certain classes of printed matter Specifies the √2 aspect ratio of the A and B paper series.
- NIST SP 811 — Guide for the Use of the International System of Units (SI) Length units and coherent quantity algebra for dimension work.