Aspect Ratio Calculator

About the Aspect Ratio Calculator

Type a width and height — in pixels, millimetres or any unit — and get the simplified ratio (1920 × 1080 → 16:9), its decimal (1.7778:1) and the nearest standard with what it is used for. Then keep that ratio and type a new width or height to get the other side, rounded to whole pixels or to even numbers for video.

The ratio is reduced with Euclid’s algorithm for the greatest common divisor, so it is exact, and decimal ratios such as cinema’s 1.85:1 and 2.39:1 work too. Presets cover 16:9, 4:3, 21:9, 1:1, 4:5, 9:16, 3:2, 1.85:1, 2.39:1 and the ISO paper ratio 1:√2. Drop a photo or video to read its size (phone photos are measured upright), copy the CSS aspect-ratio, and see how a picture fits a screen: the letterbox or pillarbox bars, or how much is cropped to fill it.

Common uses

• Checking that a photo, banner or video export has the ratio a platform expects before uploading it.
• Working out the height of a resized image or video so it is not stretched.
• Writing responsive CSS: aspect-ratio: 16 / 9 for video embeds, cards and image placeholders.
• Planning how a 4:3 or cinema-scope clip will look on a 16:9 screen.

How the ratio is worked out

An aspect ratio is width : height in its simplest form. Both sides are divided by their greatest common divisor (GCD), found with Euclid’s algorithm: replace the larger number by the remainder of dividing it by the smaller, until the remainder is 0. For 1920 and 1080: 1920 mod 1080 = 840, 1080 mod 840 = 240, 840 mod 240 = 120, 240 mod 120 = 0, so the GCD is 120 and the ratio is 16:9.

Decimal sides are first scaled to whole numbers (1.85 : 1 → 185 : 100 → 37:20). The decimal form is width ÷ height (16 ÷ 9 = 1.7778), written as 1.7778:1. Some ratios have no whole-number form: ISO paper is 1 : √2, and √2 is irrational, so it is shown as 1:1.4142.

Where the standard ratios come from

• 16:9 — HD and Ultra HD television: 1920 × 1080 in ITU-R BT.709, and 3840 × 2160 and 7680 × 4320 in ITU-R BT.2020.
• 1.85:1 and 2.39:1 — digital cinema’s “flat” and “scope” formats in the DCI Digital Cinema System Specification: 1998 × 1080 and 2048 × 858 in 2K, 3996 × 2160 and 4096 × 1716 in 4K.
• 1:√2 — ISO 216 paper (A4, A3 …): halving a sheet across its long side gives the same ratio, which only works for 1 : √2.
• 21:9 is a marketing name. 2560 × 1080 monitors are 64:27 (2.37:1) and 3440 × 1440 ones are 43:18 (2.39:1); exact 21:9 would be 2.33:1.
• A size within 0.5 % of a standard counts as that standard: 1366 × 768 laptop screens are 683:384, only 0.05 % wider than 16:9.

Rounding for video and images

Pixels are whole numbers, so a new size often has to be rounded, which changes the ratio very slightly; the calculator shows by how much. Video in the common 4:2:0 format (yuv420p) needs an even width and height — encoders such as x264 refuse odd sizes — so choose Even numbers for video. Video codecs split the picture into blocks of 8 or 16 pixels, so multiples of 8 or 16 avoid padding at the edges.

Letterbox, pillarbox and crop

When a picture and a screen have different ratios, there are two choices. Fit scales the picture until it touches the frame on two sides and leaves bars on the others: bars above and below (letterbox) for a wider picture, at the sides (pillarbox) for a narrower one. Fill scales it until the frame is covered and crops what sticks out. These are CSS’s object-fit: contain and object-fit: cover.

Limitations

• The calculator assumes square pixels, as in almost all photos and web video.
• A video’s size is the display size the browser reports; for MP4 and MOV files the browser cannot play, it is read from the file header instead. Other video formats it cannot open need the size typed in.
• Photos are measured as displayed (their EXIF rotation is applied). HEIC photos open natively in Safari; other browsers first download a free HEIC decoder (about 1.4 MB, once). SVG files are measured by the width, height or viewBox written in them.
• Ratios without a whole-number form, such as 1:√2, are rounded to 4 decimals in the CSS.

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