Why is a 1366 × 768 screen not exactly 16:9?

Because 1366 ÷ 768 is not 16 ÷ 9. It reduces to 683:384, about 0.05% wider than 16:9. It is close enough that most people call it 16:9, but this page only names a ratio when it matches exactly and shows how far off the closest one is.

Is a 2560 × 1080 monitor really 21:9?

Not exactly. It reduces to 64:27. A true 21:9 picture is 7:3, such as 2520 × 1080. The 21:9 label is a marketing name for ultrawide screens whose picture is a little wider than that (64:27 is 2.37, 7:3 is 2.33).

How accurate are the PPI and physical size?

The arithmetic is exact apart from the square root, which is worked out to 20 decimals. The inputs are the weak point: the diagonal printed on a box is usually rounded, it excludes the bezel, and square pixels are assumed. Treat the result as close, not as a measurement of your actual panel.

Is anything uploaded or saved?

No. The numbers stay in this page, nothing is sent to a server, and nothing is stored. Refreshing the page resets the boxes.

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Aspect ratio calculator

Type a width and height to get the simplest ratio, find the other side when you resize, see the black bars when a video does not fit a screen, or turn a screen's diagonal and resolution into pixels per inch and real size. Everything is worked out in this page and nothing is uploaded.

Positive numbers with up to 6 decimals, for example 1920 or 1.85. A single comma or dot is a decimal mark. A lone 1,234 is ambiguous (1.234 or 1234?), so it is not guessed: write 1234 or 1.234. Sizes up to 1,000,000,000.

Simplify a size to a ratio

Type a width and a height to get the shortest ratio.

Width

Height

Common sizes

16:9

1920 × 1080 reduces to 16:9.

That is exactly 16:9.
Landscape (wider than tall)
Width ÷ height ≈ 1.7778 (rounded to 4 decimals)
Height ÷ width = 0.5625
2073600 pixels in total (2.07 megapixels)

Find the other side from a ratio

Know the width or the height and the ratio? Get the missing side.

Ratio

Type 16:9, 16/9, 16x9 or a single number such as 1.85 (meaning 1.85:1).

I know the

Known side

Decimals

At 1080 wide with a 16:9 shape, the height is 607.5.

1080 × 607.5

That is not a whole number. Rounded to the nearest whole pixel it is 608, which makes the ratio very slightly different.

Fit a picture or video into a box

See the scaled size and the bars (letterbox or pillarbox) when the shapes differ.

Source width

Source height

Box width

Box height

How to fit

Allow enlarging the source

Off keeps a small source at its own size, so bars can appear on all four sides.

Scaled to 1080 × 607.5 (56.25% of the source size).

Letterbox: bars above and below, 1312.5 in total, 656.25 each.

Sizes that are not whole pixels are rounded for display only: 1080 × 608. Real software rounds differently, so check the result in yours.
A bar of half a pixel cannot be drawn; the two sides will differ by one pixel.

■ Picture
■ Bars

Screen PPI and physical size

From the diagonal and the resolution, get pixels per inch, the real width and height and the pixel size.

Screen diagonal

Diagonal unit

Pixels across

Pixels down

Viewing distance (optional)

Distance unit

Leave empty to skip pixels per degree.

Common resolutions

108.8 PPI

2560 × 1440 on a 27 inch diagonal is 108.8 PPI.

Physical size: 23.53 × 13.24 inches (597.7 × 336.2 mm).
Each pixel is about 0.233 mm wide.
Shape: 16:9 (exactly 16:9).
3686400 pixels (3.69 megapixels)
At 60 centimetres, about 44.9 pixels per degree.

Pixels per degree is measured at the centre of the screen, looking straight at it. A rough rule of thumb often quoted for sharp normal vision is about 60, which would be reached at about 80.3 centimetres for this screen. Eyes and content differ, so treat it as a guide, not a rule.

PPI counts pixels along the screen diagonal and assumes square pixels. Marketing diagonals are rounded, so real panels can differ a little.

How it works

•A ratio is simplified by dividing both numbers by their greatest common divisor, so 1920 × 1080 becomes 16:9. Decimals are read exactly, so 1.5 × 1 gives 3:2 with no rounding error.
•A name such as 16:9 or 4:3 is shown only when the ratio is exactly equal to it. 1366 × 768 reduces to 683:384, which is 0.05% away from 16:9, so it gets no name and the closest ratio is shown separately. The same goes for 2560 × 1080, which reduces to 64:27 and is often sold as 21:9; exactly 21:9 is 7:3, for example 2520 × 1080.
•To find the other side, the known side is multiplied or divided by the ratio. 1080 wide at 16:9 is 607.5 high, which is not a whole pixel, so rounding it changes the ratio slightly. Many video encoders need even sizes, so check the result in yours.
•Fitting scales the source by the smaller of box width ÷ source width and box height ÷ source height, which keeps all of it visible, and centres it. A 1920 × 1080 video in a 1080 × 1920 vertical box becomes 1080 × 607.5 (56.25%) with bars of 656.25 above and below. "Fill the box" uses the larger factor and crops what spills over.
•Screen PPI is the diagonal length in pixels, the square root of width² + height², divided by the diagonal in inches. The physical width and height come from the same right triangle, with 1 inch = 25.4 mm. A 27 inch 2560 × 1440 screen is 108.8 PPI and about 23.53 × 13.24 inches. Pixels per degree multiplies PPI by 2 × distance × tan(0.5°); it is a rough guide that assumes you look straight at the centre of the screen.

Questions

Why is a 1366 × 768 screen not exactly 16:9?: Because 1366 ÷ 768 is not 16 ÷ 9. It reduces to 683:384, about 0.05% wider than 16:9. It is close enough that most people call it 16:9, but this page only names a ratio when it matches exactly and shows how far off the closest one is.
Is a 2560 × 1080 monitor really 21:9?: Not exactly. It reduces to 64:27. A true 21:9 picture is 7:3, such as 2520 × 1080. The 21:9 label is a marketing name for ultrawide screens whose picture is a little wider than that (64:27 is 2.37, 7:3 is 2.33).
How accurate are the PPI and physical size?: The arithmetic is exact apart from the square root, which is worked out to 20 decimals. The inputs are the weak point: the diagonal printed on a box is usually rounded, it excludes the bezel, and square pixels are assumed. Treat the result as close, not as a measurement of your actual panel.
Is anything uploaded or saved?: No. The numbers stay in this page, nothing is sent to a server, and nothing is stored. Refreshing the page resets the boxes.