Calculate Screen Size: Measure Diagonal and Dimensions

Calculate a screen’s diagonal size from its width and height, or estimate its width and height when you know the diagonal and aspect ratio. Enter dimensions in the same unit; the calculator returns the diagonal in that unit, plus the screen’s width and height from a diagonal and aspect ratio.

Formula

The diagonal of a rectangular screen is calculated with the Pythagorean theorem:

d = √(w² + h²)

• d is the screen diagonal, in inches in this calculator.
• w is the screen’s visible width, in inches.
• h is the screen’s visible height, in inches.

For a screen whose diagonal and aspect ratio are known, first write the aspect ratio as width:height, then calculate width as w = d × a / √(a² + b²) and height as h = d × b / √(a² + b²). Here, a:b is the aspect ratio and d is the diagonal. For example, a 16:9 screen has a = 16 and b = 9. These equations assume the screen is a rectangle.

Worked Example

The calculator’s default measurements are a width of 20 inches and a height of 12 inches. Substituting those values into the diagonal formula:

1. Square the width: 20² = 400 square inches.
2. Square the height: 12² = 144 square inches.
3. Add the squares: 400 + 144 = 544 square inches.
4. Take the square root: √544 ≈ 23.3238 inches.

Rounded to two decimal places, the screen diagonal is 23.32 inches. This is the diagonal across the display area, not the width or the outside measurement of the device.

How to Measure a Screen

To measure a physical screen, use a ruler or tape measure across the visible display from one corner to the opposite corner. Measure the screen itself, excluding the bezel, case, stand, and any surrounding frame. If you can reach only the horizontal and vertical edges, measure the visible width and height and use the calculator instead.

For a laptop, monitor, television, or tablet, manufacturers generally describe screen size by its diagonal. A product’s advertised diagonal may be rounded, so a tape-measure result can differ slightly. Curved displays can also complicate a direct measurement: the advertised diagonal is typically based on the display’s rectangular geometry, while a tape laid along the curve measures a longer path. For the most comparable result, use the published specification or measure straight across the screen rather than following its surface.

Using Aspect Ratio to Find Width and Height

A diagonal alone does not uniquely determine a screen’s width and height: many rectangles can have the same diagonal. You also need the aspect ratio, expressed as width:height. Common examples include 16:9 for many widescreen displays and 4:3 for older or more nearly square formats. Check the screen’s specifications if its ratio is unknown; do not assume every display uses the same one.

For example, a 27-inch screen with a 16:9 ratio has an approximate visible width of 23.53 inches and height of 13.24 inches. The calculation uses the diagonal and ratio, not the device’s outer dimensions. Different aspect ratios at the same diagonal produce different screen shapes and viewing areas.

Aspect ratio does not tell you the display’s resolution or pixel density. Resolution is the number of pixels across and down; pixel density describes how tightly those pixels are packed. Two screens can have the same diagonal and aspect ratio but different resolutions and image sharpness.

Units, Rounding, and Common Mistakes

The calculator expects width and height in the same unit. If you measure in centimeters, calculate the diagonal in centimeters using those measurements, then convert the result to inches if needed: 1 inch equals exactly 2.54 centimeters. Do not enter a width in inches and a height in centimeters together; the formula cannot detect or correct a mixed-unit input.

• Measure the active display, not the casing. A bezel adds to the device’s outside size but is not part of the screen diagonal.
• Measure corner to corner. The diagonal is not the horizontal width, and it is not the sum of the width and height.
• Keep units consistent. Convert both measurements before calculating if they use different units.
• Round only the final result. Rounding intermediate values can introduce avoidable error, especially with small measurements.
• Distinguish size from aspect ratio. A diagonal does not reveal the screen’s shape unless the ratio is known.

The formula assumes a flat rectangular active area and exact input measurements. It does not account for rounded corners, curved surfaces, perspective, or measurement uncertainty. A result with two decimal places is a mathematical output, not a guarantee that a ruler or product specification is accurate to one-hundredth of an inch.

Diagonal Size Versus Overall Device Size

Screen diagonal is useful for comparing display sizes, but it does not tell you whether a device will fit in a desk space, bag, cabinet, or wall opening. For fit, check the full product dimensions, including the bezel, frame, stand, and any clearance needed for cables or ventilation. A screen with a larger diagonal can be narrower or shorter than another screen if their aspect ratios differ.

Diagonal is also not a direct measure of screen area. For a rectangular display, area is width multiplied by height. At a fixed diagonal, a squarer aspect ratio generally has more rectangular screen area than a wider, narrower ratio, though the actual difference depends on the ratios being compared. This is another reason to consider width, height, and aspect ratio alongside the advertised size.

Frequently Asked Questions

How do I calculate my screen size from width and height?
Measure the visible width and height in the same unit, square each measurement, add them, and take the square root: diagonal = √(width² + height²).

Is screen size measured diagonally?
Yes. Screen size usually refers to the diagonal of the visible display, from one corner to the opposite corner. It does not normally include the bezel or device frame.

How big is a 27-inch screen in width and height?
It depends on the aspect ratio. A 27-inch rectangular screen with a 16:9 ratio is approximately 23.53 inches wide and 13.24 inches high; another ratio will have different dimensions.

How do I convert a screen size from inches to centimeters?
Multiply inches by 2.54 to get centimeters. For example, a 24-inch diagonal is 60.96 centimeters. Convert the diagonal itself, or convert both width and height before calculating it.

Does a bigger diagonal always mean more screen area?
No. Aspect ratio affects the width and height at a given diagonal. To compare area, multiply the screen’s width by its height; to compare how it fits, check both dimensions and the device’s overall measurements.

Related

• Bytes Calculator
• EBITDA Margin: Formula and How to Calculate It
• Calculate Image Dimensions from Pixels and Aspect Ratio

in w | Screen width | in | 20 | 0
in h | Screen height | in | 12 | 0
out d | Screen diagonal | in | sqrt(w^2 + h^2) | 2