Calculate Screen Size From Height and Width

To calculate a screen’s diagonal size from its height and width, use the Pythagorean theorem: diagonal = √(height² + width²). Enter both measurements in the same unit; the result is the diagonal in that unit, which is the screen size commonly quoted in inches.

Formula

The diagonal is the square root of the sum of the squares of the screen’s width and height:

d = √(w² + h²)

• d is the screen’s diagonal, in the same unit as the measurements.
• w is the screen’s width, measured straight across the display area.
• h is the screen’s height, measured straight from top to bottom.

The calculator’s first result is the diagonal in inches. Its second result converts that diagonal to centimeters using 1 inch = 2.54 centimeters. The conversion does not change the size; it only changes the unit.

Worked Example

Using the calculator’s default values, the width is 20.9 inches and the height is 11.8 inches.

1. Square the width: 20.9² = 436.81.
2. Square the height: 11.8² = 139.24.
3. Add the squares: 436.81 + 139.24 = 576.05.
4. Take the square root: √576.05 ≈ 24.001 inches.
5. Round to two decimal places: the screen size is 24.00 inches.
6. Convert to centimeters: 24.001 × 2.54 ≈ 60.96 cm.

This is a 16:9-shaped display close to 24 inches diagonally. Actual product specifications may be rounded to a whole or half inch, so a calculated measurement need not match a listing’s label digit for digit.

How to Measure a Screen

Measure the visible display, not the outside of the device. The bezel, frame, stand, and case are not part of the screen diagonal. Use a ruler or tape measure and keep it straight across the active display area. If you can only measure the width and height, make both measurements along the edges of the visible rectangle.

• Measure width from the left edge of the image area to the right edge.
• Measure height from the top edge of the image area to the bottom edge.
• Use the same unit for both values. For example, do not enter a width in inches and a height in centimeters.
• Keep reasonable precision in the measurements, then round the final result to a useful number of decimal places.

If you already have the screen’s diagonal and aspect ratio, you can also work out the approximate width and height. The ratio describes the shape of the display, not its diagonal size.

What Screen Size Means

In everyday product descriptions, “screen size” usually means the length of the diagonal across the visible rectangular display. It does not mean the screen’s width, its height, or the device’s overall dimensions. A 24-inch screen therefore has a diagonal near 24 inches, even though neither edge is 24 inches long.

For a conventional rectangular screen, the diagonal follows directly from the right triangle formed by its width, height, and diagonal. The width and height are the perpendicular sides; the diagonal is the hypotenuse. This is why simply adding width and height gives the wrong answer. For instance, 20.9 + 11.8 = 32.7 inches is not the diagonal: the correct result is about 24 inches.

Rounded corners and curved displays can make the physical outline more complicated, but the quoted diagonal is generally based on the display’s nominal rectangular dimensions. A tape measure may not follow that convention exactly, particularly with curved or unusually shaped panels.

Aspect Ratio and Comparisons

Two screens with the same diagonal can have different widths and heights if their aspect ratios differ. A 16:9 screen is wider relative to its height than a 4:3 screen. As a result, the 16:9 display will be wider and shorter at the same diagonal, while the 4:3 display will be narrower and taller.

To compare screens for desk space or a particular use, look beyond the diagonal. Check the listed width and height of the display itself, and separately check the device’s external dimensions if the frame or stand matters. A diagonal alone cannot tell you whether a screen will fit a specific space.

Resolution is also separate from physical size. Resolution counts pixels, while diagonal measures length. A larger screen does not automatically have more pixels, and two screens of the same diagonal can have different resolutions and pixel densities.

Common Measurement Mistakes

• Mixing units: Convert both measurements to inches or both to centimeters before applying the formula. If width is 50 cm and height is 30 cm, calculate the diagonal in centimeters first; convert afterward if needed.
• Measuring the frame: The outer casing is larger than the active display and produces an oversized estimate.
• Adding the sides: The diagonal is not width plus height. Square each side, add the squares, and take the square root.
• Rounding too early: Keep the measured values through the calculation and round only the final result. Early rounding can shift the answer slightly.
• Assuming a nominal size is exact: Product labels commonly use rounded diagonal classes. A modest difference from a tape measurement is not necessarily an error.

The formula assumes a flat rectangular measurement and accurate inputs. It does not account for a curved panel’s surface path, a nonrectangular active area, or measurement uncertainty. For a curved screen, the diagonal is usually understood as a straight-line measurement across the display, not a distance traced along its surface.

Frequently Asked Questions

How do I calculate screen size in inches?
Measure the visible screen’s width and height in inches, square both, add the results, and take the square root: √(width² + height²). The answer is the diagonal in inches.

Can I calculate screen size from centimeters?
Yes. Measure width and height in centimeters and use the same formula. The result is in centimeters. Divide by 2.54 to convert the diagonal to inches.

Is a 24-inch screen 24 inches wide?
No. The 24-inch label refers to the diagonal, not the width. The actual width depends on the screen’s aspect ratio and can also vary slightly between models.

Should I measure the bezel?
No, not when calculating the screen’s quoted size. Measure only the visible display area. Measure the full device separately if you need to check whether it will fit on a desk or in a cabinet.

Can two screens with the same diagonal have different dimensions?
Yes. Screens with different aspect ratios have different width-to-height proportions, so their edges differ even when their diagonal measurements match.

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in width | Screen width | in | 20.9 | 0
in height | Screen height | in | 11.8 | 0
out size | Screen size (diagonal) | in | sqrt(width^2 + height^2) | 2
out diagonal_cm | Diagonal | cm | size * 2.54 | 2