What Is My Monitor Size?
A credit card is 85.6 mm wide. Hold it against your screen and drag the slider until the outline matches. If the card spans 360 CSS pixels on a 1920 x 1080 screen, the display is 20.6 inches (45.7 x 25.7 cm) across the diagonal.
How to use
- Pick a reference object. A credit card is best. A coin or a ruler works if you have no card.
- Hold the object flat against the screen and drag the slider until the outline on the page is exactly the same size. Use the arrow buttons for single-pixel steps.
- Read the diagonal in inches, the width and height in cm, and the pixels per inch. Keep the browser window on the monitor you are measuring and leave page zoom at 100 percent.
How it works
A bank card follows the ISO/IEC 7810 ID-1 standard and is 85.60 x 53.98 mm. When the outline on screen matches the card, you know how many CSS pixels equal 85.6 mm. Dividing gives millimeters per CSS pixel.
The browser reports your screen size in CSS pixels as screen.width and screen.height. Multiplying each by millimeters per pixel gives the physical width and height, and the diagonal is the square root of width squared plus height squared, divided by 25.4 to get inches.
Pixels per inch (PPI) uses the real pixel count: screen size times the device pixel ratio, then the diagonal in pixels divided by the diagonal in inches. The closest standard size is shown when your result is within 3 percent of a common panel size such as 24 or 27 inches.
Reference sizes used
| Object | Size used |
|---|---|
| Credit or bank card (ISO/IEC 7810 ID-1) | 85.60 x 53.98 mm |
| 1 euro coin | 23.25 mm diameter |
| US quarter | 24.26 mm diameter |
| Ruler option | 100 mm span |
Common monitor sizes
| Diagonal | Typical use |
|---|---|
| 13 to 16 inch | Laptops |
| 21.5 to 24 inch | Office monitors |
| 27 inch | Desktop monitors, 1440p and 4K |
| 32 to 34 inch | Large and ultrawide monitors |
FAQ
How accurate is this?
Usually within about 1 to 3 percent if you match the outline carefully. Look straight at the screen, because viewing at an angle makes the edges of the card appear shifted. A manufacturer label such as 27 inch is rounded, so a measured 26.9 is normal.
What if I do not have a credit card?
Choose a coin or the ruler. A 1 euro coin is 23.25 mm across and a US quarter is 24.26 mm. For the ruler option, draw or find a 10 cm span on a real ruler and match the bar to it. Small objects give a coarser result, so a card is better.
Why not use the resolution to find the size?
Resolution only counts pixels. A 1920 x 1080 panel can be 15.6 inches on a laptop or 32 inches on a TV, so the physical size needs a real measurement. The browser also assumes about 96 pixels per inch, which is rarely true.
Does it work with several monitors or a laptop?
Yes. Move this window to the screen you want to measure before you adjust the slider, because the browser reports the screen the window is on. On a phone, a credit card is wider than the screen, so pick the coin.