Screen Size Test — Free Online Test
Our Screen Size Test measures the diagonal, width, and height of your display in inches and centimetres using the browser's DPI reporting. It is a quick way to confirm the advertised size of a monitor or laptop matches the actual panel.
How to use the Screen Size Test
- Load the tool — measurements appear automatically.
- Compare against the manufacturer's advertised size.
How to interpret your results
Manufacturers typically advertise the diagonal in inches (e.g. 15.6"). A measured size within 0.2" of advertised is normal. Larger deviations suggest DPI misreporting by the browser.
Common issues and what they mean
Reported size differs by 20%+
The browser is guessing DPI. Manually calibrate using a physical ruler on the screen.
When to run this test
- When buying a used monitor.
- For CSS media-query planning.
How display size is measured — and where the numbers disagree
Display size is quoted as the diagonal of the active viewing area, corner to corner, in inches. It is not the diagonal of the chassis and it excludes the bezel. Because the diagonal alone says nothing about shape, two panels quoted at the same size can differ substantially in usable area: a 16:9 27-inch monitor has around 311 square inches of screen, while a 16:10 27-inch panel has closer to 324 and gives you noticeably more vertical room for documents and code.
Browsers cannot measure your panel directly. They infer physical size from the CSS pixel ratio and an assumed reference density of 96 pixels per inch, which many operating systems only approximate. That is why this tool should be treated as a sanity check — good enough to catch a monitor sold as 27 inches that is really 24 — rather than a metrology instrument. If the number matters commercially, confirm it with a tape measure across the lit area.
Scaling settings shift the result too. A Windows machine at 150% scaling reports a third fewer CSS pixels, and unless the device pixel ratio is reported correctly the calculated size moves with it. Setting scaling back to 100% temporarily before reading the measurement removes that source of error.
Choosing the right size for viewing distance
Comfortable text size depends on angular size, not physical size. At a typical desk distance of 60 to 70 centimetres, a 24-inch 1080p or a 27-inch 1440p panel puts glyphs at roughly the same apparent scale, which is why both remain the mainstream office choices. Move to a 32-inch 4K panel at the same distance and you will usually want 125% to 150% scaling to keep reading effortless.
For a laptop used on a lap or a train tray table, distance drops to 40 to 50 centimetres, so a 13 to 14-inch panel presents a similar angular field to a 24-inch desktop monitor. This is the practical reason a 17-inch laptop feels enormous rather than merely large: the extra area sits outside the comfortable central field of view and forces head movement.
When buying second-hand, measure before paying. Listings frequently repeat a model family name whose members span several sizes, and a returned or swapped panel is one of the most common discrepancies on marketplace listings. Our used-laptop testing guide walks through the full sequence to run in front of the seller.
Frequently asked questions
Is this accurate?
It is limited by browser DPI reporting, which is often imprecise — verify with a ruler for exact measurements.
Why is my 15.6-inch laptop measured as 15.4 inches?
Advertised sizes are rounded and measured across the raw panel including a millimetre or two hidden under the bezel, so small differences are normal.
Does screen size affect image quality?
Only in combination with resolution. The same 1080p signal is sharp at 21 inches and visibly coarse at 32 inches because pixel density falls as the panel grows.