What Is My Monitor Size
Your screen size in inches and centimetres, worked out as the page loads — and a one-step calibration to make it exact.
This is an estimate from your display scaling, not a measurement — no browser is told your screen's real size. Calibrate below, or enter your diagonal, for an exact figure.
- Pixel density
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- Aspect ratio
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- Screen area
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- Dot pitch
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- Device pixels
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- CSS pixels
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- Total pixels
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Calibrate for an exact size
Hold any bank, credit or ID card flat against the screen and drag until the blue card matches its width. Every such card is 85.60 mm by international standard. Saved on this device and shared with the screen ruler — do it once, both tools become accurate.
Or enter your diagonal
If you already know your screen size — it is usually in the model number — type it here for an exact result. Measure corner to corner across the panel only, ignoring the bezel.
Common sizes:
How monitor size is measured
Screen size is quoted as the diagonal of the visible panel, measured corner to corner and excluding the bezel. A "24-inch monitor" therefore has a viewing area roughly 20.9 inches wide and 11.8 inches tall in 16:9 — the diagonal is always noticeably larger than the width, which is why screens tend to look smaller in person than the number suggests.
Why browsers cannot simply tell you
A browser knows how many pixels your screen has. It is never told how large those pixels are. The usual workaround is to assume the CSS reference value of 96 pixels per inch, but that is a specification constant, not a property of your hardware — which is why the same trick reports a 27-inch 1080p monitor as 22.9 inches and a modern iPhone as roughly half its real size.
There are only three honest ways round it, and this page uses all three:
- Recognise the device. Apple hardware reports a distinctive combination of resolution and pixel ratio, so an iPhone or MacBook can be identified exactly. Android cannot — dozens of unrelated phones report identical figures, so guessing there would be data-shaped noise.
- Measure against something known. A bank card is 85.60 mm everywhere in the world, which makes it the most reliable ruler most people already have.
- Ask. If you know your diagonal, that is authoritative and everything else follows from it.
Turning pixels into inches
The diagonal in pixels comes from Pythagoras: the square root of width squared plus height squared. Dividing that by the physical diagonal gives pixels per inch, and the physical width and height follow directly. For a 24-inch 1920 x 1080 monitor the pixel diagonal is about 2,203 pixels, giving roughly 92 PPI — around 0.277 mm per pixel.
What pixel density means in practice
| Display | Resolution | PPI | Sharpness |
|---|---|---|---|
| 32″ 1080p | 1920 x 1080 | ~69 | Pixels visible at desk distance |
| 24″ 1080p | 1920 x 1080 | ~92 | Comfortable, the common baseline |
| 27″ 1440p | 2560 x 1440 | ~109 | Noticeably crisper text |
| 27″ 4K | 3840 x 2160 | ~163 | Very sharp; needs OS scaling |
| 14″ laptop 4K | 3840 x 2160 | ~315 | Retina class |
To measure something on the screen rather than the screen itself, use the online screen ruler. For the underlying pixel grid, see what is my screen resolution. There is also a longer guide on how to measure monitor size.
More screen tools
Screen Resolution
Detect your display resolution, pixel ratio and viewport instantly.
Full Monitor Test New
Run every display check in one guided sequence and get a summary.
Online Ruler
A calibrated ruler in actual size — cm, mm, inches or pixels.
FPS Test New
Live frame rate, 1% lows and stutter, with a frame-time graph.
Refresh Rate Test New
Measure the true refresh rate of your display in Hz, live.
Dead Pixel Test New
Fullscreen solid colours that expose dead, stuck and hot pixels.
Screen Colour Test New
Check colour accuracy, gradients, banding and panel uniformity.
Aspect Ratio Calculator New
Work out and resize any aspect ratio, with a live to-scale preview.
Burn-in Test New
Check OLED and LCD panels for image retention, and try to clear it.
Touch Screen Test New
Find dead zones and check multi-touch by drawing on the screen.
Backlight Bleed Test New
Find light leaking around the edges of an LCD panel.
Ghosting Test New
Check motion blur, trailing and pixel response with moving objects.
Screen Size Comparison New
Put two displays side by side, drawn to true scale.
Frequently asked questions
How do I find my monitor size?
The figure above is worked out the moment the page loads. Because no browser is told the physical size of your display, it starts as an estimate based on your display scaling — calibrate against a bank card, or type your diagonal, and it becomes exact. The label under the number always tells you which of those you are looking at.
Can a website really detect my monitor size?
Not on its own, and any site claiming otherwise is estimating. Browsers expose the resolution in pixels but never the physical dimensions, so the only accurate routes are matching a known device, measuring against an object of known size, or being told the diagonal. This page uses all three, in that order of preference.
Why was my screen size wrong on another site?
Almost every such tool assumes 96 pixels per inch, which is a fixed reference value rather than a measurement. That assumption is close on an ordinary desktop monitor and badly wrong on high-density screens — it typically reports a phone at about half its real size, and a scaled 4K monitor a third too small.
What is PPI and why does it matter?
PPI is pixels per inch: how tightly the pixels are packed. A 24-inch 1080p monitor is about 92 PPI, while a 24-inch 4K panel is about 184. Higher PPI means smoother text and finer detail. Below roughly 90 PPI individual pixels start to become visible at a normal desk distance.
How does bank-card calibration work?
Every bank, credit and ID card is manufactured to ISO/IEC 7810 ID-1, which fixes the width at 85.60 mm worldwide. Matching an on-screen shape to a real card therefore tells the page exactly how many pixels cover a known distance, which is the one fact browsers do not provide. It is stored on your device and shared with the screen ruler, so you only ever do it once.
Is a bigger monitor always better?
Not on its own. A large screen with a low resolution has a low pixel density, so text looks coarse. Size and resolution have to rise together — which is exactly what the PPI figure above tells you.