Online Ruler
A ruler in actual size, right on your screen. Hold something against it to measure in centimetres or inches, or drag along it to measure an on-screen distance in pixels.
Not calibrated yet. The ruler is drawn at the browser default of 96 pixels per inch, which is wrong on most laptops and roughly double-scale on phones. Calibrate below — it takes ten seconds and is remembered.
Calibrate with a bank card
Hold any bank, credit or ID card flat against the screen and drag until the blue card is exactly the same width. Every such card is 85.60 mm by international standard — the most reliable ruler most people already own.
Your screen is too narrow to draw the card at full width — use the screen-diagonal method instead.
Or use your screen size
If you know your screen diagonal — it is usually in the model number — type it here and everything else is worked out from it. This is the quickest route on a phone, where a bank card is wider than the screen.
Measuring a real object
- Calibrate first. Without it the ruler is drawn at a standard assumption rather than your actual screen, and on a phone that assumption is out by about half.
- Lay the object flat against the glass, square to the markings.
- Line one end up with zero and read the other end off the scale.
- Look straight on. Reading at an angle introduces parallax error larger than the calibration error you just removed.
- Use Fullscreen for anything longer than the page column, and Rotate for tall objects.
Measuring something on the screen
Drag along the ruler from one edge of what you are measuring to the other. The highlighted band shows the span and the reading updates as you go. In pixels this is exact — no calibration needed, because the browser knows precisely how many pixels it drew. Calibration only matters when you want the answer in real-world units.
Why an on-screen ruler has to be calibrated
A browser can tell you exactly how many pixels wide something is. What it is never told is how large a pixel physically is. A 13-inch laptop and a 27-inch monitor can both report 1920 x 1080 — identical in pixels, wildly different in size — and nothing in the browser distinguishes them.
So every uncalibrated on-screen ruler falls back on the same assumption: the CSS reference pixel of 96 per inch. That is a reasonable guess on a plain desktop monitor and poor everywhere else. Matching a known 85.60 mm card, or supplying your screen diagonal, replaces the guess with a measurement.
Units and conversions
| Unit | Equals | Best for |
|---|---|---|
| 1 centimetre | 10 mm · 0.394 in | Everyday objects |
| 1 millimetre | 0.1 cm · 0.039 in | Small parts, thickness |
| 1 inch | 25.4 mm · 2.54 cm | Screens, hardware, US sizing |
| 1 pixel | depends entirely on the display | Design and layout work |
That last row is the whole point: a pixel has no fixed physical size, which is why the other three need calibration and pixels do not.
Where the calibration goes
It is stored in your own browser and shared with the rest of the site, so calibrating here also makes the monitor size calculator and the screen size comparison accurate. Nothing is uploaded, and the Reset button clears it.
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Frequently asked questions
How do I use an online ruler?
Hold the object flat against your screen, line one end up with zero, and read off the markings. Calibrate first — otherwise the ruler is drawn at the browser default of 96 pixels per inch, which is wrong on most laptops and badly wrong on phones. Calibration takes about ten seconds and is remembered.
How accurate is an on-screen ruler?
After calibration, typically within half a millimetre — good enough for everyday measuring, though not a substitute for callipers. Uncalibrated it is a guess, and the page says so rather than pretending otherwise. The label under the reading always tells you which of the two you are looking at.
How do I calibrate the ruler?
Two ways, both on this page. Hold a bank card against the screen and drag the slider until the on-screen card matches it — every bank, credit and ID card is 85.60 mm wide by international standard. Or, if you know your screen diagonal, type it in and everything is worked out from that.
Can I measure in inches and centimetres?
Yes — centimetres, millimetres, inches and pixels. The summary line under the reading shows all four at once, so you never have to convert by hand.
Can I make the ruler full screen?
Yes. The Fullscreen button removes everything except the ruler, which gives you the longest possible measuring edge — useful for anything wider than the page column. Rotate switches between a horizontal and a vertical ruler.
Does it work on a phone?
It does, and the ruler is touch-driven. Calibrate first: phone screens pack far more pixels per inch than the 96 default assumes, so an uncalibrated phone reading will be roughly half the true size.
Is anything uploaded?
No. The ruler runs entirely in your browser and your calibration is stored on your own device. It is shared with the monitor size tool, so calibrating once makes both accurate.