This ruler needs JavaScript to draw. Without it, nothing is shown rather than a scale that would be wrong.
On-Screen Ruler
A ruler that is only real size once you calibrate it — which the page explains, rather than pretending a browser knows how big your screen is.
Why a screen ruler has to be calibrated
CSS — the language that sizes everything on the web — defines one inch as 96 pixels. That is a fixed convention, not a measurement of your display, and almost no screen actually puts 96 pixels in an inch. A phone might put three times that many; a large television, a third. So an on-screen ruler that trusts the convention can be out by a factor of three, and this one says so instead of quietly being wrong.
Calibrating replaces the assumption with something real. The bank card is the better of the two methods because it needs no information about your screen at all: every bank, credit and membership card in your wallet is the same size by international standard — 85.6 by 53.98 millimeters — so matching an outline to one you are holding measures the screen through the card.
Calibrating
Choose "A bank card" and hold any card flat against the outline that appears.
Drag the slider, or press the arrow keys, until the outline and the card are exactly the same width. The height follows automatically.
Read the ruler. It is now drawn from your screen's real pixel size, in centimeters or inches.
Or type your screen's diagonal instead, if you have no card to hand and you know the size it was sold as.
How accurate it is
After calibration the ruler is as accurate as the match you made and as even as your screen is: good enough for a cable, a screw, a photograph or a screen bezel, and not a substitute for a real ruler or a pair of calipers when the number matters. A millimeter out on the card becomes about a percent out along the whole ruler.
Zoom changes everything. If you zoom the page or change your display scaling after calibrating, the ruler is no longer real size — the page notices and tells you to calibrate again.
The calibration is not saved. Nothing on ScreenTail is stored between visits, and this tool keeps that promise rather than making an exception for convenience: the calibration lasts while the page is open, and takes seconds to redo.
If what you actually want is pixel density rather than a ruler, the PPI Calculator works it out from your diagonal, and the Screen Size Calculator turns a diagonal into a width and height.
Nothing you do here is stored or sent. The calibration lives in the page while it is open and disappears when you close it, and the card outline is a drawing — no card details are entered, read or kept.