Scale Ruler

An on-screen ruler you can measure with directly on your monitor, phone or tablet. Calibrate it once against a bank card, then read distances in millimetres, centimetres or inches. Set a drawing scale such as 1:50 or 1:100 and the ruler labels show the real-world size instead of the measured length.

On this page: Calibrate · Ruler · How to use · Accuracy · FAQ

Step 1 — Calibrate Your Screen

Hold any bank card, credit card or ID card against the screen and drag the slider until the outline below matches the side you picked, exactly. Every card of this type is 85.60 × 53.98 mm by international standard, so this sets your screen's real size. The setting is remembered on this device.

Match the card's

On a phone the long side may not fit across the screen. Switch to the short side, or turn the phone sideways.

85.60 mm

Or enter your screen PPI:

Step 2 — Measure

Units

Drawing scale 1 :

Leave the scale at 1 to measure the screen itself. Set it to 50 to read a 1:50 drawing in real-world size.

How to Use the Scale Ruler

Calibrate once with a bank card. The value is stored in your browser for next time.
Keep the browser zoom at 100 %. Zooming changes the size of everything on the page, including the ruler.
Choose metric or imperial.
If you are measuring a drawing displayed at a known scale, enter that scale. The ruler then labels real-world distances.
Hold the object or drawing against the screen and read the value.

For a printable scale bar to place on a drawing, use the Scale Bar Generator. To convert a measurement you have already taken, use the Architectural Scale Calculator.

How Accurate Is an On-Screen Ruler?

Accuracy depends entirely on the calibration. Once the card outline matches a real card, the ruler is typically within a fraction of a millimetre across its length. Four things break it:

Browser zoom: anything other than 100 % changes the scale. Press Ctrl+0 (Cmd+0 on a Mac) to reset it, then recalibrate.
Operating system display scaling: changing the system scaling setting changes the CSS pixel size. Recalibrate after you change it.
A different device: the calibration is stored per browser and per device.
Curved or non-flat screens: measuring a straight edge against a curved display introduces error.

For a dimension that has to be right, use a physical ruler or a calibrated print. An on-screen ruler is for quick checks and estimates.

Scale Ruler Questions

Why do I have to calibrate it?

A web page is measured in CSS pixels, and a CSS pixel is not a fixed physical size. It depends on the display's pixel density, the operating system's scaling setting and the browser zoom level. Matching a card of known width is the only reliable way to translate pixels into millimetres.

Why a bank card?

Bank cards, credit cards and most ID cards follow the ISO/IEC 7810 ID-1 standard: 85.60 × 53.98 mm. Practically everyone has one, and every one of them is the same size, which makes it a better reference than a coin or a sheet of paper.

What is a scale ruler used for?

A scale ruler reads real-world distances straight off a scaled drawing without any arithmetic. Architects, engineers, surveyors and model makers use one to check dimensions on plans, maps and technical drawings.

Can I use it to measure a drawing on screen?

Yes, if the drawing is displayed at a known scale. Enter that scale and the ruler labels the real-world size. Remember that a PDF or image shown at an arbitrary zoom level is no longer at its stated scale.

Does the calibration persist?

Yes. It is stored in your browser on this device. Clearing site data, using a different browser or using another device means calibrating again.

What PPI should I enter for my screen?

You do not need to know it — the card slider is more reliable. If you prefer a number, a standard desktop monitor is usually between 90 and 110 PPI, and a laptop between 120 and 220 PPI, but operating system scaling often changes the effective value.

Looking for a different tool? Try the scale converter and scale factor calculator or the Map Scale Calculator.