Online Ruler
Measure real objects on your screen in inches and centimetres with a calibrated on-screen ruler.
Step 1: Calibrate your screen
Every screen packs a different number of pixels into one inch, so the ruler must be calibrated before it can measure accurately. Pick a method below. This free tool runs entirely in your browser, with no sign-up and nothing uploaded.
Box width: 324 px. A standard credit card is 85.6 mm wide.
Step 2: Measure with the ruler
Click the ruler once at one end of your object, then click again at the other end. The distance between the two markers is shown below. The top ruler is in inches, the bottom ruler is in centimetres.
Your measurement
Click twice on the ruler to measure a distance.
Pixels per inch and per centimetre at common screen densities
The same inch is a different number of pixels on every screen. The 96 DPI row is the web standard used before calibration and is highlighted.
How an online ruler works
A standard credit card measures 85.60 by 53.98 millimetres under the ISO/IEC 7810 ID-1 standard (International Organization for Standardization, 2003 revision), which is why it makes such a reliable calibration object. The web standard defines 1 inch as exactly 96 CSS pixels (W3C CSS Values and Units Module, 2022), so this tool starts at 96 DPI until you calibrate it.
How does an online ruler work?
An online ruler draws a measuring scale on your screen. It converts pixels into inches or centimetres using your screen’s DPI, which is the number of pixels in one inch. Because every screen has a different DPI, the ruler must be calibrated. Once calibrated, the distance between any two points on the ruler matches a ruler held against the screen.
How do I calibrate the ruler with a credit card?
Hold a standard credit card flat against your screen and drag the slider until the blue box is exactly as wide as the card. Then press the calibration button. A credit card is 85.6 millimetres wide, so the tool divides the box width in pixels by 85.6 to work out the pixels in one millimetre.
Why is my ruler wrong before calibration?
Screens pack different numbers of pixels into one inch, from about 92 on a large desktop monitor to over 400 on a phone. Without calibration the tool has to guess, and it uses the web standard of 96 pixels per inch. That guess is close on some monitors and far off on others, so always calibrate before measuring anything important.
How do I measure an object with the ruler?
Place the object against your screen, lined up with the zero mark at the left of the ruler. Click the ruler at one end of the object, then click at the other end. Two markers appear and the tool shows the distance between them in inches, centimetres and millimetres. Press the clear button to remove the markers and measure again.
Can I measure in both inches and centimetres?
Yes. The tool shows two rulers, one marked in inches with eighths and one marked in centimetres with millimetres, and both use the same calibration. Any measurement you make is shown in inches, centimetres and millimetres at the same time, so you can switch units without measuring again or doing any conversion yourself.
What is the most accurate way to calibrate?
The credit card method is the most accurate because it measures your actual screen directly instead of trusting a specification. Screen-size presets come next, but only if you know your exact screen size and resolution. Typing a DPI number is the least reliable option, because the DPI printed on a box is often rounded. Recalibrate whenever you switch screens.
Frequently asked questions
Is this online ruler really the right size?
Yes, once it is calibrated. An uncalibrated ruler uses the 96 DPI web standard, which is only a guess for your screen. After calibrating with a credit card, one inch on the ruler is one real inch on your screen, as long as you do not change screens or zoom settings.
What can I use instead of a credit card?
Any card built to the same ID-1 standard works: a bank card, a driving licence or a store card. A paper banknote does not work because notes vary in size. If you have a real ruler, you can also use the DPI method by measuring how many pixels fit in one inch.
Does the ruler work on my phone?
Yes. Phones have very high pixel densities, so calibrate carefully with a card and measure short objects such as rings, screws or coins. The ruler scrolls sideways, so longer objects need several measurements added together.
Why does the ruler look different on each device?
Each device has its own pixel density. A 15.6 inch laptop screen is about 141 pixels per inch, while a phone can be over 400. The same inch is drawn with far more pixels on the phone, so the ruler must be calibrated separately on every device you use.
Can I print the ruler?
You can, but printers rescale pages by default, so a printed ruler is only accurate if you print at 100 percent scale with no fit-to-page option. For measuring real objects, the on-screen ruler after calibration is the reliable option.
Does the tool save my measurements or screen details?
No. The ruler runs entirely in your browser. Your calibration, clicks and measurements are never uploaded, stored or shared, and they disappear when you close the page.
What is DPI?
DPI means dots per inch, and on a screen it is the number of pixels packed into one inch. A higher DPI means a sharper screen with smaller pixels. Knowing your screen DPI is what lets this tool turn a pixel distance into inches or centimetres.
More tools you may need
Working out a past investment instead? Try the What If I Had Invested Calculator. Converting units? Our Unit Converter and Metric to Imperial Converter handle length, weight and temperature.
