Auto-detect device
Best for common phones, tablets, and laptop displays. The site estimates screen density from device signals and confirms the match before saving it.
1 cm
Online ruler in cm, mm & inches at actual size
Use this free actual-size online ruler in centimeters, millimeters, or inches. Calibrate your screen first so the on-screen scale matches real-world size.
What is an online ruler?
An online ruler is a browser-based measuring tool that shows centimeter, millimeter, and inch marks at their true physical size on your screen. After calibration, you can take quick measurements without a physical ruler, with no download or account. Learn more about how accurate an online ruler is or see the online ruler vs physical ruler comparison.
- Calibrate once with auto-detect, screen diagonal, or a credit card (85.60 mm) for actual-size accuracy.
- Switch freely between cm, mm, and inch units on the same calibrated scale.
- Keep browser zoom at 100% and measure on the same display you calibrated.
Three ways to calibrate.
Use the quickest method that fits your screen, then switch back to the ruler.
Need a deeper walkthrough? Read the online ruler calibration guide or calculate your screen density with the PPI calculator.
Method 1
Auto-detect device
The detector checks common screen classes and applies the closest physical scale it can infer.
No device detection has been applied yet.
Method 2
Screen diagonal
Enter the display diagonal in inches. This is usually listed in laptop, monitor, phone, or tablet specs.
Method 3
Credit card calibration
Place a standard card over the guide and move the slider until the on-screen card matches its physical width.
Standard card size: 85.60 mm by 53.98 mm.
85.60 mm
Built for real measuring, not just a picture of a ruler.
The ruler is drawn from your saved pixels-per-millimeter value, so its scale updates after calibration.
Screen diagonal
Best for monitors and laptops. Enter the display diagonal printed in your device specs and the ruler converts screen pixels into physical units.
Credit card calibration
Best when accuracy matters. Match the on-screen guide to a standard card width of 85.60 mm and save the measured scale.
Measure in centimeters, millimeters, or inches
The same calibrated ruler can switch between metric and imperial units, so the screen scale stays consistent while the measurement labels change.
Centimeter ruler
Use centimeter marks for school work, crafts, margins, desk objects, and quick checks where a 1 cm grid is easiest to read.
Millimeter ruler
Switch to millimeters for small parts, packaging details, jewelry, model pieces, or any measurement that needs finer tick spacing.
Inch ruler
Use inches and sixteenth-inch ticks when you are matching US paper sizes, hardware, fabric, or product dimensions.
Need a longer flexible scale? Try the online measuring tape in inches and cm.
New to ruler marks? Learn how to read a ruler in inches, centimeters, and millimeters, use the cm to inches converter, check the ruler measurement converter, or browse all measurement guides.
Use it for quick desk measurements.
Simple Online Ruler is useful for checking small objects, paper margins, craft cuts, screen mockups, packaging sizes, and school measurements when a physical ruler is not nearby.
- Calibrate on the same screen you will use for measuring.
- Keep browser zoom at 100% after calibration.
- Use credit card calibration when you need the most dependable actual-size scale.
- Recalibrate after changing displays, browser zoom, or operating-system display scaling.
How to use the online ruler
01
Align the object with zero
Place the object against the zero mark and keep it straight while reading the scale.
02
Calibrate once
Auto-detect, enter your screen diagonal, or match the credit-card guide.
03
Measure at 100%
Keep browser zoom unchanged and place the object against a selected edge.
Online ruler FAQ
How accurate is an online ruler?+
An online ruler can support quick everyday checks after you calibrate it on the same display and at the same browser zoom. Match the on-screen guide to a known physical reference, such as an 85.60 mm ID-1 card, then independently verify any measurement that matters.
How do I calibrate a screen ruler?+
Set browser zoom to 100%, then use one of three methods: auto-detect your device, enter your screen diagonal in inches, or match the on-screen guide to a standard credit card (85.60 mm wide). Save the calibration and measure on the same display.
What is the best online ruler for actual-size measurements?+
The best online ruler offers multiple calibration methods, supports centimeters, millimeters, and inches, and makes its limitations clear. Simple Online Ruler provides credit-card, screen-diagonal, and auto-detect setup options; verify the result with a physical reference before relying on it.
Is an online ruler as accurate as a physical ruler?+
A physical ruler avoids the display-scaling and browser-zoom variables that affect an online ruler. A calibrated screen ruler can help with quick checks, but use a suitable physical measuring instrument whenever the required tolerance or consequence of an error matters.
Can I use this as a millimeter ruler?+
Yes. Select mm in the unit control. The ruler still shows major centimeter marks, but the measurement readout and tick spacing are based on millimeters.
Why does browser zoom affect an actual-size ruler?+
Browser zoom changes the size of CSS pixels on the screen. A real-size ruler should be calibrated and used at 100% zoom on the same display.
What is screen PPI and why does it matter for an online ruler?+
PPI (pixels per inch) is how many CSS pixels fit in one physical inch on your screen. Online rulers need this value to draw tick marks at true size. You can calculate PPI from your screen resolution and diagonal, or let calibration determine it automatically.
What credit card size is used for calibration?+
The calibration guide uses the ISO/IEC 7810 ID-1 card size: 85.60 mm wide by 53.98 mm tall.
Can I use this online ruler on a phone?+
Yes. Phone screens can work well after calibration, especially if you use the credit card method and keep the phone browser at its default zoom.
Is this actual-size ruler free?+
Yes. Simple Online Ruler runs in the browser and does not require a download or account.