Online Ruler (cm, inch, px)
Measure on-screen lengths in centimeters, inches, or pixels. Calibrate with a credit card, A4 paper, US letter, or a custom length to match your screen. Calibration is saved locally in your browser.
Core Features
Made for on-screen measuring
Switch between cm/in and pixels, add guides, and calibrate when you need real-world scale.
- Screen Calibration
Use a known reference (card, paper size, or custom length) to adjust PPI for your screen. Recalibrate after zoom or display changes.
- cm / inch / px
Measure in centimeters, inches, or pixels. You can also show physical units and pixels together.
- Guides & Selection
Drag guidelines to check spacing, or draw a selection box to read width, height, diagonal, and area.
- Local by Default
No account needed. Calibration and preferences are stored in your browser (local storage).
About The Tool
What is Online Ruler?
Online Ruler is a screen ruler for quick on-screen measuring. It can show pixels, and it can estimate centimeters or inches after you calibrate your device. Without calibration, cm/in values use a default PPI and may be off.
Simple Steps
How to Use
- 1Calibrate
Open Calibrate and pick a reference (credit card, A4 paper, US letter, or custom length).
- 2Adjust
Adjust until the on-screen outline matches the real object. Keep browser zoom at 100%.
- 3Measure
Measure in cm, inches, or pixels. Use guides or the selection tool to check distances and sizes.
Benefits
Why Use This Ruler?
- Closer to Real Size
Calibration helps convert pixels to cm/in for your current device. Recalibrate after switching screens or changing zoom.
- Instant Access
No installation needed. This online ruler works on desktop, tablet, and mobile browsers.
- Privacy Focused
Calibration and settings stay on your device. The tool works without sending your measurements anywhere.
Measurement method
How the screen ruler calculates size
The ruler reads distances in CSS pixels. Real-world units are estimates derived from the effective PPI you set by matching a physical reference on the same screen.
CSS pixels and formula
A px reading means CSS pixels, not a count of physical display pixels. The conversion is: cm = CSS px Ă· effective PPI Ă— 2.54.
Physical-reference calibration
Match the outline to an ID-1 card (85.60 mm wide), A4 or US Letter paper, or a length you know. This accounts for the current browser and display setup better than a hardware PPI preset.
Limits and privacy
Zoom, OS scaling, monitor changes and orientation can change the result, so recalibrate after changes. Measurement coordinates, selections and calibrated PPI stay in your browser.
Method references: W3C CSS Values and Units · ISO/IEC 7810 ID-1
Everyday uses
What can you measure with it?
It is most useful for quick checks on the screen. Calibrate first when you want an estimate in cm or inches.
UI spacing
Check the width of a button, card, margin, or gap while reviewing a page.
Images on screen
Read the displayed width and height of an image without saving or uploading it.
Small objects
Estimate the size of a coin, card, label, or screw placed carefully against the screen.
Print and design checks
Compare a layout or print reference at an approximate real-world size before using a physical ruler.
FAQ
Frequently Asked Questions
- It can be close after calibration. Without calibration, the cm/in display uses a default PPI and may be off. Keep browser zoom at 100% and recalibrate after display changes.
- Centimeters (cm), millimeters (mm), inches (in), and pixels (px). You can also show physical units and pixels together.
- Yes. It works on smartphones and tablets—calibrate on that device, and avoid browser zoom if you want cm/in to match your reference.
- Calibration and settings stay in your browser (local storage). The tool does not require you to upload images or measurement data.