Calibrated for Accuracy
Easily calibrate using a standard card or custom object to be used on any display.
Multi-Unit Support
Switch seamlessly between Centimeters and Inches with a single click.
Mobile Ready
Fully responsive design that works perfectly on phones, tablets, and desktop monitors.
Mastering Precision
Get the most accurate measurements by following these simple steps.
1. Choose Calibration
You can calibrate using a standard Credit Card OR by entering your Monitor Size (diagonal inches). Both methods ensure 100% precision.
2. Calibrate
Card Mode: Place card on screen and adjust box width.
Monitor Mode: Simply enter your screen size (e.g., 24 inch) for auto-calibration.
3. Measure & Customize
Drag the ruler, switch Units (cm/inch), flip Tick Marks, or change Position (Horizontal/Vertical) using the settings menu.
The Secret of Accuracy: Calibration & Pixels
Many users ask, "Why does the online ruler look different on every monitor?" This is due to the relationship between digital 'pixels' and physical 'length'.
1. Pixels are not absolute length
While centimeters and inches are absolute units, pixels are relative. The physical size of 100 pixels on a 24-inch monitor is completely different from 100 pixels on a 6-inch smartphone.
2. Screen Density, PPI (Pixels Per Inch)
PPI is the number of pixels in one inch. Higher PPI means pixels are packed more densely. Since every monitor has a different PPI, accurate measurement is impossible without calibration.
3. The Math Behind Calibration
When you calibrate with a credit card, RulerHero calculates how many pixels that physical length (3.37 inch) occupies on your screen to find your exact PPI.
4. Why use a Credit Card?
Credit cards follow the international ISO/IEC 7810 standard (85.60mm × 53.98mm), making them the perfect universal reference object with minimal error.
Standard Dimensions & Reference Tables
Look up exact physical dimensions for credit cards, ISO paper standards, coins, and screen PPI values.
| Object Name | Standard | Millimeters (mm) | Centimeters (cm) | Inches (in) |
|---|---|---|---|---|
| Standard Credit / Debit Card | ISO/IEC 7810 ID-1 | 85.60 × 53.98 mm | 8.56 × 5.40 cm | 3.37 × 2.12 in |
| Driver's License / ID Card | ISO/IEC 7810 ID-1 | 85.60 × 53.98 mm | 8.56 × 5.40 cm | 3.37 × 2.12 in |
| East Asia Business Card | Standard Business Card | 90.00 × 50.00 mm | 9.00 × 5.00 cm | 3.54 × 1.97 in |
| US Business Card | US Standard | 88.90 × 50.80 mm | 8.89 × 5.08 cm | 3.50 × 2.00 in |
Measurement Knowledge Base & Guides
Essential guides on screen PPI calculation, paper dimensions, and reference object standards.
Complete Guide to Screen PPI & Pixel Density Calculation
Standard Paper Sizes Guide: A4, A3, B5, Letter in cm & Inches
Physical Dimensions Reference Table: Credit Cards, Coins & Badges
Step-by-Step Guide to Calibrating and Using an Online Screen Ruler
Frequently Asked Questions
Find answers to common questions about online screen rulers, PPI calibration, and measurement accuracy.