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High-Precision Online Ruler Suite

Measure any physical object using screen edge rulers, 360° protractor, 360° floating ruler, or spawn true-scale silhouettes.

Instant Screen Calibration Enter diagonal or pick preset
Align Physical Card 85.60 × 53.98 mm

Free Online Ruler — Measure in Inches, Centimeters & Millimeters at Actual Physical Size

What is OnlineRuler? OnlineRuler is a free, high-precision browser-based screen ruler that measures physical objects at true 1:1 scale in centimeters (cm), millimeters (mm), fractional inches (in), and pixels (px). By calibrating against standard physical references like a credit card, it bypasses default 96-PPI browser scaling to deliver sub-millimeter measurement accuracy on smartphones, tablets, laptops, and desktop monitors.

Whether you need to measure screws, jewelry, credit card dimensions, phone cases, or calculate angles and 2D surface areas, OnlineRuler provides an entire digital metrology toolkit directly in your browser with zero downloads, zero account requirements, and 100% client-side privacy.

How to Calibrate Your Screen for Exact 1:1 Scale

Default web browsers assume every monitor has 96 pixels per inch (PPI). However, high-density Retina displays, 4K monitors, and mobile OLED screens range from 110 PPI to over 460 PPI. To measure with physical accuracy:

  1. Open Calibration: Click the Calibrate button in the top navigation bar.
  2. Use a Physical Reference Card: Place any standard credit card, debit card, driver's license, or gift card (which adhere to the global ISO/IEC 7810 ID-1 standard of 85.60 mm × 53.98 mm) flush against the right edge of your screen.
  3. Align the Slider: Drag the calibration slider until the on-screen outline matches your physical card width.
  4. Save Settings: Click "Save Card Calibration". Your exact PPI is saved to local storage for future visits.

Metric to Imperial Conversion Quick Reference

Inches (Fractional) Inches (Decimal) Centimeters (cm) Millimeters (mm)
1/16 in0.0625"0.1588 cm1.5875 mm
1/8 in0.125"0.3175 cm3.175 mm
1/4 in0.250"0.635 cm6.35 mm
1/2 in0.500"1.27 cm12.7 mm
1 in1.000"2.54 cm25.4 mm
6 in6.000"15.24 cm152.4 mm
12 in (1 ft)12.000"30.48 cm304.8 mm

Frequently Asked Questions (FAQ)

Can I use my phone as an accurate physical ruler?

Yes. Open OnlineRuler on your iPhone or Android mobile browser, tap Calibrate to align a standard card or choose your phone model, and place small items directly onto your smartphone glass to measure lengths in millimeters, centimeters, or fractional inches.

Is there an online ruler that displays actual 1:1 physical size?

Yes. OnlineRuler calibrates against your monitor's true Pixels Per Inch (PPI). Once calibrated using a credit card or diagonal screen calculation, 1 cm on screen equals exactly 1 physical centimeter in real life.

How do I measure 2D objects and surface areas?

Click the 2D Caliper icon or press the B key. Drag the boundary handles around your object to calculate width, height, diagonal span, and surface area in cm² or in².

How do I measure angles with the 360° protractor?

Click the Angle Protractor icon or press A. Drag the protractor arm to measure acute, obtuse, and reflex angles with live degree, radian, and gradient slope readouts.