About RulerInches
We built RulerInches because we kept reaching for a physical tape measure that wasn't nearby—and every online ruler we tried gave inaccurate measurements. Standard web code operates in virtual software pixels, assuming every display has 96 pixels per inch (PPI). That legacy default originated in the 1990s for CRT desktop monitors. Modern displays vary widely: a 24-inch monitor has ~92 PPI, an iPad Pro has ~264 PPI, a MacBook Air has ~224 PPI, and an iPhone 15 Pro has ~460 PPI. Using 96 PPI on a modern phone means an on-screen "inch" actually measures under half an inch in physical reality.
RulerInches solves this problem by combining hardware-level display auto-detection with ISO 7810 credit card calibration, scaling virtual tick marks so that one inch on screen corresponds to one physical inch.
Our Engineering Methodology
RulerInches uses a dual-engine calibration architecture:
- Hardware Auto-Detection — When you open the tool, our script analyzes your display resolution, device pixel ratio (`window.devicePixelRatio`), screen aspect ratio, and hardware signatures against our database of smartphone, tablet, and laptop profiles. For recognized hardware, the exact native PPI is applied instantly without user input.
- ISO 7810 Card Calibration — For custom desktop monitors or unrecognized displays, our tool provides manual credit card calibration. Payment cards follow the international ISO/IEC 7810 ID-1 standard (85.60 mm × 53.98 mm). Simply align a credit card against the screen handles, and the tool calculates your display's exact physical PPI.
Privacy-First Architecture
RulerInches runs entirely inside your browser using client-side JavaScript and HTML5 Canvas.
- Zero Server Data Collection — We do not track your measurements, harvest device fingerprints, or store personal telemetry on external servers.
- Local Sandbox Storage — Calibration settings are saved exclusively in your browser's local sandbox (`localStorage`) so your settings persist across visits without requiring an account.
- 100% Free & Offline Ready — No sign-up, no app store download, no permissions required. Once loaded, the ruler works offline.
Get in Touch
Have a question, found a bug, or want to suggest a new device profile? Reach out via our Contact Page or send an email directly to contact@rulerinches.com.