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Credit Card Calibration

Drag a slider until the on-screen card matches your physical card. Your screen's PPI is saved permanently — calibrate once, measure forever.

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Actual Size Objects

View credit cards, passports, coins, and paper at their exact physical dimensions — based on ISO and international standards.

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Full-Screen Ruler

Expand a ruler to fill your entire screen width. Perfect for measuring string, thread, or ribbon for wrist/ring/neck size.

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Inline Unit Converter

Convert between mm, cm, inches, and feet instantly below every object. Quick presets for common sizes save time.

How to Use the Online Screen Ruler

Step 1: Calibrate Your Screen (One-Time)

Every monitor has a different pixel density — a 27-inch 4K display has a much higher PPI than a 24-inch 1080p monitor. To render objects at their true physical size, we need to know your screen's exact PPI. The most accurate method is the credit card drag method:

  1. Have any credit card, debit card, or ID card nearby (they all follow the same ISO standard: 85.6 × 53.98 mm)
  2. Place it flat on your screen next to the on-screen card shown in the calibration wizard
  3. Drag the slider left or right until both cards are exactly the same size
  4. Click “This matches my card” — done! Your calibration is saved forever

Step 2: Choose an Object to Display

After calibration, select from the object library: credit card, passport, coins, or paper sizes. Each object renders at its exact physical dimensions on your screen, complete with a millimeter ruler overlay along its edges. You can rotate cards between landscape and portrait orientation.

How to Measure Your Wrist Size at Home

One of the most popular use cases for this tool. Here's how to find your exact wrist circumference without a measuring tape:

  1. Click Full-Screen Ruler to expand the ruler across your entire screen
  2. Cut a thin strip of paper (about 1cm wide) and wrap it snugly around your wrist
  3. Mark the point where the strip overlaps with a pen
  4. Lay the strip flat on your screen, aligning the start with the 0mm mark
  5. Read the measurement at your pen mark — that's your wrist circumference

This technique also works for ring size (finger circumference), necklace length, head circumference for hat sizing, and even waist measurement for fashion.

Who Is This Tool For?

Anyone who needs a quick measurement and doesn't have a ruler handy. Common users include:

  • Online shoppers checking if a watch face is 40mm or 42mm before buying
  • Jewellery buyers finding their ring or bangle size at home
  • Students and teachers needing a quick measurement in class or homework
  • Crafters and DIY enthusiasts estimating fabric or paper dimensions
  • Anyone wondering how big an object is before they order it online

Why Credit Cards Are the Perfect Calibration Reference

Every credit card, debit card, ATM card, and government ID card in the world follows the ISO/IEC 7810 ID-1 standard, which specifies their dimensions as exactly 85.60 × 53.98 mm (3.370 × 2.125 inches). This international standard is enforced by banks and payment networks worldwide, making your everyday card the most universally available, perfectly consistent physical measurement reference you'll ever find.

Limitations to Be Aware Of

  • Screen quality matters: Very old or budget screens with inconsistent pixel density may show slight inaccuracies
  • Calibration is per-monitor: If you use multiple monitors, calibrate on each one separately
  • Physical placement: For best accuracy, objects should be placed flat on the screen surface, not held at an angle
  • Not a legal instrument: This tool is for everyday convenience, not official or medical measurement

Standard Object Sizes Reference

All objects in this tool follow internationally recognised standards:

  • Credit/Debit Card: 85.6 × 53.98 mm (ISO/IEC 7810 ID-1)
  • Passport: 125 × 88 mm (ICAO Doc 9303)
  • A4 Paper: 210 × 297 mm (ISO 216)
  • US Letter: 215.9 × 279.4 mm (ANSI standard)
  • US Quarter: 24.26 mm diameter (US Mint standard)
  • 1 Euro Coin: 23.25 mm diameter (EU standard)

Frequently Asked Questions

How accurate is this online ruler?

When calibrated carefully with a physical credit card, accuracy is typically within 0.5mm. The precision depends on how carefully you matched the calibration slider to your physical card.

Do I need to download anything?

No. This tool runs entirely in your browser using JavaScript. No installation, no signup, no download needed.

Does my calibration save between visits?

Yes. Your calibration (screen PPI) is saved in your browser's localStorage. You only need to re-calibrate if you switch to a different monitor.

How do I measure my wrist with this?

Use the Full-Screen Ruler mode. Wrap a strip of paper around your wrist, mark the overlap, then lay it on the ruler to read the circumference in mm or cm.

Will it work on mobile phones?

Yes, the tool is responsive and mobile-friendly. Calibrate using the same credit card method, then use the full-screen ruler for measurements.

Is my data private?

Completely. No data is uploaded. Everything runs locally in your browser. Nothing is ever sent to a server.

Why does a credit card work for calibration?

All credit and debit cards worldwide follow ISO/IEC 7810 ID-1 standard: exactly 85.6mm wide. This makes them a perfect universal reference object.

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