Actual Size

How to Use the Online Ruler

Last updated: 2026-10-18

Calibrate the ruler once with a known-size object, then follow these steps to mark, measure, and print.

Not calibrated yet? Go to: the online ruler

Calibrate in 4 Steps

1. Choose a reference object

In the “Reference Object” panel, choose a known-size item or enter a custom size. See recommended reference objects below.

2. Align the object with the 0 mark

Place the object flat against the screen.

Align its edge with the ruler’s 0 mark, not the outer edge of the ruler graphic; the 0 mark is the actual starting point.

3. Drag the green calibration handle

Drag the green calibration handle until it lines up with the other edge of your reference object.

4. Click Calibrate

When both edges are aligned, click Calibrate. The ruler calculates your screen's scale and displays measurements at true size, ready to use.

What Can You Use for Calibration?

The best reference objects are rigid, flat, and manufactured to a known standard.

Recommended reference objects

These are generally convenient and easy to verify:

  • Credit or debit card: ISO/IEC 7810 ID-1 — 85.60 × 53.98 mm
  • Passport: ICAO ID-3 — 125 × 88 mm
  • AA battery: 50.5 mm long, 14.5 mm diameter
  • AAA battery: 44.5 mm long
  • A4 paper: 210 × 297 mm
  • US Letter paper: 215.9 × 279.4 mm
  • CD / DVD: 120 mm diameter

Banknotes

Banknotes can also be used as reference objects, but make sure you select the correct country, currency, and denomination.

Banknote dimensions vary between countries, and in some currencies they also vary between denominations.

For example, US banknotes use a common physical size, while euro banknotes increase in size with denomination.

Coins

Coins can work, but they are less convenient than cards, batteries, or paper because their dimensions vary by country and currency.

Examples:

  • US quarter: about 24.26 mm
  • US penny: about 19.05 mm
  • €1 coin: 23.25 mm
  • €2 coin: 25.75 mm
  • UK £1 coin: about 23.43 mm
  • UK £2 coin: about 28.4 mm

Always make sure you are using the correct coin and currency.

Approximate references only

These objects are useful for rough comparison, but their dimensions can vary:

  • Smartphones
  • Business cards
  • Bottle caps
  • Playing cards

For better calibration, use a standardized object whenever possible.

Why Does the Ruler Need Calibration?

Browsers cannot infer physical screen size from pixels alone, and display settings vary. Aligning a known-size object sets the scale directly, so there is no need to enter a PPI or DPI value.

Once you calibrate the ruler, it calculates the screen’s measurement scale from your reference object. Millimeters, centimeters, and inches are then displayed using that calibrated scale. If you need to convert between centimeters and inches, use the cm to inches converter.

Tips for Better Accuracy

For more consistent measurements:

  • Keep browser zoom at 100% when calibrating.
  • Do not change the browser zoom after calibration.
  • Recalibrate when switching to another monitor.
  • Recalibrate after changing display or scaling settings.
  • Keep the reference object flat.
  • Do not bend or press the object against the screen.
  • Use a rigid object with a clearly defined edge whenever possible.

Print a Real-Size Ruler

Need a physical ruler?

Create an A4 or US Letter printable ruler and print at 100% / Actual Size with page fitting disabled. Check the test square before using it.

Using Mark and Measure

The online ruler also includes two simple measurement helpers.

Mark

Place a marker at any position on the ruler.

This is useful for remembering a point or marking the start and end of an object.

Measure

Select two points on the ruler and the tool will calculate the distance between them in your selected unit.

Both tools require calibration first.

Fullscreen Measuring Mode

Need to measure something larger than the ruler itself?

Fullscreen Mode places rulers along all four edges of the screen, turning the entire display into a larger measuring area.

It can be useful for checking:

  • Larger objects
  • Images
  • Windows
  • Documents
  • Content displayed on a touchscreen

Common Mistakes

Measuring from the edge of the ruler graphic

Do not use the outer edge of the ruler as the starting point.

Always align your object with the 0 mark.

Changing the browser zoom

If you calibrate at 100% and later change the browser zoom to 110%, 125%, or another value, the original calibration may no longer match.

Recalibrate after changing the zoom.

Using an object with an uncertain size

Phones, bottle caps, business cards, and similar objects may vary by model, manufacturer, or country.

Use a standardized object whenever possible.

Treating it as a precision instrument

An online ruler is useful for everyday measurements and quick size checks, but it is not a caliper.

For tight tolerances, machining, engineering, or other precision work, use a physical ruler, caliper, or other appropriate measuring instrument.

For quick answers, see the online ruler FAQ.

Sources

The information on this page is based on published standards and reference works. For verification: