Actual Size

How to Use the Online Ruler

Last updated: 2026-10-15

This on-screen ruler needs to be calibrated once before it can display real-world measurements accurately.

The easiest method is to use a physical object with a known size. For example, a standard ID-1 bank or credit card is 85.60 mm long. Place the card against the ruler, align the edges, and adjust the calibration handle to match its actual size.

Not calibrated yet? Go to: real-object calibration

Calibrate in 3 Steps

1. Choose a reference object

In the “Reference Object” panel, choose an object you have nearby with a known size.

  • Credit or bank card
  • Passport
  • Coin (e.g. a quarter, €1, or £2)
  • AA or AAA battery
  • A4 paper
  • US Letter paper
  • US dollar or euro banknote

You can also enter a custom size if you have another object with a known measurement.

2. Align the object with the 0 mark

Place the object flat against the screen.

Align its left edge with the ruler’s 0 mark, not with the outer edge of the ruler graphic.

There may be a small amount of visual space before the 0 mark. This is part of the ruler’s layout and is not part of the measurement.

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.

When both ends are aligned, click Calibrate.

The ruler will then calculate the scale for your current screen and display measurements using that calibrated scale.

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 / bank 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?

A browser cannot determine the exact physical size of your screen simply from the number of pixels.

Different monitors, laptops, phones, resolutions, and display settings can cause the same number of screen pixels to represent different physical distances.

That’s why this ruler does not require you to enter a PPI or DPI value.

Using a real object with a known size is usually simpler and less error-prone.

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.

If you change the browser zoom or display setup, calibrate again.

Print a Real-Size Ruler

Need a physical ruler?

Use the printable ruler to create a ruler on A4 or US Letter paper.

When printing, select 100% / Actual Size and disable options such as “Fit to Page,” “Scale to Fit,” or automatic resizing.

After printing, you can cut out the ruler and use it as a physical measuring tool.

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 more questions, see the online ruler FAQ.

Frequently asked questions

Why does the online ruler need calibration?

A browser cannot work out the physical size of your screen from the number of pixels alone. Different monitors, resolutions and display settings mean the same number of pixels can represent different physical distances, so the ruler has to learn the scale from a real object of known size.

Do I need to enter my screen's PPI or DPI?

No. The ruler calibrates by lining up a real object whose size you already know — such as a credit card at 85.60 mm — which avoids the common mistake of entering the wrong DPI value.

Do I have to recalibrate after changing zoom or switching monitors?

Yes. Keep the browser zoom at 100% while calibrating and do not change it afterwards. Recalibrate whenever you switch monitors, change display settings, or change OS scaling.

Can I print a physical ruler instead?

Yes. Use the printable ruler to make one on A4 or US Letter paper, then print at 100% / Actual Size with “Fit to Page” and automatic resizing turned off.