A centimeter ruler on your screen
This page opens the ruler in centimeters, with a small mark for every millimeter and a longer mark at every half centimeter, just like a school ruler. After calibration, 1 cm on screen matches 1 real centimeter.
Related guides: How to Read a Ruler (Inches and Centimeters) · How to Measure Without a Ruler
How to use it
- Tap Calibrate and match a bank card for the best accuracy, or let the ruler detect your device.
- Place the object against the ruler with one end at 0.
- Read the last number passed in cm, then count the small marks after it in mm.
Good to know
- 1 cm = 10 mm ≈ 0.39 inches.
- The longer mark halfway between two numbers is 0.5 cm (5 mm).
- On a computer, hover over the ruler to see the exact position, such as 7.4 cm.
Centimeters and millimeters at a glance
1 cm = 10 mm = 0.3937 in. The numbered lines on this ruler are centimeters, and the small lines between them are millimeters. Everyday objects you can hold against the ruler to check it:
| Object | Size (cm) | Size (inches) |
|---|---|---|
| Bank card | 8.56 × 5.40 | 3.37 × 2.13 |
| US quarter (across) | 2.43 | 0.955 |
| 1 euro coin (across) | 2.33 | 0.915 |
| AA battery (length) | 5.05 | 1.99 |
| A4 sheet | 21.0 × 29.7 | 8.27 × 11.69 |
| US Letter sheet | 21.6 × 27.9 | 8.5 × 11 |
- To convert centimeters to inches, divide by 2.54: 10 cm is 3.94 in.
- Measure from the 0 line, not from the edge of the screen.
- For very small parts, the mm ruler numbers every millimeter.
Three ways to calibrate
Auto-detect your device
The ruler recognizes about 130 phones, tablets, laptops and monitors, including iPhones, Galaxy phones, Pixels and MacBooks.
Enter your screen size
Enter your screen's diagonal size (for example, 15.6" for a laptop), and the ruler works out the scale from your screen resolution.
Calibrate with a bank card
Hold any bank card against the outline on screen and move the slider until they match. Bank cards are the same size everywhere: 85.60 × 53.98 mm.
How accurate is the ruler?
How accurate the ruler is depends on how well it knows your screen's pixel density, meaning how many pixels fit in an inch. There are three ways to set it, from most to least accurate:
Bank card (±0.5%): Bank and credit cards are made to one worldwide size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Because every card is exactly the same, matching one on screen gives the most accurate result.
Auto-detect (±1%): The ruler looks up your device's published screen size and resolution. Screens of the same model can differ very slightly, so this is a little less exact. It covers about 130 devices.
Screen size (±2%): You type in your screen size, and the ruler works out the rest from your resolution. Advertised sizes are rounded (a "15.6-inch" screen may really be 15.59 inches), so this is the least exact method. It's a good fallback when you don't have a card.
Real-life objects to check your measurements
Use these everyday objects to confirm the ruler is calibrated correctly.
Bank or credit card: 85.60 × 53.98 mm (3.37 × 2.13 in)
A4 paper: 210 × 297 mm (8.27 × 11.69 in)
US Letter paper: 8.5 × 11 in (215.9 × 279.4 mm)
US quarter: 24.26 mm (0.955 in) across
1 euro coin: 23.25 mm (0.915 in) across
AA battery: about 50.5 mm long and 14.5 mm wide
More online rulers
Measuring guides
- How to Read a Ruler (Inches and Centimeters)
- How to Measure Without a Ruler
- How to Use a Protractor
- Free Printable Ruler Worksheets with Answer Keys
- How to Show the Ruler in Word
- How to Show the Ruler in Google Docs
- How to Show the Ruler in PowerPoint
- Is an Online Ruler Accurate? How to Calibrate It
- How Big Is 1 cm and 1 mm? Real-Life Comparisons
- How to Measure PD (Pupillary Distance) for Glasses
- Ear Gauge Size Chart: Gauge to mm and Inches
- Bra Size Chart and How to Measure for a Bra
- All guides →