A full 30 cm ruler at actual size
The 30 cm ruler is the standard long ruler in schools and offices around the world. This page opens a centimeter ruler across the top of your screen for the longest straight edge. Most 15.6-inch laptops and desktop monitors are wide enough to show the full 30 cm.
Related guides: How to Measure Without a Ruler
How to use it
- Tap Calibrate and match a bank card so each centimeter is real size.
- Use Fullscreen for the longest ruler, with browser zoom at 100%.
- Lay the object along the top edge with one end at 0 and read the length in cm and mm.
Good to know
- 30 cm = 300 mm ≈ 11.8 inches.
- A 15.6-inch laptop screen is about 34.5 cm wide, so the full 30 cm fits.
- An A4 sheet is 29.7 cm long, just under the length of a 30 cm ruler.
Will 30 cm fit on your screen?
30 cm is 11.81 inches. It fits across most laptop screens of 14 inches and up. Screens are sold by their diagonal size, so here is the actual width of common screens:
| Screen (diagonal) | Shape | Screen width | Fits 30 cm? |
|---|---|---|---|
| 13.3″ | 16:9 | 29.4 cm (11.6 in) | No |
| 13.3″ | 16:10 | 28.6 cm (11.3 in) | No |
| 14″ | 16:9 | 31.0 cm (12.2 in) | Just |
| 15.6″ | 16:9 | 34.5 cm (13.6 in) | Yes |
| 16″ | 16:10 | 34.5 cm (13.6 in) | Yes |
| 24″ | 16:9 | 53.1 cm (20.9 in) | Yes |
- The browser window uses a little of the width, so turn on fullscreen for the most room.
- On a smaller screen, measure in two steps: mark where the ruler ends, slide the object along and add the lengths.
- Things about 30 cm long: an A4 sheet (29.7 cm) and a standard school ruler.
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