A full-foot ruler at actual size
The standard school and office ruler is 12 inches (1 foot, 30.48 cm) long. This page opens the ruler in inches along the top of your screen, so you get the longest possible straight edge. On most 15.6-inch laptops and desktop monitors, the whole 12 inches fits across the screen.
Related guides: How to Read a Tape Measure · How to Measure a TV or Monitor Screen Size
How to use it
- Tap Calibrate and match a bank card so each inch is real size.
- Use Fullscreen for the longest ruler, and keep browser zoom at 100%.
- Hold the object along the top edge with one end at 0 and read the length in inches.
Good to know
- 12 inches = 1 foot = 30.48 cm.
- A 15.6-inch laptop screen is about 13.6 inches wide, so 12 inches fits with room to spare.
- Phones and small laptops can't show 12 inches at once; measure in sections instead.
Will 12 inches fit on your screen?
A 12-inch ruler is 1 foot, or 30.48 cm. It fits across a laptop or monitor only if the screen is wide enough. Screens are sold by their diagonal size, so here is the actual width of common screens:
| Screen (diagonal) | Shape | Screen width | Fits 12 in? |
|---|---|---|---|
| 13.3″ | 16:9 | 11.6 in (29.4 cm) | No |
| 13.3″ | 16:10 | 11.3 in (28.6 cm) | No |
| 14″ | 16:9 | 12.2 in (31.0 cm) | Just |
| 15.6″ | 16:9 | 13.6 in (34.5 cm) | Yes |
| 16″ | 16:10 | 13.6 in (34.5 cm) | Yes |
| 24″ | 16:9 | 20.9 in (53.1 cm) | Yes |
- The browser window uses a little of the width, so turn on fullscreen for the most room.
- If an object is longer than your screen, mark where the ruler ends, slide the object along and add the two lengths.
- Things about 12 inches long: a US Letter sheet (11 in on its long side), an A4 sheet (11.7 in) 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