Turn your phone into a ruler
Your phone is always with you, so it makes a handy ruler. Open this page in your phone's browser and a ruler appears along the side of the screen. It detects about 130 phone and tablet models, including iPhone, Samsung Galaxy, Google Pixel, Xiaomi and OnePlus, and you can fine-tune it with any bank card.
Related guides: How to Measure Without a Ruler · Is an Online Ruler Accurate? How to Calibrate It
How to use it
- Open this page on your phone and give it a moment to detect your model.
- For the best accuracy, tap Calibrate and match a bank card placed on the screen.
- Lay the object on or against the screen next to the ruler and read the size.
Good to know
- Most phones show about 13–15 cm (5–6 inches) along the long edge.
- Turn on fullscreen to hide the browser bar and gain a little more length.
- Your calibration is saved in your browser, so the ruler is ready next time.
Getting accurate results on a phone
A phone screen is small but very sharp, so it makes a precise ruler once it knows its own size. The longest ruler you can show depends on the screen:
| Phone screen (diagonal) | Long side | Good for |
|---|---|---|
| 5.4″ | 12.5 cm (4.9 in) | Rings, coins, screws, SIM cards |
| 6.1″ | 14.1 cm (5.5 in) | Cards, batteries, small parts |
| 6.7″ | 15.5 cm (6.1 in) | Anything up to 15 cm / 6 in |
| 6.9″ | 15.9 cm (6.3 in) | Anything up to 15 cm / 6 in |
- Auto-detect covers recent iPhone, Samsung Galaxy, Google Pixel, Xiaomi and OnePlus models. If the status shows the wrong phone, pick yours from the list or calibrate with a bank card.
- Keep the page at normal size: pinch-zooming makes the ruler larger than real life.
- Lay the object flat on the screen and look straight down at the mark.
- For longer items, use the 30 cm ruler on a laptop or print a 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