Online Ruler (Actual Size)
Measure small objects right on your screen. Calibrate once with a credit card, then use the ruler in inches, centimeters or millimeters.
How to use the Online Ruler (Actual Size)
- Set browser zoom to 100%.
- Hold a standard credit card against the screen and adjust the calibration until it matches 85.6 mm (3.37 in).
- Choose inches or centimeters and the ruler length, then measure.
How it works
Web pages define 1 inch as 96 CSS pixels, but real screens have different pixel densities, so an uncalibrated ruler can be off by 10–30%. A credit card (ISO/IEC 7810 ID-1) is exactly 85.60 × 53.98 mm — use it as a reference to set the calibration percentage. The inch scale has marks every 1/16 inch; the centimeter scale has millimeter marks.
Reference objects
| Object | Size |
|---|---|
| Credit card | 85.6 × 53.98 mm (3.37 × 2.13 in) |
| US quarter | 24.26 mm (0.955 in) diameter |
| US penny | 19.05 mm (0.75 in) diameter |
| US dollar bill | 156 × 66.3 mm (6.14 × 2.61 in) |
| A4 sheet | 210 × 297 mm |
| US Letter sheet | 8.5 × 11 in |
Frequently asked questions
Is an online ruler accurate?
After calibration, typically within about 1 mm over a few inches; for precision, use a physical ruler.
How do I calibrate the ruler?
Match a credit card’s width (85.6 mm) by adjusting the calibration percentage.
Does it work on phones?
Yes, but the screen is short; calibrate on each device separately.
Why is the ruler the wrong size?
Browser zoom is not 100% or the calibration has not been set for your screen.