How to Use an Online Ruler (and Get an Accurate Measurement)

An online ruler turns your screen into a measuring tool, but only if you set it up properly. Here's how to go from "roughly right" to accurate within a millimeter.

1. Reset your browser zoom

Zoom stretches everything on the page, including the ruler. Press Ctrl + 0 on Windows or Chromebook, or ⌘ + 0 on a Mac, so the page is at 100%.

2. Calibrate your screen

Browsers don't know how big your screen physically is, so an uncalibrated ruler can be 10–40% off. Open the calibration panel and choose a method:

• Auto-detect: quickest on popular iPhones, iPads, Pixels, Galaxies and MacBooks.
• Pick your device: precise when your model is in the list.
• Screen diagonal: type the exact size from your spec sheet.
• Credit card: the most accurate on any screen. Resize the outline until it matches a real card (85.60 × 53.98 mm).

3. Check it with a real object

Use the reference object checker: put a coin or card on its outline. If it fits exactly, you're calibrated.

4. Align the object at zero

Place one end of the object on the 0 mark, which is the ruler's left edge, not the screen bezel. Hold it flat and look straight down to avoid parallax.

5. Read the measurement

Read where the other end falls, or tap that point on the ruler to get an exact readout in centimeters, millimeters, inches and fractions. For long or diagonal distances, open full screen (press F) and drag from end to end.

6. Use guides for tricky shapes

Turn on an edge ruler, then click the ruler to drop a dashed guide line. Drop one at each end of the object and the tool shows the distance between them.

Common mistakes

• Measuring at 90% or 110% zoom.
• Calibrating on one monitor, then measuring on another.
• Pressing hard on the screen. Rest objects gently.
• Starting at the screen edge instead of the 0 tick.