How to Use an Online Ruler — Free Screen Ruler (2026)
By Rui Barreira · Last updated: 18 June 2026
You can measure objects on your screen with pixel-accurate precision using the brevio Online Ruler — free, no signup, runs entirely in your browser. Enter your screen diagonal size to calibrate it, then click to place a measurement marker anywhere on the ruler.
How to Use the Tool
- Open the brevio Online Ruler. No account required.
- Enter your screen diagonal in inches. This is the measurement from corner to corner of your screen. Common sizes: 13.3" (MacBook Air), 15.6" (most laptops), 27" (desktop monitors). You can find this in your device specifications.
- Choose your unit — centimetres or inches.
- Hold an object up to the screen and click on the ruler to place a red measurement marker. The measurement appears in real-time.
How Screen Calibration Works
Digital rulers must account for two unknowns: the physical size of your screen and the pixel density (pixels per inch, or PPI). The tool uses your reported screen resolution (from window.screen.width and window.screen.height) combined with the diagonal you provide to calculate PPI using the Pythagorean theorem: PPI = √(w² + h²) / diagonal.
This calculation is precise — a 1920×1080 display with a 15.6" diagonal gives 141 PPI. Once PPI is known, any pixel position on the ruler maps directly to a physical measurement. The most common calibration error is entering the wrong diagonal, particularly confusing the screen diagonal with the monitor frame diagonal, which is slightly larger.
Common Screen Diagonal Sizes
Smartphones typically range from 5.5" to 6.9". Tablets range from 9.7" to 13". Laptops are most commonly 13.3", 14", 15.6", and 16". Desktop monitors span from 21.5" to 34" for ultrawide models. If you are unsure, check your device's product page or settings — search for "display specifications" in your system settings.
Frequently Asked Questions
- How accurate is this ruler?
- Accuracy depends on the precision of the screen diagonal you enter. With the correct diagonal, measurements are typically accurate to within 0.5mm. Retina/HiDPI displays (where the device pixel ratio is not 1) may introduce a small offset — the tool reads the logical resolution, not physical pixels.
- Why does my ruler look bigger/smaller than a real ruler?
- The most likely cause is an incorrect screen diagonal. Double-check the number against your device's technical specifications. Browser zoom settings (anything other than 100%) also affect accuracy.
- Can I use this on a phone?
- Yes, though phone screens vary widely in PPI. Enter your phone's diagonal from the manufacturer's specifications for best results.
- Is this free to use?
- Yes, completely free with no signup required. All measurements happen in your browser — no data is sent to any server.
Frequently Asked Questions
- How accurate is this ruler?
- Accuracy depends on the precision of the screen diagonal you enter. With the correct diagonal, measurements are typically accurate to within 0.5mm. Retina/HiDPI displays may introduce a small offset.
- Why does my ruler look bigger/smaller than a real ruler?
- The most likely cause is an incorrect screen diagonal. Double-check the number against your device's technical specifications. Browser zoom settings (anything other than 100%) also affect accuracy.
- Can I use this on a phone?
- Yes, though phone screens vary widely in PPI. Enter your phone's diagonal from the manufacturer's specifications for best results.
- Is this free to use?
- Yes, completely free with no signup required. All measurements happen in your browser — no data is sent to any server.