Frequently Asked Questions
How accurate is this ruler?
Accuracy depends on your calibration method and screen quality. Object-match calibration and device picker give the highest consistency, while screen diagonal gives medium consistency and auto-detection is only a starting estimate. The confidence badge in the toolbar shows your current consistency level.
Why do I need to calibrate?
Every screen has a different pixel density. A 100-pixel line might be 5 mm on one screen and 8 mm on another. Calibration determines your screen's exact pixels-per-inch (PPI) so we can draw accurate measurements.
Which calibration method should I use?
Object-match is the most reliable method because you're directly comparing a physical object to its on-screen representation. If you have a credit card or A4 paper handy, use that. Device picker is the next best option if your device is in our database.
Does browser zoom affect accuracy?
Yes. Browser zoom changes the effective pixel density, which can throw off measurements. The online ruler detects zoom changes and shows a warning banner. If you see the warning, recalibrate for best results.
Is my data saved anywhere?
No. Everything runs locally in your browser. Your calibration profile, unit preference, and theme setting are stored in localStorage — they never leave your device.
What objects can I use for calibration?
We support four standard reference objects:
- Credit/debit card — 85.6 × 53.98 mm (ISO/IEC 7810 ID-1)
- US dollar bill — 156 × 66.3 mm
- A4 paper — 210 × 297 mm (ISO 216)
- US Letter paper — 215.9 × 279.4 mm
Can I use this on my phone?
Yes. The online ruler works on phones, tablets, and desktops. It's a responsive tool that adapts to any screen size. You can also install it as a PWA (Progressive Web App) for offline use.
What is the command bar?
Press / to open the command bar. You can type natural commands like:
show 3cm— see how big 3 cm is on your screencompare 2in to 5cm— compare two measurementsconvert 100mm to in— convert between units
How do guide lines work?
Click the guide lines button in the toolbar (or press G), then click anywhere on the screen to drop a reference line. Drag to reposition, double-click to remove, or press Esc to clear all.
What does the rectangle tool do?
The rectangle tool lets you measure width, height, diagonal, and area of an object placed on your screen. Enable it with the toolbar button (or press R), then click and drag to draw a measurement box.