Frequently Asked Questions
Find answers to common questions about our service
1How accurate is the online ruler?
Accuracy depends on calibration and zoom. Calibrate with a known reference (ID-1 card, A4 short edge, US Letter width, or a custom length) and keep browser zoom at 100% for the closest 1:1 scale. For critical work, verify with a physical ruler—screens and scaling can vary.
2Why does the scale change when I zoom or change display settings?
Browser zoom and OS display scaling change the on-screen pixel size, which breaks 1:1 measurements. Set zoom to 100%, avoid changing display scaling, and recalibrate after switching monitors, resolutions, or orientation.
3What units are supported?
You can measure in centimeters (cm), millimeters (mm), inches (in), and pixels (px). Pixel (px) readings are always available; cm/mm/in readings are based on your calibrated PPI. Switching units updates instantly.
4Can I convert pixels to inches or centimeters?
Yes, after calibration. Once your PPI is calibrated, px measurements can be estimated as inches or centimeters at a closer real-world scale. If you only need screen dimensions, px works without calibration.
5What are Guides and the Selection tool?
Guides add draggable lines for aligning edges on your screen. The Selection tool lets you drag a rectangle and read width, height, diagonal, and area in your selected units.
6Why calibrate with a physical reference instead of a device PPI preset?
Hardware PPI does not reliably map to CSS pixels because browser zoom, operating-system scaling, and devicePixelRatio affect the page scale. Match a real card, paper edge, or known length on the same screen instead.
7How do I check that browser zoom is 100%?
Open your browser menu and look for Zoom. Set it to 100% before calibrating. On most desktop browsers, Ctrl+0 on Windows or Command+0 on Mac resets the page zoom.
8Do I need to change Windows or macOS display scaling?
No. Leave your normal display setting in place and calibrate the ruler in that setup. If you later change display scaling, resolution, or browser zoom, calibrate again.
9Which calibration reference should I choose?
A bare ID-1 bank or ID card is easiest on phones and small screens. A4 and US Letter work better on a large monitor. Use Custom when you have a physical object with a length you already know.
10Should I recalibrate after rotating my phone or tablet?
Check the scale after rotation. Some browsers change the relationship between CSS pixels and the screen when orientation changes. If the reference no longer matches, calibrate again.
11What if the blue reference shape will not match?
First confirm 100% browser zoom. Then choose a smaller reference, remove any card case, and make sure the physical object is on the same display as the browser window. If you changed monitors or display settings, start a fresh calibration.
12Can I use this for manufacturing, medical, or safety-critical work?
No. A screen ruler is a convenient estimate, not a certified measuring instrument. For cutting, machining, medical sizing, construction, jewelry, or any task where an error matters, verify with an appropriate physical tool.
13Does this work on mobile?
Yes. It works in mobile browsers. For best results, calibrate on the same device (PPI differs per screen), keep zoom at 100%, and recalibrate after rotation or display setting changes.
14Is my data private?
Yes. The ruler runs locally in your browser. Calibration and preferences are stored on your device (local storage) so you don’t have to recalibrate every visit, and the app doesn’t upload images or measurement data.
15Why isn't my calibration saved?
Calibration is saved in your browser's local storage. It can reset if you clear site data, use private/incognito mode, block storage, or open the site in a different browser or profile.
Still have questions?
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