Online Ruler
A precise, calibratable virtual ruler that measures real-world objects directly on your screen. Calibrate once using your screen size, a card, a coin, or a sheet of paper, then measure in true centimeters or inches.
Hover to see a live reading, or click and drag along the ruler to measure a span. Calibrate above for true real-world accuracy on your specific screen.
Recommended Settings
Pro Tips
- •Every browser assumes 96 pixels equal one inch by default, regardless of your screen's actual pixel density - that's why calibration is essential for a physically accurate ruler
- •Calibrating by screen size is more accurate and convenient than matching a physical object, since it uses your device's exact resolution rather than how precisely you can eyeball a match
- •Calibration is saved in your browser and remembered next time you visit this tool on the same device
- •If you change browser zoom level after calibrating, measurements will be off - reset to 100% zoom before measuring, or recalibrate at your preferred zoom level
Most Popular
Most users calibrate once by screen size, then switch to inches or centimeters depending on what they're measuring
When to Use This Tool
Quickly check the size of a small item without getting up to find a physical ruler.
Compare a product's listed dimensions against a real-world reference before buying.
Get an approximate measurement for schoolwork, crafts, or quick sketches.
Check that an image or design element renders at its intended physical size.
How It Works
Calibrate once by selecting your screen's diagonal size, or by matching a card, coin, or sheet of paper to a resizable box, which computes exact CSS pixels per real-world inch for your specific screen
The ruler renders tick marks and labels scaled to that calibration, so 1 cm on screen equals 1 real-world cm
Hover for a live position readout, or click and drag to measure the exact distance between two points
100% Private
Files never leave your device. All processing happens locally in your browser.
Lightning Fast
Powered by Client-side pixel calibration using screen resolution or a physical reference object for optimal performance on modern browsers.
Open Source
Built with verified, open-source libraries. Fully transparent.
Frequently Asked Questions
Is an online ruler actually accurate?
Only after calibration. Browsers don't expose your screen's real pixel density, so an uncalibrated ruler is just a rough estimate based on a 96 DPI assumption. Once you calibrate using your screen size, a card, a coin, or a sheet of paper, this ruler is accurate to within a millimeter or so on most displays.
Which calibration method is most accurate?
Screen size is generally the most accurate and easiest, since it's calculated from your device's exact pixel resolution rather than relying on you visually matching a box to an object. Use the card, coin, or paper method instead if you don't know your screen's diagonal size - a larger object (like a sheet of paper) tends to give a more precise match than a small one, since tiny alignment errors matter less at larger sizes.
Why do I need to recalibrate after changing browser zoom?
Zooming in or out changes how many CSS pixels fit on your screen, which changes the effective pixel density the ruler is drawn at. Reset your browser zoom to 100% before measuring, or recalibrate at whatever zoom level you plan to use.
Does this work on my phone or tablet?
Yes, though mobile screens vary widely in reported resolution and zoom behavior across browsers, so accuracy can vary more than on desktop. Calibrating by screen size using your device's known diagonal size gives the best results.
Is my data sent to a server?
No. All measurement and calibration happens locally in your browser, and your calibration is stored only in your own browser's local storage.