A free online screen ruler that lets you measure physical objects by calibrating against a known reference (credit card, bank card, A4 paper, US dollar bill, or coin). Features a realistic dual-scale ruler (cm + inches), draggable calipers for precise measurements, crosshair mode, and auto-detection of screen DPI with manual calibration override. Supports millimeters, centimeters, inches (with fractional display), and pixels. Calibration is saved locally for your device.
Open Calibration and select a reference object (credit card, A4 paper, or coin).
Place the physical object flat against your screen, then adjust the slider until the on-screen box matches its real size.
Choose Caliper mode and drag the red handles to span the object you want to measure.
Read the measurement in cm, inches, and pixels simultaneously.
After calibration with a credit card, accuracy is typically within 1-2mm on most displays. The key is to carefully match the on-screen rectangle to your physical reference object using the calibration slider.
You can calibrate using A4 paper (210mm wide), a US dollar bill (156mm), a US quarter (24.26mm diameter), a 1 Yuan coin (25mm), or select "Custom" to enter any known size.
Yes. Your calibrated PPI is saved to your browser's local storage. You only need to recalibrate if you switch monitors or change screen resolution.
Switch to Caliper mode, then drag the red triangular handles on the vertical lines. The distance between them updates in real time. You can also click anywhere in the measurement area to snap the nearest handle to that position.
Free online tool — works in your browser, no sign-up required. Start using it right now.
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