MM Ruler — Actual Size Online
A scrollable millimeter ruler at actual size, calibrated to your screen. Every tick mark is 1 mm. Drag to measure any span.
Place a physical credit card on your screen. Drag the slider until the dashed box matches the card exactly, then click Apply.
Common Objects at Actual Size
Hold the real object against your screen to verify calibration.
About This MM Ruler
This online millimeter ruler renders 500 mm of measurement at your screen's exact pixel density. The top edge shows millimeter ticks with markers at every 1 mm, 5 mm, and 10 mm interval. The bottom edge shows the corresponding inch scale for reference.
Millimeters are the go-to unit for precision work: ring sizing, PCB layout, 3D print tolerances, jewellery, and any engineering task below 1 cm. Calibrate to your screen for measurements accurate to a fraction of a millimetre.
Frequently Asked Questions
Is this millimeter ruler actual size?
Yes — when calibrated to your screen. Click "Calibrate" and use a credit card to set your screen's exact PPI. Once calibrated, every mm tick mark represents exactly 1 physical millimeter.
How many mm are in 1 cm?
There are exactly 10 millimeters in 1 centimeter. On this ruler, every 10th small tick mark aligns with a centimeter number. The formula: mm ÷ 10 = cm.
How many mm are in 1 inch?
There are exactly 25.4 millimeters in 1 inch. You can see both scales on this ruler — mm on the top, inches on the bottom. So 25.4 mm = 1 inch exactly.
What can I measure in mm?
Millimeters are ideal for precise small measurements: ring sizes (inner diameter), PCB component spacing, jewellery, phone bezels, screw diameters, and any engineering or craft work where sub-centimeter precision matters.
How do I drag to measure a span?
Click and hold anywhere on the ruler, drag to the end of what you want to measure, then release. The measurement badge above the ruler shows the exact span in mm and inches.
How accurate is this online mm ruler?
After calibration, accuracy is within a fraction of a millimeter for objects placed directly on screen. Without calibration the ruler uses 96 PPI (web standard), which is accurate on most 24–27 inch desktop monitors but may be off on high-DPI laptops and phones.