The mm ruler online shows millimeter tick marks at actual size on your screen for high-precision measurement. After credit-card calibration, you get readings accurate to plus or minus 0.5 mm, enough for ring sizing, screw and bolt identification, jewelry crafting, watch repair, and small electronics work. People also search for this tool as "online ruler mm" or "ruler online mm" or "millimeter ruler online", all of which describe this same calibrated MM ruler.
Millimeters give 10 times the precision of centimeters and are the standard unit for fine engineering, medical, and scientific measurement. This online ruler in MM works on phone, tablet, or desktop, and the calibration is shared with our online ruler in CM and inches modes.
Use the online millimeter ruler for: ring inner diameter, watch strap width (18, 20, 22 mm), screw and bolt sizing (M3, M4, M5, M6), bead and chain dimensions, pill diameter, small electronics, and any task where centimeters are too coarse.
How to Use the MM Ruler Online
- Calibrate once with a credit card. The standard ISO/IEC 7810 ID-1 card is 85.6 mm wide, which is the calibration anchor for all online rulers.
- Confirm MM mode is selected using the unit toggle. MM is the default on this page.
- Place the object against the screen, aligned with the red 0 mm line at the left edge.
- Read directly from the millimeter tick marks, or drag the green marker to the right edge for a precise digital readout to one decimal place.
When to Use the MM Ruler
- Ring sizing. Ring inner diameters are measured in mm. See our at-home ring sizing guide.
- Screws, bolts, nuts. Metric fasteners are specified in mm (M3, M4, M5, M6, M8, M10). Diameter and thread pitch both use mm.
- Jewelry crafting. Bead diameters, jump ring sizes, wire gauges, chain links.
- Watch repair. Strap width (lug width) in mm: 18, 19, 20, 22, 24 are the most common.
- Electronics. Component dimensions, PCB hole spacing (2.54 mm pitch is standard), connector sizes.
- Glasses and contact lenses. Pupillary distance, frame width, lens diameter all in mm.
- Medical and pharmaceutical. Pill diameter, syringe markings, blister-pack dimensions.
MM Quick Reference
- 1 mm equals 0.1 cm, or 0.0394 inches (about 1/25 of an inch)
- 10 mm equals 1 cm, or 0.3937 inches
- 25.4 mm equals exactly 1 inch (by international definition)
- 100 mm equals 10 cm, or 3.937 inches
- 500 mm equals 50 cm, our online measuring tape maximum
- 1000 mm equals 1 meter, or 39.37 inches
Common MM Sizes You Might Need
The mm ruler online is the right tool for these common items:
- Adult ring sizes: inner diameter 15 to 22 mm covers most sizes.
- Watch strap widths: 16, 18, 20, 22, 24 mm are the most common lug widths.
- Pen tips: ballpoint is typically 0.7 to 1.0 mm, gel pen 0.5 to 0.7 mm.
- Earphone jacks: 3.5 mm headphone jack, 6.35 mm guitar jack.
- USB connectors: USB-C is 8.4 by 2.6 mm, micro-USB is 6.85 by 1.8 mm.
- Metric screws: M2 (2 mm diameter), M3 (3 mm), M4, M5, M6, M8.
For Sub-Millimeter Precision
The online millimeter ruler can resolve down to about 0.5 mm depending on screen DPI. For finer measurements below 0.5 mm, use a physical digital caliper which resolves to 0.01 mm. Screen rulers are not suitable for engineering tolerances tighter than 0.5 mm.
Need Different Units?
For larger measurements, switch the same tool to CM mode, or use our online measuring tape for objects up to 50 cm. For imperial units, see the online inch ruler.
Get a Credit Card
Grab any standard bank card. Every credit card and debit card worldwide is exactly 85.6 mm wide by ISO/IEC 7810 ID-1 standard.
Click "Calibrate"
The amber Calibrate button sits in the ruler toolbar. Click it to open the calibration overlay on top of the ruler.
Drag to Match
Drag the blue calibration box right edge until its width exactly matches the long edge of your card.
Confirm and Done
Click Confirm. Your screen pixel density is saved to your browser. All future measurements display at real-world actual size.
Width: 85.6 mm (8.56 cm or 3.37 inches) · Height: 53.98 mm (5.4 cm or 2.13 inches)
Every bank card worldwide follows this exact standard.
Unit Conversion Reference
| Centimeters (cm) | Millimeters (mm) | Inches (fraction) | Inches (decimal) |
|---|---|---|---|
| 0.1 cm | 1 mm | 1/16" | 0.0394" |
| 0.5 cm | 5 mm | 3/16" | 0.1969" |
| 1 cm | 10 mm | 3/8" | 0.3937" |
| 2 cm | 20 mm | 13/16" | 0.7874" |
| 2.54 cm | 25.4 mm | 1" | 1.0000" |
| 5 cm | 50 mm | 1 15/16" | 1.9685" |
| 10 cm | 100 mm | 3 15/16" | 3.9370" |
| 15 cm | 150 mm | 5 7/8" | 5.9055" |
| 20 cm | 200 mm | 7 7/8" | 7.8740" |
| 25 cm | 250 mm | 9 13/16" | 9.8425" |
| 30 cm | 300 mm | 11 13/16" | 11.8110" |
| 50 cm | 500 mm | 19 11/16" | 19.6850" |