🔬 What Is a Millimeter?
A millimeter (mm) is one-tenth of a centimeter and one-thousandth of a meter. It equals approximately 0.0394 inches. The millimeter is the smallest unit on a standard ruler and is used when centimeter-level precision is not enough, in electronics, jewelry making, watchmaking, dentistry, and 3D printing.
🔧 How to Use the Online MM Ruler
- Calibrate: Click Calibrate Screen and match the blue box to a credit card (85.6 mm wide). Even a 1mm calibration error affects results, so be precise.
- Select MM mode: The ruler is preset to millimeters. Major ticks appear every 10 mm (1 cm), with sub-ticks at 5 mm and 1 mm intervals.
- Place your object: Align the left edge to the red zero line. For small objects like screws or coins, hold them directly against the screen surface.
- Drag the green marker: Move it to the right edge of the object. The measurement displays in whole millimeters with one decimal place.
- Scroll for larger objects: Swipe or scroll right to extend the ruler up to 500 mm.
💍 Ring Sizing with the MM Ruler
One of the most popular uses for the MM ruler is determining ring size. Ring sizes are determined by the inner diameter of the ring in millimeters:
- US Size 5 = 15.7 mm inner diameter
- US Size 6 = 16.5 mm inner diameter
- US Size 7 = 17.3 mm inner diameter
- US Size 8 = 18.2 mm inner diameter
- US Size 9 = 19.0 mm inner diameter
- US Size 10 = 19.8 mm inner diameter
Place an existing ring on the screen with its inner edge against the zero line and drag the marker to the opposite inner edge.
🔩 Measuring Screws, Bolts & Small Hardware
Screws and bolts are specified by their thread diameter in millimeters. Common metric screw sizes: M2, M3, M4, M5, M6, M8, where the number is the thread diameter in mm. To identify an unknown screw: measure the thread diameter with the MM ruler, then compare to standard sizing.
Other useful reference dimensions: pencil diameter ≈ 7 mm; standard biro cap ≈ 9–10 mm; SD card = 24 mm × 32 mm; nano SIM = 12.3 mm × 8.8 mm.