Millimeters to Inches Converter

Multiply mm by 0.0394 to get inches (the exact factor is 0.0393700787, derived from 1 in = 25.4 mm exactly). For a quick mental estimate, multiply by 0.04 and round down by about 1.5 percent. Examples: 10 mm is about 10 x 0.04 = 0.4 in (actual 0.394 in), 25 mm is about 25 x 0.04 = 1.0 in (actual 0.984 in, close to 1 in), 100 mm is about 4 in (actual 3.937 in). For the reverse, multiply inches by 25 in your head for a fast underestimate (actual factor 25.4), so 6 in is roughly 150 mm (actual 152.4 mm).

No. A 10 mm socket measures 10 mm across the flats, while 3/8 in is 9.525 mm. The smaller 3/8 in socket may not fit a 10 mm fastener, and substituting nearby sizes can damage the fastener or tool. Use the specified metric or SAE size.

No, not safely. Most metric-to-SAE pairs sit at least 0.1 to 0.5 mm apart, which is enough slop to round the corners of a hex head under torque. The handful of pairs that are within 0.05 mm (19 mm vs 3/4 in, 23 mm vs 29/32 in, 27 mm vs 1-1/16 in) are the only ones you can swap in an emergency. A modern car built since the 1990s is almost entirely metric, even US-built models, so a metric socket set is the practical default in any general toolbox. Older US iron (pre-1980s Ford, GM, Chrysler) and US agricultural equipment are still mostly SAE.

Aluminum sheet for hobby and trim work usually comes in 1.5 mm (0.059 in, about 16 gauge), 2 mm (0.079 in, 14 gauge), and 3 mm (0.118 in, 11 gauge). Mild steel sheet for chassis, bracketry, and weldments stocks at 1 mm (20 gauge), 1.5 mm (16 gauge), 2 mm (14 gauge), and 3 mm (11 gauge). Stainless sheet follows the same gauges. Acrylic and polycarbonate cast sheet stocks at 2 mm, 3 mm, 5 mm, 6 mm, and 10 mm. Always confirm against the supplier spec sheet because nominal sheet metal gauges run about 5 to 10 percent thinner than their fractional inch labels suggest.

The required tolerance depends on diameter, material, temperature, load, assembly method, and the intended clearance or interference. ISO 286 fit designations pair a hole tolerance with a shaft tolerance, so H7 alone does not define the complete fit. Use the fit specified on the engineering drawing or calculate one for the application.