MM to Inches Calculator

The formula

1 inch is defined as exactly 25.4 mm, so the conversion is inches = mm / 25.4 — an exact figure by international agreement since 1959, not a rounded estimate. The calculator updates the result on every keystroke and shows three decimal places.

Worked example

10 mm converts to 10 / 25.4 = 0.394 inches. This is a common check figure since 10 mm (1 cm) is a frequent reference point in metric drawings that need an imperial equivalent.

Reference conversions for common sizes

M6 bolt threads are 6 mm, which is 0.236 in. A credit card is about 0.76 mm thick, or 0.030 in. A sheet of standard printer paper (80gsm) runs roughly 0.1 mm, or about 0.004 in. Nominal "1/4 inch" hardware is 6.35 mm exactly (0.25 × 25.4).

Precision notes for machining and drawings

Because 1 inch = 25.4 mm exactly, converting mm to inches never introduces rounding error from the definition itself — only from how many decimal places you keep. For fine tolerances (machining, PCB design), three decimal places (thousandths of an inch) as shown here is standard; for rougher carpentry work, rounding to the nearest 1/16 inch (0.0625) is more practical than reading the raw decimal.