Läpimitta

Läpimitta is a Finnish term used in mechanical engineering and manufacturing to denote the inner diameter of a hollow circular feature such as a hole, bore, tube, or pipe. It describes the width of the interior opening and is measured across the interior perpendicular to the axis. The term is commonly used in technical drawings and specifications and is equivalent to the inner diameter (ID) in English.

In drawings, läpimitta is shown with the diameter symbol (Ø) and a numeric value, typically given in millimetres. Tolerances and fits are specified separately and follow standards such as ISO, DIN, or ASME to define permissible deviations from the nominal value. Measurement is done with bore gauges, inside micrometers, or calipers depending on the required precision.

For a hollow cylinder, läpimitta is influenced by the outer diameter and the wall thickness: läpimitta equals the outer diameter minus twice the wall thickness at the measured section. If the wall thickness varies along the length, the specified läpimitta may include a minimum and maximum value. The concept also applies to holes and bores in solid materials where the interior opening governs passing parts.

Etymology: the word combines läpi meaning through and mitta meaning measurement. In Finnish technical language, läpimitta is the standard term for this dimension, while sisähalkaisija is sometimes used as a synonym in general descriptions.