Tolerance

Tolerance is the manufacturing band within which a resistor's measured value must fall when checked at 25 °C with no power applied. Standard E-series tolerances run from ±0.005 % (ultra-precision foil) through ±0.01 %, ±0.1 %, ±1 %, ±5 %, ±10 % up to ±20 % for old composition parts. The tighter the tolerance, the more expensive the part — but tight tolerance alone does not guarantee long-term accuracy if the TCR, drift and voltage coefficient are loose.

For power resistors the dominant grade is ±5 % or ±10 %, because the part is intended to dissipate energy rather than set a precision ratio. Wirewound and metal-film parts can be supplied at ±1 % or ±0.1 % when used as current shunts, divider arms or feedback elements in regulators. Foil resistors push tolerance below ±0.01 %.

When specifying a circuit, total error is the root-sum-square of nominal tolerance, TCR-induced drift over operating temperature, voltage-coefficient deviation, self-heating shift and long-term aging. A 0.1 % resistor that drifts 0.5 % per 1000 h is no better than a 1 % part at end of life. Always specify both initial tolerance and the long-term stability spec.