Definition
Tolerance
A tolerance is the allowed range around a target measurement or made value.
What it means
A target says what you are aiming for. A tolerance draws the allowed zone around it. Values inside that zone are accepted; values outside it are not. The zone can be the same size on both sides of the target or different on each side.
Remember it
Put the target in the middle; the tolerance is the allowed zone around it.
Example
A peg is specified as 50 mm ± 1 mm. Its accepted interval is 49 mm to 51 mm, including both limits. A 50.6 mm peg is within tolerance, but a 51.4 mm peg is outside it.
Watch for
Saying a length is “about 50 mm” is not a tolerance unless the acceptable limits are stated.
Common mix-up
Tolerance is chosen for a specification; measurement error is the difference found between a measured value and a reference value.
Exact meaning
In measurement and making, a tolerance specifies the values accepted around a target or nominal value, often by upper and lower limits.