Tolerance in engineering is an allowance, given as a permissible range, in the nominal dimension or value specification of a manufactured object. The purpose of a tolerance is to specify the allowed leeway for imperfections in manufacturing the part or component.
The tolerance may be specified as a factor or percentage of the nominal value, a maximum deviation from a nominal value, an explicit range of allowed values, be specified by a note or published standard with this information, or be implied by the numeric accuracy of the nominal value. Tolerance can be symmetrical, as in 40±0.1, or asymmetrical, such as 40+0.2/-0.1.
It is good engineering practice to specify the largest possible tolerance while maintaining proper functionality, since closer or tighter tolerances are more difficult to manufacture and hence cost more to either build or buy.
Tolerance is different from safety factor, but an adequate safety factor will take into account relevant tolerances as well as other possible variations.
Electrical component tolerance
An electrical specification might call for a resistor with a nominal value of 100Ω (ohms), but will also state a tolerance such as "±1%". This means that any resistor with a value in the range 99Ω to 101Ω is acceptable. It would not be reasonable to specify a resistor with a value of exactly 100Ω in any case, because the exact resistance will vary with temperature, current and other factors beyond the control of the designer.
Mechanical component tolerance
Tolerance is related to, but different from fit in mechanical engineering, which is a designed–in clearance or interference between two parts. For example, if a shaft with a nominal diameter of 10 millimeters is to have a sliding fit within a hole, the shaft might be specified with a tolerance range from 9.964 to 10 millimeters and the hole might be specified with a tolerance range from 10.04 to 10.076 millimeters. This would provide a clearance fit of somewhere between 0.04 millimeters (largest shaft paired with the smallest hole) and 0.112 millimeters (smallest shaft paired with the largest hole). In this case the size of the tolerance range for both the shaft and hole is chosen to be the same (0.036 millimeters), but this need not be the case in general.