The tolerance of a resistor refers to the allowable deviation in its resistance value from the specified nominal or ideal resistance. It indicates the range within which the actual resistance of the resistor can vary from the stated nominal value. Tolerance is expressed as a percentage of the nominal resistance value and is typically indicated by a tolerance band or value printed on the resistor itself.

A tolerance of 5% means that the actual resistance of the resistor can vary by up to 5% above or below the nominal resistance value specified. For example, if a resistor is labeled as 100 ohms with a 5% tolerance, its actual resistance can range from 95 ohms to 105 ohms. This tolerance level is common in standard resistors and allows for reasonable variations in resistance due to manufacturing processes and materials.

When a resistor is specified with a tolerance of 10%, it means that the actual resistance can deviate by up to 10% from the nominal value stated. For instance, a resistor labeled as 100 ohms with a 10% tolerance can have an actual resistance ranging from 90 ohms to 110 ohms. Resistors with higher tolerances are typically less precise but may be suitable for applications where exact resistance values are not critical or where cost considerations are important.

The primary purpose of specifying the tolerance of a resistor is to ensure reliability and consistency in circuit design and operation. By knowing the tolerance, circuit designers can select resistors that meet the required accuracy for their applications. For critical circuits where precise resistance values are essential for proper operation, low-tolerance resistors (such as 1% or even lower) are chosen to minimize variations and ensure consistent performance. In less critical applications, where exact resistance values are less crucial, resistors with higher tolerances may be used to achieve cost savings without significantly impacting circuit performance.

A 3-band resistor does not inherently specify tolerance directly through its color bands. Typically, 3-band resistors use color codes to denote the resistance value and sometimes the multiplier. The third band typically represents the tolerance, but in 3-band resistors, this is not standard. Instead, the tolerance is often assumed or specified separately. In practice, 3-band resistors are less common today, with 4-band and 5-band color codes being more prevalent, which explicitly include the tolerance band to clearly indicate the allowable variation in resistance.