Tolerance

The first set of numbers above represents standard resistance values available in tolerances of plus or minus 10 percent. This means that the resistance might be as much as 10 percent more or 10 percent less than the indicated amount. In the case of a 470-ohm resistor, for example, the value can be off by as much as 47 ohms and still be within tolerance. That’s a range of 423 to 517 ohms. The tolerance is calculated according to the specified value of the resistor, not the actual value. You might measure the value of a 470-ohm resistor and find it to be 427 ohms, and it would be within 10 percent of the specified value; if it measures 420 ohms, it’s outside the 10-percent range and is a “reject.”
The second set, along with the first set, of numbers represents standard resistance values available in tolerances of plus or minus 5 percent. A 470-ohm, 5-percent resistor will have an actual value of 470 ohms plus or minus 24 ohms, or a range of 446 to 494 ohms.
Some resistors are available in tolerances tighter than 5 percent. These precision units are employed in circuits where a little error can make a big difference. In most audio and radio-frequency oscillators and amplifiers, 10-percent or 5-percent tolerance is good enough. In many cases, even a 20-percent error is all right.