# Resistors power and tolerance?

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I am relatively new to electronic projects building and I have noted that some projects do not state the power and tolerance of the resistors. How should I proceed, are there any guide lines to establish the tolerance and wattage? Thatnks.

## Discussions

Generally, if the project description doesn't specify wattage or tolerance, that means that the circuit isn't very picky. Quarter-watt 10% resistors are usually the default assumption; they're cheap, easily available, and good enough for most purposes.

Tolerance is a measure of precision. If you have a 1 kohm resistor with 10% tolerance, then you are guaranteed that the true resistance is between 900 and 1100 ohms. But see below for an amusing caveat...

Power rating tells you the maximum current that you can put through the resistor before it burns up or explodes. For a resistor, P = I

^{2}R, so I(max) = sqrt(P/R). If that 1 kohm resistor above is rated at 1/4 watt, then the maximum current allowed is sqrt(0.25/1000) = sqrt(2.5×10^{-4}) = 1.58×10^{-2}= 15.8 mA.If the project you're buiding tells you how much current is involved, then you can work out what rating resistor you need.