# EIA Resistor Values Explained

Have you ever wondered why standard 5% resistors have strange values, like 330 and 470 Ohms, instead of nice round numbers like 300 or 500 Ohms?

It turns out that standard resistor values form a preferred number series defined by the EIA.  5% values are part of a standard called E24.  The standard is based on a geometric series – each value is approximately 1.1 times the previous one in the set.

This scheme ensures that the resistance values are separated by an amount approximately equal to twice their tolerance.  Since a 5% tolerance resistor could actually be plus or minus 5% of the nominal value, the E24 range spaces the values by 10%.  That way, where the tolerance range of one value leaves off, the next higher value picks up, with the smallest possible overlap or gaps in resistance.

For example, 330 Ohms + 5% = 347 Ohms.  The next highest E24 value is 360 ohms, and 360 Ohms – 5% = 342 Ohms.  There is a small overlap of 5 ohms because the values don’t follow the geometric series exactly (due to rounding to the nearest 10 Ohms).  Spacing resistances significantly closer than their tolerance range would be silly – a 330 Ohm resistor could in reality be larger than a resistor marked 335 Ohms if both resistors had a 5% tolerance.

Here is a chart of the E24 resistor values between 100 Ohms and 1k: