Preferred Value Finder

In short Resistors are not made in every value. They are made in the IEC 60063 preferred series — E6, E12, E24, E48, E96 — where the number says how many values fill each decade, and each series is spaced so that consecutive values just cover one another at the matching tolerance: E24 pairs with ±5%, E96 with ±1%. Making every value would be pointless, because parts within tolerance would overlap. The series ought to be a clean geometric progression of ten to the power k over n, and E24 almost is and interestingly is not. Thirteen of its twenty-four values sit more than one per cent off that ideal, and the deviations are not random: the middle of the decade was rounded to friendlier two-digit numbers, so 2.61 became 2.7, 2.87 became 3.0, 3.16 became 3.3 and 3.48 became 3.6, while the ends of the decade stayed nearly exact. The consequence is the part that affects a design. An evenly spaced E24 would step 10.1% every time; the real series steps between 6.7% from 1.5 to 1.6 and 15.4% from 1.3 to 1.5. So how close you can get to a value you want depends on where in the decade it falls, and the nearest E24 part can be more than seven per cent away — worse than the tolerance of the part itself. When that matters the options are an E96 value or, usually cheaper and faster, two ordinary resistors in series or parallel.