How to use it
- Enter the resistance your formula gave, for example 350 Ω or 4.7 kΩ.
- Choose the unit and the E series you can buy: E6, E12, E24, E48 or E96.
- Read the nearest standard value and the values just below and above, with the percentage difference of each.
How it works
The E series divide each decade into n steps of equal ratio 10^(1/n): E6, E12, E24, E48 and E96 have 6, 12, 24, 48 and 96 values. E6 to E24 use the historical rounded values of IEC 60063 (for example 2.7, 3.3, 4.7); E48 and E96 are 10^(i/n) rounded to three significant figures (1.00, 1.02, 1.05…).
The calculator scales the series to your decade, finds the largest value at or below your resistance and the smallest at or above it, and picks the one with the smaller relative error. Deviation = (standard − target) / target × 100 %.
Worked example
Your LED resistor came out as 350 Ω. In E24 the neighbours are 330 Ω and 360 Ω: 330 is 20/350 = 5.71 % low, 360 is 10/350 = 2.86 % high, so the nearest is 360 Ω. In E12 the choice is 330 Ω or 390 Ω (+11.43 %), and 330 Ω wins.
For 4.7 kΩ in E96 the values around it are 4.64 kΩ (−1.28 %) and 4.75 kΩ (+1.06 %), so 4.75 kΩ is nearest. 4.7 kΩ itself is an E12 and E24 value, but not an E96 one.
Tips
- Combine two resistors in series or parallel when no single value is close enough.
- Mind the tolerance: a 5 % resistor marked 360 Ω may measure anywhere from 342 to 378 Ω, so a deviation of 2 to 3 % from the target is usually lost in it.
- Color bands: use the resistor color code tool to read or check the bands of the value you picked.