Standard resistor values: the E-series and how to hit a value that is not in them
Resistors do not come in every value. They come in preferred numbers: 1.0, 1.5, 2.2, 3.3, 4.7, 6.8 and their multiples of ten in the coarsest series, with more steps added as the tolerance tightens, up to 96 values a decade for 1% parts. A calculation that asks for 2791.57 Ω therefore ends with a choice: the nearest standard value, a tighter series, or two parts that combine to something closer. This calculator shows all three, with the error of each.
How to use the standard value calculator
- Type the value your calculation gave (2791.57, 15.9k, 1.2M). Capacitors and inductors use the same series, so 4.7u or 100n work too.
- Pick the series you buy from: E24 for an ordinary 5% drawer, E96 for 1% metal film, E12 for most capacitors.
- The number line shows one decade of that series with your value and the nearest marked; the three cards give the nearest single value, the best two in series and the best two in parallel, each with its error; the ladder shows the nearest value in every series.
- Click a card for its details, use the neighbour buttons to step through the series, and the Decade table for every value with its colour code and tolerance window.
Why 1.0, 1.5, 2.2, 3.3, 4.7, 6.8?
Because of tolerance. A ±20% resistor marked 1.0 may really be anything from 0.8 to 1.2; the next useful marking is the one whose band starts where that one ends, about 1.5 (1.2 to 1.8), then 2.2, 3.3, 4.7, 6.8, and 10 again. Six values cover the decade with no gaps and little overlap: the E6 series. Halve the tolerance and you need twice the values, so E12 is ±10%, E24 ±5%, and the precision series E48, E96 and E192 are ±2%, ±1% and ±0.5%. Each value is the previous one multiplied by the decade root, 101/6 ≈ 1.47 for E6, 101/24 ≈ 1.10 for E24, rounded to two or three figures. The standard is IEC 60063.
The series
| Series | Tolerance | Per decade | Step | Typical parts |
|---|---|---|---|---|
| E6 | ±20% | 6 | ×1.47 | Electrolytic and ceramic capacitors, old carbon resistors |
| E12 | ±10% | 12 | ×1.21 | Most capacitor ranges, budget resistor kits, inductors |
| E24 | ±5% | 24 | ×1.10 | Carbon-film resistors, the common drawer |
| E48 | ±2% | 48 | ×1.049 | Some metal-film ranges |
| E96 | ±1% | 96 | ×1.024 | Metal-film resistors, thin-film SMD, precision work |
E6 is a subset of E12, which is a subset of E24; but E24 and E96 are separate ladders: 4.7 is in E24, the E96 neighbours are 4.64 and 4.75. A 1% part marked 4.7 kΩ is sold too, as a convenience.
Nearest value and error
error = (standard − wanted) / wanted
The nearest value is the one with the smallest ratio to your target, which can differ from the smallest difference in ohms near the top of a decade. The error tells you the systematic offset you accept by using it; the part's tolerance adds a random spread on top. If your value lies inside the nearest part's tolerance window, no better marking is guaranteed to help and the single part is the right answer unless you measure and select.
Making a value from two
Two resistors in series add; in parallel they combine as R1R2 / (R1 + R2). The calculator tries every value of the series as the first part and the two candidates either side for the second, and keeps the pair with the least error. The convenient shapes are a large part plus a small one in series (the small one trims and its tolerance hardly matters) and a slightly-large part with a much larger one in parallel. A pair is never better than its parts' tolerance in the worst case, but it removes the systematic error, which for a gain or a timing value is usually the point.
Your value, step by step
- Nearest E24: 2.7 kΩ (−3.3%); neighbours 2.7 kΩ and 3 kΩ; window 2.565 kΩ … 2.835 kΩ, your value inside.
- Series pair: 2.7 kΩ + 91 Ω = 2.791 kΩ (−0.02%).
- Parallel pair: 18 kΩ ∥ 3.3 kΩ = 2.789 kΩ (−0.10%).
- Other series: E6 3.3 kΩ (+18.2%); E12 2.7 kΩ (−3.3%); E48 2.74 kΩ (−1.8%); E96 2.8 kΩ (+0.30%).
Worked example: 2791.57 Ω for an LED
An LED calculation asks for 2791.57 Ω. In E24 the neighbours are 2.7 kΩ and 3 kΩ; the nearest is 2.7 kΩ, −3.3% low, and its ±5% window of 2.565 kΩ … 2.835 kΩ contains the target. In E96 the nearest is 2.8 kΩ (+0.30%). Two E24 parts in series, 2.7 kΩ + 91 Ω, give 2.791 kΩ (−0.02%); in parallel, 18 kΩ ∥ 3.3 kΩ give 2.789 kΩ (−0.10%). For an LED, where a few percent of current is invisible, the single 2.7 kΩ is the right choice; for a precision divider you would take the pair or the E96 part.
Questions
Why is 4.7 kΩ not in the E96 list?
E96 is built from 10n/96 rounded to three figures, which gives 4.64 and 4.75 but not 4.70. Manufacturers add the popular E24 values to their 1% ranges anyway, so you can buy a 1% 4.7 kΩ; the calculator sticks to the standard ladders.
Should I round up or down?
Neither by rule: pick the smaller error, unless the circuit cares about direction (an LED resistor is safer rounded up, a current-limiting resistor for a motor driver safer rounded down). The neighbour buttons let you choose.
Do the same series apply to capacitors?
Yes: capacitors are made in E6 and E12 (sometimes E24 for small ceramics), inductors in E12 or E24. Type the value in farads or henries and read the mantissas; the ohm sign is just a label.
What about E192?
±0.5% and ±0.1% parts use E192, 192 values a decade. They are rare outside precision instruments; if you need them you are probably trimming or selecting anyway.