Series and Parallel Calculator
Combine resistors, capacitors or inductors either way, split the voltage and power across them, and find two standard parts that hit any value.
Type values however you like — 4k7, 4.7k, 100n,
0.1u, 220. Case matters for one letter only:
M is mega, m is milli.
Find two standard parts that hit a value
Capacitors combine backwards
Resistors and inductors add in series and divide in parallel. Capacitors do exactly the opposite: two 100 nF parts in parallel make 200 nF, and the same two in series make 50 nF. The reason is physical rather than arbitrary — putting capacitors side by side gives the charge more plate area to sit on, while stacking them end to end effectively widens the gap.
- A parallel combination is always smaller than its smallest part. Useful as a sanity check: if your answer came out bigger than any single resistor, you used the wrong rule.
- Series capacitors do not simply add their voltage ratings. The applied voltage splits in inverse proportion to capacitance, so the smallest capacitor takes the largest share. Stack a 100 nF with a 10 nF across 100 V and the small one sees 91 V. Real stacks carry balancing resistors for exactly this reason.
- Parallel resistors share power unevenly. The smaller one takes more current and dissipates more heat, so rating both for the total is not enough — check the smaller one on its own.
- Two parts usually beat one. A single E12 resistor can be 5% away from an arbitrary target. Two of them in the right configuration usually get inside 1%, which is why the search above exists.
How to use
- Pick resistors, capacitors or inductors — the rules differ and the tool applies the right one.
- Type values in any notation: 4k7, 4.7k, 100n, 0.1u, 220.
- Add an applied voltage to see how it splits and how much each part dissipates.
- Or enter a target value below and get the closest pair from the series you stock.
Frequently asked questions
Why do capacitors combine backwards?
Because capacitance depends on plate area and the gap between plates. Wiring capacitors side by side gives the charge more area, so parallel adds. Stacking them end to end effectively widens the gap, so series divides. Two 100 nF parts make 200 nF in parallel and 50 nF in series, which is exactly opposite to resistors, and applying the resistor rule to capacitors produces an answer that looks entirely plausible and is wrong.
Is there a quick way to check my arithmetic?
Yes. A parallel resistor combination is always smaller than the smallest part in it, and never by more than the number of parts. Two resistors in parallel land between half the smaller one and the smaller one itself. If your answer came out bigger than any single resistor, you used the series rule by mistake.
Can I stack capacitors to get a higher voltage rating?
Not by itself, and this catches people out. The applied voltage divides in inverse proportion to capacitance, so the smallest capacitor takes the largest share. Put a 100 nF in series with a 10 nF across 100 V and the small one sees 91 V, not 50 V. Leakage varies between parts too, which drifts the split further over time. Real series stacks carry balancing resistors across each capacitor for this reason.
Do parallel resistors share power equally?
Only if they are equal. Current divides in inverse proportion to resistance, so the smaller resistor takes more current and dissipates more heat. Rating the pair for the total is not enough — work out the smaller one on its own and rate that. The tool shows the per-part figures when you enter an applied voltage.
Why bother with two parts when one nearly works?
Because an arbitrary target usually falls between standard values. E12 steps by about 20%, so a single part can be 10% away from what you want. Two E12 parts in the right configuration are usually inside 1%, and often exact — 3 kΩ is not an E12 value, but 1.8 kΩ and 1.2 kΩ in series is precisely 3 kΩ.
What are E12, E24 and E96?
Preferred value series. Each is a geometric progression across a decade, so E12 has twelve values per decade spaced about 21% apart, E24 has twenty-four, and E96 has ninety-six spaced about 2.4% apart. They exist so that a manufacturer sorting parts by tolerance can cover every value with no gaps and no wasteful overlap. E12 is what most hobby kits contain; E96 is the 1% precision range.
Does this account for tolerance?
No — it works with nominal values. A pair of 5% resistors combining to an exact target on paper will land somewhere in a band around it in reality, and combining two parts does not reduce the spread. If the value genuinely matters, use tighter parts or a trimmer rather than a clever combination of loose ones.
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