Resistor Calculator
Series and parallel totals for up to four resistors.
Series and parallel totals for up to four resistors. Resistors add in series and combine reciprocally in parallel.
Optional.
Optional.
In series (Ω)
200
In parallel, 50 Ω
Resistors add in series and combine reciprocally in parallel. The parallel total is always below the smallest member — 50 Ω here — because adding another path can only ever make it easier for current to flow.
How the Resistor Calculator works
Resistors add in series and combine reciprocally in parallel. The parallel total is always below the smallest member, because adding another path can only ever make it easier for current to flow, which is a useful sanity check on any answer.
Also known as: resistors in parallel · resistors in series · equivalent resistance calculator · parallel resistance formula
Series and parallel
Resistors in series add: current must pass through each in turn, so the oppositions accumulate. Two 100 Ω resistors make 200 Ω.
In parallel the reciprocals add. Two 100 Ω give 50 Ω; three give 33.3 Ω.
The sanity check
A parallel total is always below the smallest member, and that is worth remembering as a check on any answer.
The reason is that each additional resistor gives current another route. More paths can only ever make it easier for current to flow, so the total opposition must fall — even if the added resistor is enormous.
The asymmetric case makes it vivid: 10 Ω in parallel with 1,000 Ω gives 9.9 Ω. The large resistor barely participates, because almost all the current takes the easy path.
Where to go next
The Resistor question rarely arrives on its own. These are the ones that usually come with it:
- Capacitor Calculator — Series and parallel, which are the reverse of resistors.
- Voltage Divider Calculator — Output voltage, current and the power each resistor dissipates.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
- Acceleration Calculator — Acceleration in m/s² and in g, which is the legible one.
Frequently asked questions
How do I calculate resistors in series?
Add them. Two 100 Ω resistors in series make 200 Ω.
How do I calculate resistors in parallel?
Add the reciprocals and invert. Two 100 Ω in parallel give 50 Ω; three give 33.3 Ω.
Why is parallel resistance always lower?
Because each extra resistor offers current another route. More paths means less total opposition, so the result must fall below even the smallest single resistor.
What about two different values in parallel?
The product over the sum works for exactly two. The result sits closer to the smaller value — 10 Ω with 1,000 Ω gives 9.9 Ω, barely different from the 10 alone.
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