Series Resistor Calculator
Enter two resistor values and get their total resistance in series — the single number behind current limiting, voltage dividers, and load design.
Two values, one answer
Enter both resistor values in ohms and the calculator returns the series total by simply adding them, R = R1 + R2.
Always larger
The series total is always larger than either of your two resistors — the same current must push through both in turn.
What does series resistance mean?
Two resistors, one combined value
Two resistors are wired in series when they sit end to end on a single path, so the same current flows first through one and then through the other. Together they behave like a single resistor whose value you get by adding them, written R = R1 + R2. Because the current meets both obstacles one after the other, the pair always offers more resistance than either resistor on its own. That is the opposite of a parallel connection, where the resistors share both ends and the total drops below the smaller of the two. Series combinations show up constantly in real circuits — limiting current to protect a part, raising a load's resistance, and building voltage dividers where the supply splits across the resistors.
Enter both resistances in ohms to get the combined series resistance instantly.
One short formula, built from the two resistor values R1 and R2.
R = R1 + R2Add the two resistances together — that is the whole calculation. The two-resistor case is just the general series rule (R = R1 + R2 + R3 + …) stopped at two terms, so for any number of resistors on a single path you keep adding their values.
Suppose you wire a 100 Ω resistor in series with a 220 Ω resistor.
First resistor
Start with R1 = 100 Ω — the first value on the single path.
Add the second
Add R2 = 220 Ω — the next resistor the same current flows through.
Total
100 + 220 = 320 Ω — the combined series resistance, larger than either part.
The combined value tells you how the pair behaves as a single resistor, and the first thing to notice is that it is always larger than the larger of your two resistors. That is not a quirk of the formula — it is the physics. The same current has to push through both resistors one after the other, so their opposition to its flow adds up, and the total resistance climbs. Two equal resistors are the easy case: the series total is exactly double one of them, which is why 100 Ω and 100 Ω give 200 Ω, and 10 Ω with 10 Ω give 20 Ω. When the two values differ, the total simply sits above their sum's larger part — a 4.7 Ω resistor in series with a 2.2 Ω resistor gives 6.9 Ω, more than either one, because the current meets both in turn. This is the exact opposite of a parallel connection, where resistances combine to less than the smaller one. Whenever you need to raise a resistance or limit current along a single path, reaching for a series pair is the move.
The addition rule is exact for two ideal resistors, but a couple of practical points are worth keeping in mind.
Two resistors, ideal and ohmic
This calculator combines exactly two resistors. For three or more in series, keep adding their values (R = R1 + R2 + R3 + …). Keep both values in the same unit — ohms here — and remember the formula assumes ideal, ohmic resistors; real components carry a tolerance, and wires and connections add a small resistance of their own. Diodes, LEDs, and other non-ohmic parts do not follow this rule.