Series and parallel resistors: total resistance and current
Calculate equivalent resistance for simple circuits and check power before choosing parts.
How it works
For resistors in series, add resistance values: Rtotal = R1 + R2. For two parallel resistors, reciprocal resistance adds, so Rtotal = R1 × R2 ÷ (R1 + R2). Ohm’s law gives source current I = V ÷ Rtotal for an ideal source and resistive load. In series, each resistor carries the same current; in parallel, each branch sees the same voltage.
Use the inputs from your own situation and keep the units consistent. A calculator gives an arithmetic estimate under the assumptions you enter; compare it with real records before making a decision.
Worked example
At 12 V, two 6 Ω resistors in series total 12 Ω and draw 1 A. The same two in parallel total 3 Ω and draw 4 A, with 2 A in each branch. Each resistor in parallel dissipates V² ÷ R = 144 ÷ 6 = 24 W; a small quarter-watt part would be dangerously underspecified for that illustrative load.
What to check
Real supplies have limits; wires, tolerances and resistor heating matter. A short circuit or high power can cause fire or injury. Do not use a basic DC resistor example to design mains wiring or ignore component ratings. Consult a qualified professional for electrical installation.
Practical next step
Draw the circuit and identify whether components share a path or two nodes. Calculate resistance, source current and power per component, then check voltage and power ratings with an appropriate safety margin.
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Save your assumptions and repeat the calculation if an input changes. The linked article explains a neighboring decision that this single formula does not settle.