Equivalent Resistance Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
In series, the equivalent resistance is the sum of the two resistances (R1 + R2). In parallel, it is calculated with (R1 × R2) ÷ (R1 + R2). For 100 Ω and 200 Ω, this gives 300 Ω in series and about 66.7 Ω in parallel.
Explanation
Wiring two resistors in an electrical circuit changes the circuit's total (or equivalent) resistance depending on how they're arranged. In series (one after the other, a single path for the current), the equivalent resistance is simply the sum of the two resistances: it is always greater than the larger of the two. In parallel (both resistors connected between the same two points, offering the current two paths), the equivalent resistance is calculated with the product-over-sum formula: it is always smaller than the smaller of the two resistances, since adding an extra path always makes it easier for current to flow. This calculator displays both results at once, regardless of which arrangement is actually used in your circuit. For the voltage or power of a circuit, see our Ohm's law calculator and our electrical power calculator.
Example: R1 = 100 Ω, R2 = 200 Ω
Inputs
Resistance 1: 100 Ω. Resistance 2: 200 Ω.
Calculation
In series: 100 + 200 = 300 Ω. In parallel: (100 × 200) ÷ (100 + 200) = 20,000 ÷ 300 ≈ 66.67 Ω.
Result
The equivalent resistance is 300 Ω in series and about 66.67 Ω in parallel.
Frequently asked questions
Why is the equivalent resistance in parallel always lower?
Because a parallel arrangement offers current several paths to flow between two points, which always makes it easier for current to pass compared to a single path. The equivalent resistance in parallel is therefore always lower than the smallest individual resistance in the arrangement, never the other way around.
How do I calculate the equivalent resistance of more than two resistors?
In series, simply add up all the resistances, however many there are. In parallel, the general formula uses the reciprocal of the equivalent resistance: 1/Req = 1/R1 + 1/R2 + 1/R3... This calculator is limited to two resistors to keep things simple; for three or more resistors in parallel, apply the general formula or combine the results two at a time.
How do I know whether my circuit is in series or in parallel?
In a series arrangement, the components are connected end to end, forming a single loop: the same current flows through each resistor. In a parallel arrangement, the components are connected between the same two points (nodes), forming several loops: the voltage is the same across each resistor, but the current splits between them.