What is the Series & Parallel Resistor Calculator?
Use this calculator to find the equivalent resistance of a group of ideal resistors connected entirely in series or entirely in parallel. Unlike an identical-value shortcut, each of the 2–20 entries may have a different resistance. The result compares both arrangements and highlights the one you selected.
An optional applied DC voltage adds total source current and power, plus each resistor’s voltage drop, current and dissipation in the calculation steps. It does not solve an arbitrary mixed schematic. For a two-resistor output node and its load, use the separate Voltage Divider Calculator.
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Optional digital multimeters (paid link) can help check isolated resistance values before entering them. No purchase is required.
Enter different resistor values and choose the connection
- Select series or parallel.
- Enter 2–20 resistor values, separated by commas, semicolons or new lines.
- Choose the default Ω, kΩ, MΩ or GΩ unit for numbers without a suffix.
- Optionally use explicit suffixes for mixed units, such as 100, 2.2k, 1MΩ.
- Optionally enter applied DC voltage from 0 to 60 V to calculate current, voltage drops and power, then calculate.
Explicit suffixes override the default unit. Uppercase M means megohms; lowercase m and milliohm suffixes are not accepted. Scientific notation such as 1e3 is supported. Use decimal points, and do not use commas as thousands separators because commas separate components. Blank entries are not zero-ohm resistors.
Changing the default unit changes the interpretation of unsuffixed values and clears the result; it does not rewrite the list. If you want each value to retain an explicit unit, include suffixes. Optional resistor assortments (paid link) should be checked against their stated tolerance and ratings.
Series and parallel resistance formulas
Parallel: Req = 1 / (1/R1 + 1/R2 + … + 1/Rn)
Two in parallel: Req = R1 × R2 / (R1 + R2)
Total current with voltage: I = V / Req
In series, the same current flows through every resistor and voltage drops add to the applied voltage. In parallel, every branch has the same voltage and branch currents add to the source current. For each resistor, power equals its own voltage drop times its own current, not necessarily the full supply voltage times the total source current.
For positive resistors, a parallel equivalent is below the smallest branch resistance, while the series equivalent is their sum. Identical values are a special case: n resistors of R give n × R in series or R/n in parallel. The existing Speaker Impedance Calculator handles a different nominal-speaker workflow; real speaker impedance varies with frequency.
Parallel resistor calculator example with 3 resistors
Take 100 Ω, 200 Ω and 300 Ω. In series, Req = 100 + 200 + 300 = 600 Ω. In parallel, conductance is 1/100 + 1/200 + 1/300 = 11/600 siemens, so Req = 600/11 ≈ 54.54545 Ω. These are two alternative wiring arrangements for the same parts.
With 12 V across the parallel arrangement, the branch currents are 0.12 A, 0.06 A and 0.04 A. Their sum is 0.22 A. Branch powers are 1.44 W, 0.72 W and 0.48 W, totaling 2.64 W. These are arithmetic examples, not a recommendation to operate a component at those powers.
With the same 12 V across the series arrangement, source current is 12/600 = 0.02 A. Voltage drops are 2 V, 4 V and 6 V; powers are 0.04 W, 0.08 W and 0.12 W. The steps expand to show each entered resistor when voltage is supplied.
Zero-ohm branches and optional voltage
A zero-ohm entry contributes zero to a series sum. In a parallel network, a zero-ohm branch makes the ideal equivalent resistance zero. You can calculate that resistance with the voltage field blank.
If the selected equivalent is zero and a voltage is entered, the ideal model does not determine a finite current. The calculator reports a clear error instead of showing infinity or inventing a source limit. A zero-volt source across an ideal short also does not uniquely determine its current in this ideal model. Real source resistance and current limiting must be modeled separately.
Optional solderless breadboards (paid link) are intended for suitable prototype connections, not as a guarantee that an ideal-short example is an acceptable physical circuit. Positive entries must represent at least 0.000001 Ω and at most 1 trillion Ω. Zero is allowed explicitly; negative values and unsupported suffixes are rejected.
Current, voltage drop and power: useful limits
The applied-voltage option assumes an ideal DC source with at most 60 V input and purely resistive elements. No current limit, resistor tolerance, thermal derating, lead resistance, voltage rating or frequency-dependent impedance is included. A calculated power is dissipation in the model, not a selected safe wattage rating.
For a mixed network, reduce known series or parallel groups one at a time and then re-enter their equivalents only when the topology permits that reduction. This tool does not inspect a wiring image, find unknown connections or automatically solve bridges. Use the Ohm’s Law Calculator for a single resistive relationship and the Electrical Power Calculator for a separate power check.
Optional magnifying lamps (paid link) may help inspect small component labels and connections. Results display up to seven significant digits. Series addition uses compensated summation, and the parallel method scales by the smallest resistance to reduce numerical error; rounded displayed numbers should not be treated as exact component values.
Series & Parallel Resistor Calculator FAQs
Add their resistances in the same unit. This calculator accepts 2–20 values and compares that sum with the parallel alternative.
Add their reciprocal resistances, then take the reciprocal of that sum. For 100, 200 and 300 ohms, the equivalent is 600/11, about 54.54545 ohms.
Yes. Choose a default unit for unsuffixed numbers, or add Ω, kΩ, MΩ or GΩ suffixes to individual entries. Uppercase M means megohms.
Yes. Enter optional applied DC voltage for the selected series or parallel arrangement. The steps show total source current and each resistor’s voltage, current and power.
The ideal parallel resistance is zero. Resistance-only mode supports it, but an entered voltage produces an error because the ideal model cannot determine a finite short-circuit current.
No. It handles one all-series or all-parallel group. Mixed networks require valid step-by-step group reductions or a more complete circuit solver.
Sources and methodology
Independent circuit equations and implementation, reviewed October 24, 2026. Manufacturer datasheets control real component ratings. The linked organizations do not endorse this tool.