Parallel Resistance Calculator
In a parallel circuit, the equivalent resistance is always less than the smallest individual branch resistor. Enter 2–10 values to calculate Req and optional branch currents.
Preferred-Value Series/Parallel Resistor Search
Compare a single resistor, two in series and two in parallel for a target resistance. Lock one resistor or write a manually entered pair’s total back to the target.
Inputs and reverse calculation
Calculated results
| Nearest single preferred value | 10 kΩ; deviation 0% |
|---|---|
| Rs1 | 8.2 kΩ |
| Rs2 | 1.8 kΩ |
| Rp1 | 11 kΩ |
| Rp2 | 110 kΩ |
| Series total | 10 kΩ; deviation 0% |
| Parallel total | 10 kΩ; deviation 0% |
How to interpret the result
The existing multi-branch parallel calculator remains available. This section adds preferred-value search. With no lock, enter an actual pair and use its button to write the total to the target, preserving that pair and refreshing candidates on the other side.
Formulas and units
Series Rs=R1+R2; parallel Rp=R1R2/(R1+R2). With one series resistor fixed, the ideal other value is R−Rfixed. For a fixed parallel resistor it is Rfixed×R/(Rfixed−R). The other resistor is then replaced by the nearest selected preferred value. Deviation=(actual total−target)/target×100%.
Worked example
For a 10000 Ω target with series Rs1 fixed at 8200 Ω, E24 selects Rs2=1800 Ω: total 10000 Ω and 0% deviation. For a 10000 Ω target with parallel Rp1 fixed at 20000 Ω, E24 selects Rp2=20000 Ω, also totaling 10000 Ω.
Scope and limitations
Preserves the floor-candidate and relative-error algorithm for E6–E192. Targets are limited to 0.1–10⁸ Ω to avoid the reference parallel search’s fixed 10⁹ Ω ceiling. Impossible locked combinations are rejected rather than using the App’s ÷10000/×1000 substitutes. A locked value is preserved exactly. Total buttons preserve the manually entered source pair, correcting refresh conditions that contradict the reference module’s comments. Ratings and component tolerances are not checked.
Common questions
Why can a fixed 5000 Ω parallel resistor not produce 10000 Ω?
The parallel equivalent of two positive resistors is below either branch resistance. The fixed resistor must exceed the target; otherwise an error is shown and the previous valid result is retained.
Why does editing Rs1 not immediately change the target?
With no lock, manual edits update the actual total first. The target changes only when “Write series total to target R” is pressed, which then refreshes the other solution.
Related tools
Formula and data sources
Rebuilt from the extracted Electrodoc Pro 5.2 specification. Added, inferred and corrected behavior is identified in the limitations. Updated 2026-11-13.
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