Skip to main content

Free calculator

Use this calculator to

  • Combine up to six resistors in series or parallel and read the total instantly
  • Hit a non-standard resistance target using stocked E24 values
  • Share power dissipation across several parallel resistors instead of one hot part
  • Check the effect of adding or removing a branch in an existing network

Series & Parallel Resistor Calculator

Calculate combined resistance for series or parallel resistor networks.

Enter values for each resistor (leave blank to exclude).

Ω
Ω
Ω
Ω
Ω
Ω
Result
—

Enter values for each resistor (leave blank to exclude).

Ω
Ω
Ω
Ω
Ω
Ω
Result
—

Formulas

  • Series: R_total = R1 + R2 + R3 + ...
  • Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
  • Two parallel: R_total = (R1 × R2) / (R1 + R2)

Related Calculators

Ω

Ohm's Law

Calculate V, I, R, or P — enter any 2 values

Open calculator
V÷

Voltage Divider

Calculate output voltage for R1/R2 resistor dividers

Open calculator
🎨Ω

Resistor Colour Code

Decode 4-band and 5-band resistor colour codes

Open calculator

Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

In a commercial building, resistor networks turn up far more often than people expect: building management system (BMS) sensor scaling, signage retrofit projects, EV charger control boards, fan coil unit speed taps, and the protective dividers in metering modules. A maintenance engineer staring at a faulty input card needs the equivalent resistance of the divider chain in seconds, not a re-creation of the original design from scratch. Having a fast reference at the panel saves rolled trucks, saves second visits, and keeps small controls jobs in-house rather than escalating to the OEM at full call-out rates.

For in-house engineering teams supporting retrofit projects, the resistor maths matters most when you are scaling a 0 to 10 V sensor signal into a 0 to 5 V ADC input on a new controller, or dropping a 24 V signal to a 3.3 V GPIO without burning the gate. The wrong values either clip the signal (so the BMS reports a flat-lined sensor) or over-volt the input (so the controller dies). Quick, accurate equivalent-resistance maths is the difference between a clean swap-over and a head-scratching afternoon with smoke escaping from a £400 module.

Resistor networks also creep into compliance work that Purely Energy clients deal with all the time: half-hourly metering retrofits, sub-metering for SECR reporting, and aggregated demand visibility ahead of MEC or MIC reviews. The pulse-output isolators, current-loop interfaces and CT burden circuits on those installs all use simple resistor dividers under the hood. Verifying the divider values during a commissioning walk-through avoids reporting a sub-meter that drifts 2% high for a year and then has to be retro-corrected back through twelve months of CRC and reconciliation submissions.

Common questions

How do I calculate total resistance in series?

Resistors in series simply add: R total = R1 + R2 + R3 and so on, because the same current passes through each one and their voltage drops stack up. 100 Ω, 220 Ω and 330 Ω in series make 650 Ω. The total is always larger than the biggest individual resistor, and the largest resistance in the chain dominates the result.

How do I calculate resistors in parallel?

Take the reciprocal of the sum of reciprocals: 1 / R total = 1/R1 + 1/R2 + 1/R3. For exactly two resistors there is a quicker form, product over sum: R total = (R1 × R2) / (R1 + R2), so 470 Ω with 1 kΩ gives about 320 Ω. The calculator handles up to six values; leave unused fields blank to exclude them.

Why is the parallel total always smaller than the smallest resistor?

Each parallel branch adds another path for current, so the combination conducts more readily than any single branch on its own. More conduction means less resistance, which is why the total must come out below the smallest individual value. The limiting case is useful intuition: putting a low resistance in parallel with anything pulls the total down towards that low value.

What is the quick rule for equal resistors in parallel?

Divide one resistor's value by the number of resistors: N equal resistors of value R in parallel give R / N. Three 330 Ω resistors in parallel make 110 Ω, and two 100 Ω resistors make 50 Ω. This is a common trick for halving a resistance with stocked parts, and it also shares the power dissipation across the group, so each part runs cooler.

Why combine resistors instead of buying the exact value?

Standard E24 stock does not include every value, and a precise target can often be hit faster by combining two parts already in the workshop than by ordering a special. Combining also spreads power: two 100 Ω, 0.5 W resistors in parallel behave as a 50 Ω resistor able to dissipate a full watt. In production designs, sticking to preferred values keeps the bill of materials cheap and easy to second-source.

Related calculators

The conversions that procurement and engineering use alongside this one.

Series & Parallel Resistor Calculator | Purely Energy