Series & Parallel Resistor Calculator
Enter your resistors to get the series and parallel total instantly, build a mixed series-parallel network, or find standard E12/E24 values and two-resistor combinations that hit a target.
Current, voltage and power per resistor
Wired in series
| R | Value | Voltage | Power |
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Wired in parallel
| R | Value | Current | Power |
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Build a network from groups. Each group is either series or parallel and can contain resistors and further groups.
| E12 | E24 |
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| Combination | Result | Error |
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How to use the Series & Parallel Resistor Calculator
- Enter each resistor, typing a value like 470, 4k7 or 2.2M, or a number with the unit menu. Use + Add resistor for more.
- Read the series and parallel totals with the formulas worked out underneath.
- Add a voltage to see the current, voltage drop and power for each resistor in both wirings.
- Use Mixed network to nest series and parallel groups and get the equivalent resistance of the whole circuit.
- Use Standard values to find the nearest E12/E24 resistor and the best pair to make an odd value.
Series and parallel resistance formulas
- Series: Rtotal = R1 + R2 + … + Rn. Same current through each; voltage drop Vi = I × Ri.
- Parallel: 1 ÷ Rtotal = 1/R1 + 1/R2 + … + 1/Rn. Same voltage across each; branch current Ii = V ÷ Ri.
- Two in parallel (product over sum): Rtotal = R1R2 ÷ (R1 + R2).
- n equal resistors: n × R in series, R ÷ n in parallel.
- Power in each resistor: P = I²R = V²/R. Choose a resistor rated for at least twice the power it dissipates.
Worked example: 100 Ω, 220 Ω and 470 Ω on 5 V. In series the total is 790 Ω, so 5 ÷ 790 = 6.33 mA flows and the drops are 0.633 V, 1.392 V and 2.975 V. In parallel, 1/100 + 1/220 + 1/470 = 0.016673 S, so the total is 59.98 Ω and the branch currents are 50 mA, 22.7 mA and 10.6 mA (83.4 mA total). The 100 Ω resistor dissipates 0.25 W, so a ¼ W part would be at its limit.
Solving mixed series-parallel circuits
Reduce the circuit step by step from the inside out: combine resistors that are purely in series or purely in parallel into one equivalent value, then repeat. For R1 = 100 Ω in series with (R2 = 220 Ω ∥ R3 = 330 Ω): 220 ∥ 330 = 72,600 ÷ 550 = 132 Ω, then 100 + 132 = 232 Ω. The Mixed network tab does this for any nesting of groups.
E12 and E24 standard resistor values
Each value repeats in every decade (×1, ×10, ×100 … up to MΩ):
| Series | Tolerance | Values per decade |
|---|---|---|
| E12 | ±10% | 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 |
| E24 | ±5% | 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 |
Combining two standard resistors gets much closer to an exact value than picking the nearest single one. Put two in series to fine-tune upwards (a large value plus a small trim), or two in parallel to come down from a value just above the target. Remember that the tolerance of each part still applies: two 5% resistors make a combination that is also only accurate to about 5%.
Practical tips
- Series resistors share the power in proportion to their values; parallel resistors share it in inverse proportion. Paralleling two equal resistors doubles the power rating.
- A 0 Ω link in parallel shorts the whole group, and an open (missing) resistor in series breaks the whole chain.
- Real circuits add wire and contact resistance (milliohms), which only matters for very low values such as current-sense resistors.
Frequently asked questions
How do you calculate resistors in series?
Add them: R_total = R1 + R2 + R3 + … For example 100 Ω + 220 Ω + 470 Ω = 790 Ω. The same current flows through every resistor, and the voltage divides in proportion to each value.
How do you calculate resistors in parallel?
Add the reciprocals and invert: 1/R_total = 1/R1 + 1/R2 + … For two resistors this simplifies to (R1 × R2) ÷ (R1 + R2). 100 Ω, 220 Ω and 470 Ω in parallel give about 59.98 Ω.
Why is the parallel total smaller than the smallest resistor?
Each resistor added in parallel gives the current another path, so the combination always conducts better than any single branch. Two equal resistors in parallel give exactly half the value.
What does 4k7 mean on a resistor?
It is the RKM code used on schematics: the letter replaces the decimal point and sets the multiplier. 4k7 = 4.7 kΩ, 1R5 = 1.5 Ω, 2M2 = 2.2 MΩ. Here M (or m) is read as mega-ohms.
What are E12 and E24 values?
They are the standard series of preferred resistor values defined in IEC 60063: 12 or 24 values per decade, spaced so that resistors of 10% (E12) or 5% (E24) tolerance cover the range without big gaps.
How do I make a resistor value that isn't standard?
Combine two standard resistors. Use the Standard values tab: for 5 kΩ, 4.7 kΩ + 300 Ω in series or 10 kΩ ∥ 10 kΩ in parallel give exactly 5 kΩ.