A resistor calculator finds the total, or equivalent, resistance of resistors connected in series or in parallel. Formula: Series: Req = R1 + R2. Parallel: Req = 1 / (1/R1 + 1/R2). 100 Ω and 220 Ω make 320 Ω in series and 68.75 Ω in parallel.
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How to use the resistor calculator
- Enter R1 and R2 in ohms.
- Choose series or parallel.
- Read the equivalent resistance.
How the resistor calculator works
The resistor calculator finds the equivalent resistance of two resistors connected in series or in parallel. You enter R1 and R2 in ohms and choose the connection. The formulas are:
- Series: R = R1 + R2
- Parallel: R = 1 ÷ (1/R1 + 1/R2) = R1 × R2 ÷ (R1 + R2)
Both values must be above zero. The result is shown to four decimal places. The tool handles exactly two resistors; for more, combine them two at a time.
Resistor calculator worked example
A 100 Ω and a 220 Ω resistor in series give 320 Ω, because the current must pass through both. In parallel, the current has two paths, so the total is lower than either: 100 × 220 ÷ 320 = 68.75 Ω. The resistor calculator returns both figures, one per mode. Two equal resistors in parallel always give half the value, so two 1 kΩ resistors make 500 Ω.
| R1 | R2 | Series | Parallel |
|---|---|---|---|
| 1,000 Ω | 1,000 Ω | 2,000 Ω | 500 Ω |
| 100 Ω | 220 Ω | 320 Ω | 68.75 Ω |
| 470 Ω | 330 Ω | 800 Ω | 193.875 Ω |
| 1,000 Ω | 10,000 Ω | 11,000 Ω | 909.0909 Ω |
Series circuits
In series, the same current flows through every resistor, and the voltage divides in proportion to resistance. With 9 V across 100 Ω and 220 Ω in series, the current is 9 ÷ 320 ≈ 28.1 mA, and the voltage across the 220 Ω resistor is about 6.19 V. This voltage divider is one of the most common uses of series resistors. Adding resistors in series always increases the total.
Parallel circuits
In parallel, every resistor has the same voltage across it, and the currents add. The equivalent resistance is always smaller than the smallest resistor. A small resistor in parallel with a much larger one is dominated by the small one: 1 kΩ in parallel with 10 kΩ is about 909 Ω. Household outlets on a circuit are wired in parallel so each device gets full voltage.
Three or more resistors
For a series chain, add all values. For a parallel group, add the reciprocals and take the reciprocal of the sum; three 300 Ω resistors in parallel give 100 Ω. To use the resistor calculator for more than two, combine the first pair, then combine that result with the next resistor, and so on. For mixed networks, simplify the innermost series or parallel group first and work outward.
Standard values and tolerance
Resistors come in standard series such as E12 and E24, so not every value is sold. Combining two standard parts is a common way to reach an odd value. Each part also has a tolerance, often 1% or 5%, so the real equivalent may differ slightly from the calculated one. Check power ratings too: each resistor must handle the power P = I²R it dissipates.
Colour codes
Through-hole resistors are marked with colour bands. On a four-band part, the first two bands are digits, the third is the multiplier and the fourth is tolerance. Brown-black-red-gold is 10 × 100 = 1,000 Ω at 5%. This page does not decode bands; read the value first, then enter it into the resistor calculator.
Safety
Low-voltage hobby circuits are forgiving, but mains wiring is not. Leave household wiring to a licensed electrician.
Tips
- Enter values in ohms: 4.7 kΩ is 4700.
- Check that a parallel result is below the smallest resistor.
- Measure real parts with a multimeter when precision matters.
- Use the resistor calculator together with Ohm’s law for current and voltage.
For study and planning only. Check lab, engineering and safety work against your course, instrument or a qualified professional.
Related converters and calculators
Related calculators:
- Ohm’s law calculator: voltage and current
- Voltage drop calculator: wire losses
- Electricity calculator: energy cost
- Scientific calculator: general math
Sources
OpenStax College Physics 2e, 21.1: Resistors in series and parallel.
Formula and method
Series: Req = R1 + R2. Parallel: Req = 1 / (1/R1 + 1/R2).
Two positive resistances only. Mode select chooses series or parallel. Headline includes an ohm symbol.
Questions about this calculator
How do I add resistors in series?
Add the values: R = R1 + R2.
How do I combine resistors in parallel?
R = R1 × R2 / (R1 + R2) for two.
Why is parallel resistance smaller?
The current has more paths.
Can it handle three resistors?
Combine them two at a time.
What unit does it use?
Ohms.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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