In every electronic circuit you encounter resistors – in series or in parallel. Knowing how the total resistance is made up from individual components is the basis of any circuit design. Our calculator supports up to five resistors and instantly calculates both series and parallel configurations.
Step by Step: How to Use the Resistor Calculator
- Choose the number of resistors: 2, 3, 4 or 5 resistors.
- Enter the values: For example R1 = 100 Ω, R2 = 220 Ω, R3 = 470 Ω.
- Choose the circuit type: Series or parallel.
- Read the result: Series: R_total = 100+220+470 = 790 Ω. Parallel: 1/R = 1/100+1/220+1/470 → R_total = 64.7 Ω.
- Current and voltage: Continue calculating with the total resistance using Ohm's Law.
Practical Examples
Calculate LED dropping resistor: 9V circuit, LED 2V/20mA: R = (9−2)/0.02 = 350 Ω → choose the nearest E12 standard value of 390 Ω. 2 LEDs in parallel: each needs its own dropping resistor!
Voltage divider: R1 = 1kΩ, R2 = 1kΩ with 12V supply: voltage across R2 = 12 × 1,000/(1,000+1,000) = 6V. Widely used for sensor interfaces.
Parallel combination of equal resistors: Two 100Ω resistors in parallel: R_total = 100/2 = 50Ω. N identical resistors R in parallel: R_total = R/N.
Resistance Formulas
- Series circuit: R_total = R1 + R2 + R3 + ...
- Parallel circuit: 1/R_total = 1/R1 + 1/R2 + 1/R3
- Two resistors in parallel: R_total = (R1 × R2) / (R1 + R2)
Frequently Asked Questions (FAQ)
Why is the parallel resistance always smaller than the smallest individual resistor?
In a parallel circuit, current flows through multiple paths simultaneously – more paths means less total resistance. The combined parallel resistance is always smaller than the smallest individual resistor in the group.
What are E-series resistor values?
Resistors are not made in arbitrary values but in standardised series: E12 (12 values per decade), E24 (24 values), E96 (96 values). E12: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (in Ω/kΩ/MΩ).
How do I read a resistor value from the colour code?
4-band code: bands 1+2 = digits, band 3 = multiplier, band 4 = tolerance. Gold = ×0.1; Silver = ×0.01. Example: Brown-Black-Red-Gold = 1-0-×100-±5% = 1,000 Ω = 1 kΩ.
