Resistor Calculator: Series and Parallel Resistance with Ohm’s Law
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Enter your resistor values and choose series or parallel.
Resistors 100.00 Ω, 220.00 Ω, 470.00 Ω connected in series give a total resistance of 790.00 Ω.
Voltage across each resistor (series)
| Resistor | Volts | Power (mW) |
|---|---|---|
| 100.00 Ω | 1.5190 | 23.0732 |
| 220.00 Ω | 3.3418 | 50.7611 |
| 470.00 Ω | 7.1392 | 108.44 |
Show the steps
- Series: add the values. R = 100.00 + 220.00 + 470.00 = 790.00 Ω.
- Ohm’s law: I = V ÷ R = 12.0000 ÷ 790.00 = 15.1899 mA.
Resistors
Series: R = R1 + R2 + … + Rn
Parallel: 1/R = 1/R1 + 1/R2 + … + 1/Rn
Ohm’s law: V = I × R, P = V × I
- Parallel resistance is always smaller than the smallest single resistor.
Estimates for planning and learning, not financial, tax, legal or medical advice. Check figures with your lender, tax preparer or doctor before you act.
About this calculator
When resistors are connected, their combined resistance depends on how they are wired. In series, resistances add. In parallel, the reciprocals add, and the total is always smaller than the smallest resistor.
Enter any number of values, such as 100, 2.2k and 4.7M, choose series or parallel and, if you like, a supply voltage. The calculator gives the total resistance, the current, the power, and the voltage or current through each resistor.
How to use it
- Type the resistor values in ohms, separated by commas or spaces. Use k for thousands and M for millions.
- Choose series or parallel.
- Optional: enter the supply voltage to see current and power.
Resistor formulas
- Series: R = R1 + R2 + … + Rn
- Parallel: 1/R = 1/R1 + 1/R2 + … + 1/Rn
- Two in parallel: R = R1 × R2 ÷ (R1 + R2)
- Ohm’s law: V = I R; power P = V I = V² ÷ R
Example
100 Ω, 220 Ω and 470 Ω in series give 790 Ω. The same three in parallel give about 59.6 Ω.
Series versus parallel
In series, the same current flows through each resistor and the voltage divides. In parallel, each resistor gets the full voltage and the current divides.
Sources
Quick check: test yourself
Two resistors of 200 Ω and 100 Ω are in series. What is the total?
Correct answer: 300 Ω. Series resistances add: 200 + 100.
Two equal resistors of 100 Ω are in parallel. What is the total?
Correct answer: 50 Ω. R = 100 × 100 ÷ 200 = 50 Ω.
A 12 V supply drives 60 Ω. What is the current?
Correct answer: 0.2 A. I = V ÷ R = 12 ÷ 60 = 0.2 A.
Pick an answer for each question.
Common questions
How do I add resistors in series?
Add their values. 100 Ω + 220 Ω = 320 Ω.
How do I add resistors in parallel?
Add the reciprocals, then take the reciprocal of the sum. Two equal resistors in parallel give half the value.
Why is parallel resistance smaller?
Each added path gives the current another route, so the total opposition falls.
Can I type 4.7k?
Yes. k means thousand and M means million, so 4.7k is 4,700 Ω.
How do I find the power in a resistor?
P = V² ÷ R or I² × R. Make sure the resistor’s power rating is higher than this.
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