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    Resistors in Series Calculator

    GCSE PhysicsAQAEdexcelOCRFoundation & Higher

    Find the total resistance of up to three resistors in series, or a missing resistor from the total, using Rtotal = R1 + R2 + R3.

    Quick answer

    In series, total resistance = the sum of the individual resistances. For example, 12 Ω and 18 Ω in series give 12 + 18 = 30 Ω. Convert kΩ to Ω (× 1000) before adding.

    By Engr. Mehmood Kakar · Updated

    Rtotal = R1 + R2 + R3
    Find:

    Only two resistors? Leave this at 0.

    Simulation

    Resistors in series share the pd

    Set three resistors (put one at 0 Ω to use two) and the supply pd. The same current flows through each one, and the pd is shared in proportion to the resistance.

    10 Ω
    20 Ω
    30 Ω
    12 V

    In series the total resistance is the sum of the resistances, the current is the same everywhere, and the pds add up to the supply pd.

    Put these numbers in the calculator above

    How to add resistors in series

    When resistors are connected in series, one after another in a single loop, the same current flows through each one. Their resistances simply add up:

    Rtotal = R1 + R2 (+ R3 …)

    • All resistances are in ohms (Ω)

    A 12 Ω resistor in series with an 18 Ω resistor gives a total resistance of 12 + 18 = 30 Ω. Adding more resistors in series always increases the total resistance.

    To find one unknown resistor, subtract the known ones from the total. If the total is 60 Ω and two resistors are 30 Ω and 10 Ω, the third is 60 − 30 − 10 = 20 Ω.

    In a series circuit the potential difference of the supply is shared between the components. Once you know the total resistance, you can find the current with V = IR. For resistors side by side, see the parallel resistance calculator.

    Common mistakes that cost marks

    1. Adding kΩ and Ω directly. 2.2 kΩ + 470 Ω is 2200 + 470 = 2670 Ω, not 472.2.
    2. Using the series rule for parallel resistors. Resistors side by side have a total smaller than the smallest one.
    3. Forgetting a resistor. Count every component in the loop.

    Practice questions

    Try each one on paper first, then reveal the worked answer.

    Q1A 12 Ω resistor and an 18 Ω resistor are connected in series. Calculate the total resistance.[1 marks]
    1. R = R₁ + R₂
    2. R = 12 + 18
    3. R = 30 Ω
    Q2Three resistors in series have a total resistance of 60 Ω. Two of them are 30 Ω and 10 Ω. Calculate the third.[2 marks]
    1. R₂ = R − R₁ − R₃
    2. R₂ = 60 − 30 − 10
    3. R₂ = 20 Ω
    Q3A 2.2 kΩ resistor is in series with a 470 Ω resistor. Calculate the total resistance in ohms.[2 marks]
    1. Convert: 2.2 kΩ = 2200 Ω
    2. R = 2200 + 470
    3. R = 2670 Ω

    Endless practice

    A new question every time, marked instantly. Work it out on paper, then type your answer.

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    Frequently asked questions

    How do you calculate resistance in series?

    Add the resistances together: Rtotal = R1 + R2 + R3. All values must be in ohms.

    Does adding resistors in series increase resistance?

    Yes. Every resistor added in series increases the total resistance, so the current from the same supply gets smaller.

    What happens to current and potential difference in a series circuit?

    The current is the same through every component. The supply potential difference is shared between the components.

    What about resistors in parallel?

    In parallel the total resistance is smaller than the smallest individual resistor. AQA GCSE asks you to explain this rather than calculate it.