This tool calculates the total resistance, current, voltage and power for a fictional series or parallel circuit with up to three resistors. It is for students who know the formulas but are unsure which one to apply at each step.
Free tool
Simple circuit calculation tutor
You can quote Ohm's law but lose marks when a circuit has more than one resistor.
Everything you enter stays on this device.
Who is this tool for?
It helps Form 4 and Form 5 students meeting Ohm’s law in Science or Physics. It also helps students who get the circuit type right but slip on the arithmetic.
The answer is never shown alone. Each result comes with the rule behind it.
How do I use it?
- Choose series, parallel or one resistor.
- Enter the cell voltage, from above 0 V to 24 V.
- Enter the resistance of each resistor in ohms. The third resistor is optional.
- Press Calculate and read the table, the diagram and the working.
- Read the consistency checks, then change one value and compare.
How does it work?
Ohm’s law gives V = I × R. In a series circuit the resistances add, the same current flows everywhere, and the cell voltage is shared.
In a parallel circuit, 1/R = 1/R1 + 1/R2 + 1/R3. Every branch has the full cell voltage, and the branch currents add to the total. Power for each resistor is P = V × I.
A short worked example
This is a worked example. A 12 V cell is connected to a 4 Ω resistor and a 6 Ω resistor in parallel.
The total resistance is 1/R = 1/4 + 1/6 = 5/12, so R = 2.4 Ω. The total current is 12 ÷ 2.4 = 5 A.
The branch currents are 12 ÷ 4 = 3 A and 12 ÷ 6 = 2 A, and 3 + 2 = 5 A. That match is the consistency check. A common mistake is to add 4 + 6 and call the total 10 Ω, which is the series answer.
What are the limits?
The circuit is fictional, with an ideal cell and ideal wires. The tool allows up to 24 V and up to three resistors.
It does not cover real wiring, mains electricity or building hardware. Circuits that mix series and parallel parts in one diagram are not covered either.
Where does this help in your subjects?
Start with applying potential difference, resistance and Ohm’s law and solving series and parallel circuits. The electricity practice then gives exam-style questions.
For circuit reasoning without numbers, try predicting what an open branch changes. If you want a teacher beside you, see SPM Physics tuition and the one-hour trial class (from RM50).
Common questions
Why is the total resistance smaller in a parallel circuit?
Each new branch gives the current another path, so more current flows for the same cell voltage. More current for the same voltage means lower total resistance. That is why the total is always smaller than the smallest branch.
What does the consistency check tell me?
It adds up the voltages in a series circuit or the currents in a parallel circuit and compares them with the cell voltage or total current. If they match, your method holds together. Small differences come from rounding.
Why does the tool reject a resistance of 0 or a negative value?
A zero resistance would give an undefined current in Ohm's law, and a negative resistance has no meaning for an ordinary resistor. The tool shows a message instead of a number that looks believable.
Can I use this to plan a real circuit?
No. It models a fictional low-voltage circuit with an ideal cell and ideal wires. It is for practising calculations and gives no advice on real wiring or mains electricity.
If you can follow the steps here but freeze on an exam diagram with a mix of series and parallel parts, a one-to-one Physics lesson can practise reading the diagram first, at your pace.
- Online one-to-one lessons for your child with an experienced teacher.
- Your first class is a one-hour trial, from RM50. The fee is agreed before you book.
- Happy with the teacher? Continue with lessons of about 1.5 hours. If not, ask for another teacher.