Ohm's Law Calculator
Solve for voltage, current, resistance, or power from Ohm's Law, or find the total resistance of a series or parallel resistor network.
Calculator verified • Last updated: August 2026
Not sure which mode? Click here.
- Ohm's Law — you know two of voltage, current, and resistance for a single component, and want the third (plus power).
- Series / Parallel Circuit — you have several resistors wired together and want the total resistance of the network, with each resistor marked as series or parallel.
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Circuit Diagram
The double-bar symbol on the left is the battery (red long bar = +, blue short bar = −); the zigzag shapes are resistors, labeled with their value. In Series/Parallel mode, each resistor after the first says which existing resistor it attaches to and whether it joins in series or parallel with it — that's enough to build any nesting, not just one flat chain: attaching R3 in parallel with R2 pairs just those two, and a later resistor can attach in series with that same R2 to nest a series pair inside the parallel bank, for example. The diagram and the steps below the result both draw from that same structure, so they always match exactly.
How to Use Ohm's Law
Ohm's Law relates voltage, current, and resistance in an electrical circuit — know any two, and you can always find the third.
V: voltage, in volts (V).
I: current, in amperes (A).
R: resistance, in ohms (Ω).
P: power, in watts (W).
For a network of resistors, series and parallel combinations add up differently:
Rtotal: the combined resistance of the network, in ohms (Ω).
R1, R2, …: the individual resistor values, in ohms (Ω).
Worked Example: Solving for Resistance
Using the calculator's own defaults — a 12 V source driving 2 A of current — the resistance is 12 / 2 = 6 Ω, and the power dissipated is 12 × 2 = 24 W. Change any two of the three values and the third recalculates automatically.
Worked Example: Series vs. Parallel Resistors
Three 6 Ω resistors in series combine to 6 + 6 + 6 = 18 Ω — three times any single resistor, since current has to push through all three one after another. The same three resistors in parallel combine to 1 / (1/6 + 1/6 + 1/6) = 2 Ω — a third of any single resistor, since current now has three paths to flow through at once. Parallel combinations always end up smaller than the smallest individual resistor; series combinations always end up larger than the largest.
A Brief History of Ohm's Law
German physicist Georg Ohm published the relationship between voltage, current, and resistance in 1827, based on careful experiments measuring how different wire lengths and thicknesses affected current flow. The work was initially met with skepticism and even hostility from parts of the German scientific establishment, and it took over a decade before its significance was widely recognized — Ohm was eventually honored by having the unit of electrical resistance, the ohm (Ω), named after him in 1881, well after his death in 1854.
Common Ohm's Law Mistakes
Mixing up series and parallel formulas is the most frequent error — series resistances add directly, but parallel resistances add as reciprocals, and applying the wrong one gives a wildly incorrect total. Forgetting that power has three equivalent forms (VI, I²R, V²/R) sometimes leads to unnecessary extra calculation steps when one of the simpler forms would work directly from the values already known. Assuming resistance is constant regardless of temperature is a subtler mistake — real resistors' resistance can shift somewhat as they heat up under load, something this idealized calculator doesn't account for.
Electricity Terms You Should Know
Voltage (V) — the electrical "push" driving current through a circuit, measured in volts.
Current (I) — the rate of electric charge flow through a circuit, measured in amperes (amps).
Resistance (R) — a material's opposition to current flow, measured in ohms (Ω).
Power (P) — the rate of energy conversion in a circuit, measured in watts.
This calculator assumes ideal components and constant resistance. Consult an electrical engineering reference for circuit design or safety-critical applications.
Frequently Asked Questions
What is Ohm's Law in simple terms?
Ohm's Law says that voltage equals current multiplied by resistance (V = IR). Push harder (more voltage) and more current flows; add resistance and less current flows for the same push. Knowing any two of the three lets you calculate the third.
What is the difference between series and parallel circuits?
In a series circuit, components are connected end to end along a single path, so the same current flows through all of them and resistances simply add up. In a parallel circuit, components are connected across the same two points, giving current multiple paths to flow — this always lowers the total resistance below any single branch.
Why is total resistance in parallel lower than the smallest individual resistor?
Every additional parallel branch gives current another path to flow through, so the circuit as a whole conducts more easily — even adding a large resistor in parallel can only decrease total resistance, never increase it, since it's one more path current can take.