Ohm's Law Calculator
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What Your Result Means
- Voltage (V): The electrical pressure pushing current through the circuit, measured in volts. Think of it as the "push" — household outlets provide 120 V in the U.S. and 230 V in most of Europe.
- Current (A): The flow rate of electric charge, measured in amperes. Higher current means more electrons flowing per second. A typical phone charger draws about 1–2 A.
- Resistance (Ω): How much the circuit opposes current flow, measured in ohms. A lower resistance means current flows more easily. Wire thickness, material, and temperature all affect resistance.
- Power (W): The rate at which electrical energy is converted to heat, light, or work, measured in watts. A 60 W light bulb converts 60 joules of electrical energy per second.
How This Calculator Works
You enter any two of the four quantities — voltage, current, resistance, or power — and the tool derives the other two using Ohm's Law (V = IR) and the power equation (P = VI). It identifies which two values you've provided, applies the appropriate rearrangement of the formulas, and displays all four. It assumes a simple DC resistive circuit with no reactance, capacitance, or inductance.
Quick Questions
Does Ohm's Law work for AC circuits?
The basic V = IR relationship applies to AC circuits with purely resistive loads (heaters, incandescent bulbs). For circuits with capacitors or inductors, you need impedance (Z) instead of simple resistance, which accounts for phase differences between voltage and current.
Why do I only need two values?
Ohm's Law and the power equation give you two independent relationships among four variables. Two equations with two unknowns are fully solvable, so any pair of known values determines the other two.
What if I enter more than two values?
The calculator uses the first two non-zero values it finds (in the order voltage, current, resistance, power) and derives the rest. If the extra values you entered are inconsistent, the calculated results will override them.
How is this useful for sizing components?
If you know the voltage and the current a device needs, Ohm's Law tells you the resistance to use. The power result tells you the wattage rating the resistor needs to handle without overheating — always choose a resistor rated above the calculated power.
What about series and parallel circuits?
This calculator solves for a single resistive element. For series circuits, add resistances (R_total = R1 + R2). For parallel, use 1/R_total = 1/R1 + 1/R2. Apply Ohm's Law to the total resistance and the source voltage to find current.
Sources
- Wikipedia — Ohm's Law (derivation, history, limitations)
- All About Circuits — Ohm's Law (practical examples and circuit analysis)
- NIST — SI Units for Electricity (official unit definitions for volt, ampere, ohm, watt)
Method & review
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.