One Formula Runs Every Circuit You Will Ever Touch
Every phone charger, flashlight, and circuit board answers to the same question: how are voltage, current, and resistance related? The answer, discovered nearly 200 years ago, fits on one line — and once you know any two of the values, the rest of the circuit stops being a mystery.
This free Ohm’s law calculator solves for voltage (V), current (I), resistance (R), or power (P). Pick what you want to find, enter the two values you know, and get all four — with the exact formula and steps shown, so you see how the answer was reached.
How to Use This Calculator
- Choose what to find — voltage, current, resistance, or power. The two input fields update to match.
- Enter your two known values — for example, a 12 V battery and a 4 Ω resistor.
- Click “Calculate” — your answer appears big at the top, with all four values and a step-by-step breakdown below.
- Click Reset to clear everything and start a fresh calculation.
What Is Ohm's Law?
In 1827, the German physicist Georg Simon Ohm published experiments showing that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to the conductor's resistance. Put simply:
- Voltage (V) is the electrical "pressure" pushing charges along — measured in volts.
- Current (I) is the rate of charge flow — measured in amperes (amps).
- Resistance (R) is how strongly the circuit opposes that flow — measured in ohms (Ω).
Raise the voltage and current rises with it. Raise the resistance and current falls. Everything else in basic electronics — including power (P), the rate at which energy is used or dissipated as heat, measured in watts — is built from these three.
The Formulas
Power: P = V × I = I²R = V² ÷ R
Any two known values unlock all twelve rearrangements. The most-used ones:
| To find | Formula | Example pair needed |
|---|---|---|
| Voltage (V) | V = I × R | Current + resistance |
| Current (I) | I = V ÷ R | Voltage + resistance |
| Resistance (R) | R = V ÷ I | Voltage + current |
| Power (P) | P = V × I | Voltage + current |
Worked example
A 12 V battery powers a bulb that measures 4 Ω of resistance. How much current flows, and how much power is used?
- Current: I = V ÷ R = 12 ÷ 4 = 3 A
- Power: P = V × I = 12 × 3 = 36 W
- Cross-check: P = I²R = 3² × 4 = 36 W — the same, as it must be.
So the bulb draws 3 amps and dissipates 36 watts — the kind of quick check that decides whether a battery or resistor choice is safe.
Everyday Numbers: Ohm’s Law in Real Devices
Three devices you already own, solved exactly as the calculator would solve them:
| Device | What you know | What the math gives |
|---|---|---|
| LED indicator | 5 V, 20 mA | R = 250 Ω, P = 0.1 W |
| USB charging | 5 V, 2.4 A | R = 2.08 Ω, P = 12 W |
| Space heater | 120 V, 12.5 A | R = 9.6 Ω, P = 1,500 W |
Note the conversion in the first row: 20 mA must be entered as 0.020 A (1 mA = 0.001 A) and 4.7 kΩ as 4,700 Ω (0.020 × 4,700 = 94 V). Convert before you multiply: 1 mV = 0.001 V, 1 MΩ = 1,000,000 Ω.
Who This Is For — and How to Get It Right
- Students — physics and electronics coursework is built on these four variables; seeing all three power forms agree is a great way to check homework.
- Hobbyists and makers — picking a resistor for an LED or checking a battery's load before wiring it up.
- Electricians and technicians — quick sanity checks on the job without reaching for a meter.
Tips
- Keep units consistent. Volts, amps, ohms, and watts all belong to the same system — don't mix milliamps with amps in one calculation.
- Watch the power rating. A resistor rated for 1 W will burn out in the 36 W example above — calculating P is a safety step, not a formality.
- Ohm's law assumes simple conductors. Real circuits with capacitors, inductors, or AC behave more complexly; this calculator covers the DC fundamentals.
Frequently Asked Questions
What are the three formulas of Ohm's law?
The core formula V = I × R rearranges into I = V ÷ R and R = V ÷ I. Together they let you solve for any one of voltage, current, or resistance when you know the other two.
How do you calculate power with Ohm's law?
Power is P = V × I. Using Ohm's law you can rewrite it as P = I²R or P = V² ÷ R, which lets you find power from any two known values. This calculator shows all three forms agreeing.
What is an example of Ohm's law in everyday life?
A phone charger, a flashlight, and an electric heater all obey it. For instance, a 12 V battery connected to a 4 Ω bulb draws I = 12 ÷ 4 = 3 A of current and uses P = 36 W of power.
What are the units of Ohm's law?
Voltage is measured in volts (V), current in amperes or amps (A), resistance in ohms (Ω), and power in watts (W). Keep all four in the same base units and the formulas work directly.
Can I solve for power with only voltage and current?
Yes — choose "Power (P)" in the calculator and enter your voltage and current. The tool computes P = V × I and also derives the matching resistance R = V ÷ I for completeness.
Does Ohm's law work for AC circuits?
The basic form applies to DC and to purely resistive AC loads. Circuits with capacitors or inductors need impedance calculations, which this calculator does not cover.
Why do the three power formulas give the same answer?
Because they are the same relationship rewritten: substituting V = I × R into P = V × I gives P = I²R, and substituting I = V ÷ R gives P = V² ÷ R. Agreement between them is a built-in error check.
How do I handle milliamps and kilohms in Ohm's law calculations?
Convert everything to base units first — volts, amps, ohms, watts: 20 mA = 0.020 A, 4.7 kΩ = 4,700 Ω. Then apply the formulas. Skipping this step is the most common real-world error.
What power rating should my resistor have?
At least double the dissipated power shown — this margin is called derating. For a 0.25 W result, use a 1/2 W resistor or higher. One run near its limit runs hot, drifts, and can fail.
Final Thoughts
Ohm's law is one of those rare ideas that is both simple and everywhere. Two numbers in, and the rest of the circuit tells its story — including how much heat your components must handle. Try the calculator above next time you're sizing a resistor or checking a battery: the cross-checked power result is the part most people skip, and it's the part that keeps projects from burning out.
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