Ohm’s Law Calculator
Solves voltage, current, resistance and power from any two of them.
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Your details
Fill in exactly two of the four.
Result
- Voltage
- 12 V
- Current
- 3 A
- Resistance
- 4 Ω
- Power
- 36 W
- Energy in an hour
- 0.036 kWh
Heat dissipated, or work done per second
What a meter would record running continuously
Two knowns fix the whole circuit…
V = I × R, P = V × I
V = 12 V, I = 3 A
R = V ÷ I = 12 ÷ 3
P = V × I = 12 × 3
P = 36 W
Formula — written step by step
12 volts across 4 ohms…
V = I × R, P = V × I
I = V ÷ R = 12 ÷ 4 = 3 A
P = V × I = 12 × 3
I = 3 A, P = 36 W
V is voltage in volts, I current in amperes, R resistance in ohms and P power in watts. Any two of the four determine the other two, because there are only two independent relationships between them.
A worked example
Sizing a resistor for an LED
- Supply
- 9 V
- LED forward voltage
- 2 V
- Desired current
- 20 mA
- The resistor drops the difference: 9 − 2 = 7 V
- R = V ÷ I = 7 ÷ 0.02
- R = 350 Ω — use the nearest standard value, 360 Ω
- P = V × I = 7 × 0.02 = 0.14 W
A 360 Ω resistor rated at ¼ W
The power figure is the reason the calculation matters. A quarter-watt resistor handles 0.14 W comfortably, but the same circuit at 100 mA would dissipate 0.7 W and destroy it. Sizing on resistance alone is how components burn out.
How to use this calculator
- Fill in exactly two of the four boxes — the calculator solves for the rest.
- Use base units: volts, amperes, ohms and watts. Convert milliamps to amps first, so 20 mA is 0.02.
- Read the power figure whenever you are choosing a physical component; it decides the rating you need.
- The kilowatt-hour row shows what the same load would cost to run continuously, which pairs with the electricity cost calculator.
What Ohm’s law states
Ohm’s law says the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: I = V ÷ R. Push harder and more current flows; add resistance and less does.
Power is a separate relationship that happens to share the same three quantities. P = V × I gives the rate at which energy is converted — into heat in a resistor, into light in a lamp, into motion in a motor. Combining the two gives P = I²R and P = V² ÷ R, which are the forms used most often in practice.
Why power matters more than resistance
Resistance tells you how much current will flow. Power tells you whether the component survives. Every resistor carries a wattage rating, and exceeding it is the most common way a circuit fails — not with a dramatic failure but with a slow drift as the part cooks and its value changes.
Because power goes as the square of current, the margin disappears quickly. Doubling the current through a resistor does not double the heat, it quadruples it. A part comfortably within its rating at 50 mA is well past it at 100 mA.
Where the law does not hold
Ohm’s law describes ohmic conductors — materials whose resistance is constant regardless of the voltage applied. Metals at a steady temperature behave this way, which is why it works so well for resistors and wiring.
Many components do not. A diode conducts almost nothing below its forward voltage and then very freely above it. An incandescent bulb’s resistance rises sharply as the filament heats, so its cold resistance is several times lower than its working value. Applying a single fixed resistance to either gives an answer that is wrong by a wide margin.
Formula & methodology
What this calculator assumes
- Assumes an ohmic conductor — one whose resistance does not change with the applied voltage. Metals at a steady temperature behave this way; diodes, filament lamps and thermistors do not.
- Direct current only. For AC you need impedance rather than resistance, and real power depends on the power factor.
- Ignores the temperature coefficient of resistance. A filament’s hot resistance can be several times its cold value.
Sources
- Horowitz & Hill, The Art of Electronics, 3rd edition, Cambridge University Press, 2015 — foundations.
The four forms of Ohm’s law
| To find | From V and I | From V and R | From I and R |
|---|---|---|---|
| Voltage (V) | — | V = ? | V = I × R |
| Current (I) | — | I = V ÷ R | — |
| Resistance (R) | R = V ÷ I | — | — |
| Power (P) | P = V × I | P = V² ÷ R | P = I² × R |
The three power forms are the same equation with Ohm’s law substituted in. P = I²R is the one to remember for heating: doubling the current quadruples the heat.
Frequently asked questions
Can I enter milliamps?
Convert to amperes first: 20 mA is 0.02 A. The calculator works in base units throughout, so mixing in milliamps would silently scale every result by a thousand.
Why do I need exactly two values?
There are two independent relationships between the four quantities, so two knowns fix the other two. One leaves the system underdetermined; three risks a contradiction.
What is the difference between watts and watt-hours?
Watts measure the rate of energy use; watt-hours measure a quantity of energy. A 36 W load running for one hour consumes 36 Wh, or 0.036 kWh.
Does this work for AC circuits?
Only approximately. For AC you need impedance rather than plain resistance, and real power depends on the power factor. For purely resistive AC loads with RMS values, the results hold.