Ohm's Law Calculator
Calculate voltage, current, resistance, or power by entering any two known values. The calculator uses Ohm's Law and Watt's Law to find the two missing values instantly.
Enter Two Values
Enter exactly two values to calculate the others.
Results
How to use this calculator
- Choose two known values. Decide which two of voltage (V), current (I), resistance (R) and power (P) you already know.
- Enter them. Type each value into its field using base units: volts, amps, ohms and watts. Leave the other two fields empty.
- Read the results. The two missing values appear instantly, with a "Calculated" badge when the input is valid.
- Start over. Press Clear All to empty every field and try another combination.
Enter exactly two values. If you enter one, three or four, the calculator asks you to check your values. Convert units first: 5 mA is 0.005 A and 2.2 kΩ is 2,200 Ω.
The results are exact for DC circuits and for purely resistive AC loads such as heaters. For motors and other AC loads with a power factor, use the watts to amps calculator instead.
Formulas
Ohm's Law links voltage, current and resistance, and Watt's Law adds power. Together they give a formula for every pair of known values:
| You know | The calculator finds |
|---|---|
| V and I | R = V / I, P = V × I |
| V and R | I = V / R, P = V² / R |
| I and R | V = I × R, P = I² × R |
| P and V | I = P / V, R = V² / P |
| P and I | V = P / I, R = P / I² |
| P and R | V = √(P × R), I = √(P / R) |
Common mistakes
- Entering three values. Only two are needed, and the other two are calculated.
- Mixing units, for example typing milliamps or kilo-ohms without converting to amps and ohms.
- Using the result for an AC motor without allowing for power factor, which makes real power lower than V × I.
- Expecting a measured resistance to match the working value, because resistance changes with temperature.
Worked Example
1. Voltage and current known
A 12 V supply drives a current of 5 A.
- Resistance: R = 12 / 5 = 2.4 Ω
- Power: P = 12 × 5 = 60 W
2. Voltage and resistance known
A heating element of 46 Ω is connected to 230 V.
- Current: I = 230 / 46 = 5 A
- Power: P = 230² / 46 = 52,900 / 46 = 1,150 W
3. Power and voltage known
A 1,000 W heater runs on 230 V.
- Current: I = 1,000 / 230 ≈ 4.35 A
- Resistance: R = 230² / 1,000 = 52.9 Ω
Once you know the current of a circuit, use the voltage drop calculator to check that the cable is large enough for the length of the run.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. It is expressed as V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms.
What is the difference between Ohm's Law and Watt's Law?
Ohm's Law (V = I × R) relates voltage, current and resistance. Watt's Law (P = V × I) relates power to voltage and current. Combining the two gives P = I² × R and P = V² / R, which this calculator uses whenever power is one of your two known values.
Why do I only need two values?
Voltage, current, resistance and power are linked by fixed relationships, so knowing any two of them is always enough to calculate the other two.
Does Ohm's Law work for AC circuits?
Yes, for resistive loads such as heaters and incandescent lamps, using RMS values of voltage and current. For loads with inductance or capacitance, such as motors, resistance is replaced by impedance, and real power also depends on the power factor, so P = V × I gives apparent power rather than real power.
Can I enter negative values?
No. This calculator requires positive values. Negative voltage or current can show polarity or direction in advanced analysis, but basic resistance and power calculations use magnitudes.
Why does the resistance of a bulb change when it is on?
The resistance of most conductors rises with temperature. A hot filament has a much higher resistance than a cold one, so a value measured with a meter on a switched-off bulb will not match the value calculated from its rated voltage and power.
Disclaimer
This calculator is provided for educational and estimation purposes only. Always consult a licensed electrical professional and adhere to your local electrical codes and standards before making any decisions based on these calculations.
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