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 standard units: volts, amps, ohms, and watts. Leave the other two fields empty.
- Read the results. The two missing values appear instantly, along with the specific formulas used for the calculation.
Formulas
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points:
Watt's Law defines the relationship between power, voltage, and current:
Combining the two yields expressions for all four variables:
- Voltage: V = I × R = P / I = √(P × R)
- Current: I = V / R = P / V = √(P / R)
- Resistance: R = V / I = V² / P = P / I²
- Power: P = V × I = V² / R = I² × R
Worked Example
Suppose you have a 230 V circuit supplying a heating element with a measured resistance of 23 Ω:
- Given: V = 230 V, R = 23 Ω
- Current: I = V / R = 230 / 23 = 10 A
- Power: P = V × I = 230 × 10 = 2300 W (2.3 kW)
Entering 230 in the Voltage field and 23 in the Resistance field produces 10 A of current and 2300 W of power.
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.
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 purely resistive loads like heaters and incandescent lamps. For inductive or capacitive loads, impedance (Z) replaces resistance (R), and the power factor must be considered.
Can I enter negative values?
No. This calculator requires positive magnitudes. Magnitude calculations are standard for basic circuit analysis.
Why does the resistance of a bulb change when it is on?
The resistance of conductors rises with temperature. A hot glowing filament has much higher resistance than a cold filament measured with a multimeter.
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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