Ohm's Law Calculator
Use this free ohms law calculator to solve any resistive circuit: enter any two known values — voltage (V), current (A), resistance (Ω), or power (W) — and it instantly calculates the other two. Every result is labeled as entered or calculated, so you always know what the tool solved for.
Enter your numbers
Fill in exactly two of the four values and leave the rest blank. Results update as you type.
Your results
Fill in any two values above and the other two will be solved automatically.
How to use this calculator
Enter any two of the four values — voltage, current, resistance, or power — and leave the other two fields blank. The solved values appear instantly as you type; you can also press Calculate to recompute on demand. No sign-up, no downloads, and it works on any phone or computer.
Exactly two values are required. With four circuit quantities and two governing equations, any pair determines the other two — but one value (or three) leaves the circuit underdetermined, and the calculator will ask you to enter exactly two known values.
The results table shows all four values and marks each one as Entered or Calculated, so there is never any doubt about which numbers the tool solved for. If a combination can't be computed — for example, dividing by a zero value — you'll get a clear message instead of a wrong answer.
How it works
Ohm's Law ties voltage, current, and resistance together in one equation: V = I × R. Voltage (volts) is the electrical "pressure", current (amps) is the flow rate of charge, and resistance (ohms) opposes that flow. From that one equation you get the other two arrangements: I = V ÷ R and R = V ÷ I.
Electrical power adds a second equation: P = V × I, the rate at which energy is converted to heat or work. Substituting V = I × R into it gives two more useful forms: P = V² ÷ R and P = I² × R. From resistance and power alone, you can even work backwards: I = √(P ÷ R) and V = √(P × R).
Because there are two independent equations, any two known quantities fully determine a circuit. For example, 12 volts across 4 ohms gives I = 12 ÷ 4 = 3 amps, and P = 12² ÷ 4 = 36 watts.
This calculator assumes ideal resistive DC circuits: components behave linearly and their resistance doesn't change as they heat up. Real devices (LEDs, diodes, motors), AC circuits with capacitors or inductors, and high-current heating effects need different models — and for mains-voltage work, always confirm with a licensed electrician.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is V = I × R: voltage (volts) equals current (amps) times resistance (ohms). It applies to ideal resistive DC circuits — real components can heat up and behave non-linearly at high currents.
How do you calculate power with Ohm's Law?
Electrical power equals voltage times current: P = V × I. Combined with V = I × R, that gives two more forms — P = V² ÷ R and P = I² × R. Knowing any two of the four values (V, I, R, P) is enough to solve for the other two, which is exactly what this calculator does.
What are the units in Ohm's Law?
Voltage (V) is measured in volts, current (I) in amps (amperes), resistance (R) in ohms (Ω), and power (P) in watts. The ohm is named after German physicist Georg Simon Ohm, who published the relationship in 1827.
Can I use Ohm's Law for AC circuits?
Only for pure resistors. In AC circuits with inductors or capacitors, the opposition to current (impedance, Z) depends on frequency, so V = I × Z replaces V = I × R. This calculator assumes ideal resistive DC circuits.