Ohm's Law Calculator
Enter any two values to find the third. V = IR — voltage, current and resistance calculator for physics students.
Fill any two fields — the third will be calculated automatically. Clear a field to recalculate.
Ohm's Law Formulas
Circuit Diagram
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current through it, with resistance as the constant of proportionality. Mathematically: V = IR, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
What units are used in this calculator?
Voltage is in Volts (V), Current in Amperes (A), Resistance in Ohms (Ω), and Power in Watts (W). If you are working with milliamps, convert to amperes first (1 mA = 0.001 A).
Does Ohm's Law apply to all materials?
Ohm's Law applies to ohmic conductors — materials where resistance stays constant regardless of voltage or current (like most metals at constant temperature). Non-ohmic devices like diodes, transistors, and thermistors do not follow Ohm's Law as their resistance changes with voltage or temperature.
What is the relationship between power and resistance?
Power can be expressed three ways: P = VI (basic), P = I²R (when you know current and resistance), and P = V²/R (when you know voltage and resistance). Doubling the current quadruples the power dissipated (since P = I²R).
How do I find resistance if I only know voltage and power?
Use the formula R = V²/P. For example, if a device runs at 230V and consumes 1000W, its resistance is 230²/1000 = 52,900/1000 = 52.9 Ω. You can also find current first: I = P/V = 1000/230 ≈ 4.35 A, then R = V/I = 230/4.35 ≈ 52.9 Ω.