Ohm's Law Calculator
Enter any 2 of the 4 values — Voltage, Current, Resistance, Power — and the calculator instantly solves the other 2 using Ohm's Law and Watt's Law.
Fill any 2 fields. The other 2 will be calculated automatically.
Voltage
— V
Current
— A
Resistance
— Ω
Power
— W
Quick-Reference Formulas
The Ohm's Law wheel — every way to find V, I, R, and P:
Voltage
V = I × R
Voltage
V = P / I
Voltage
V = √(P × R)
Current
I = V / R
Current
I = P / V
Current
I = √(P / R)
Resistance
R = V / I
Resistance
R = V² / P
Power
P = V × I
Power
P = I² × R
Power
P = V² / R
Resistance
R = P / I²
About Ohm's Law
Ohm's Law (V = IR) describes the relationship between voltage, current, and resistance in an electrical circuit. Combined with Watt's Law (P = VI), you can derive any two electrical quantities from the other two. This calculator covers all 12 formula variations so you never need to rearrange equations by hand. It is useful for electricians, electronics hobbyists, students, and engineers working with DC circuits.
FAQ
What is Ohm's Law?
Ohm's Law states that the voltage (V) across a conductor is equal to the current (I) flowing through it multiplied by its resistance (R): V = I × R. It was formulated by Georg Simon Ohm in 1827 and is the foundation of electrical circuit analysis.
What is Watt's Law and how does it relate?
Watt's Law relates electrical power to voltage and current: P = V × I. By substituting Ohm's Law into Watt's Law, you get additional formulas: P = I²R and P = V²/R. Together, the two laws let you solve for any quantity if you know two others.
Can I use this for AC circuits?
This calculator assumes DC (direct current) circuits or purely resistive AC loads. For AC circuits with inductors or capacitors, you need to account for impedance (Z) instead of simple resistance, and power calculations involve the power factor (cos φ).
Practical example: LED resistor sizing
To power a 2V LED at 20 mA from a 12V supply, you need a resistor to drop the remaining 10V. Enter V = 10 and I = 0.02: the calculator gives R = 500 Ω and P = 0.2 W. You would choose a standard 470 Ω or 510 Ω resistor rated for at least 0.25 W.