⚡ Ohm's Law Calculator
Solve V = I × R for any variable. Also calculates power P = V×I.
How to Use This Calculator
Select the variable you want to find from the dropdown, enter the two known values, and click Calculate. The results always show all four values: voltage, current, resistance, and power, so you get a complete picture of the circuit in one step.
Select what to solve for: Voltage (V), Current (A), or Resistance (Ω).
Enter voltage in volts. Common values: 1.5 V (AA battery), 9 V (PP3 battery), 12 V (car battery), 230 V (UK mains).
Enter current in amperes. For small electronics you may be working in milliamps: 20 mA = 0.02 A.
Read all four results at once: V, I, R, and power P in watts. Power tells you how much energy the component consumes per second.
Ohm's Law Formula
V is voltage in volts, I is current in amperes, and R is resistance in ohms. The four power formulas all give the same result and you pick whichever uses the two variables you already know. For a 12 V car lamp drawing 2 A: R = 12 / 2 = 6 Ω and P = 12 × 2 = 24 W.
Worked Examples
Where This Comes Up in Real Life
Every time you connect a component in an electronics project, you are applying Ohm's Law. A microcontroller pin typically outputs 3.3 V or 5 V and can safely supply only about 20-40 mA. To drive an LED that needs 20 mA with a forward voltage of 2 V from a 5 V pin, you need a series resistor: R = (5 - 2) / 0.02 = 150 Ω. Skipping this resistor burns out the LED in seconds.
Electrical wiring in buildings is sized using Ohm's Law and power calculations. A circuit running at 230 V with 2.5 mm² cable (resistance about 7.4 Ω per km) to a socket 20 m away has a loop resistance of 2 × 20 × 7.4 / 1000 = 0.296 Ω. At 13 A, the voltage drop is 0.296 × 13 = 3.85 V, or about 1.7% of 230 V. UK wiring regulations allow a maximum of 3% voltage drop, so this cable is acceptable for that run length.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that voltage (V) = current (I) × resistance (R). It describes the relationship between these three fundamental electrical quantities.
What is power in an electrical circuit?
Power P = V×I = I²×R = V²/R. It is the rate at which energy is consumed, measured in watts (W).
What are standard units?
Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), power in watts (W).
When does Ohm's Law not apply?
Ohm's Law applies to ohmic (linear) conductors. Non-linear devices like diodes and transistors do not follow it strictly.
Example: What current flows through 100Ω at 12V?
I = V/R = 12/100 = 0.12 A = 120 mA. Power = V×I = 12×0.12 = 1.44 W.