Ohm's Law & Electric Power Calculator
Ohm's Law is the fundamental law of electronics. Easily determine circuit voltage, current draw, required resistor sizing, and wattage ratings.
How it works
State equations evaluate any pair of parameters (Voltage, Current, Resistance) to solve for the missing quantities and the total electrical power dissipation.
Formula
V = I × R, I = (V / R), R = (V / I), P = V × I = I^2 R = (V^2 / R)
Relates electrical voltage, current flow, circuit resistance, and thermal power dissipation in direct current (DC) circuits.
| Symbol | Variable | Unit | Description |
|---|---|---|---|
| V | Electric potential difference | V | |
| I | Electric current | A | |
| R | Circuit electrical resistance | Ω | |
| P | Electric power dissipation | W |
Worked example
12V DC power supply through a 6Ω resistor
- Voltage:
- 12 V
- Resistance:
- 6 Ω
Current I = 12 / 6 = 2.0 A (2,000 mA). Power P = 12 × 2 = 24.0 Watts. A resistor rated for at least 25W is recommended.
Limitations
Does not apply to non-linear components such as semiconductor diodes, transistors, or gas discharge lamps.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance.
How do I choose the correct power wattage rating for a resistor?
Calculate P = I²R or P = V²/R and select a resistor with a wattage rating at least 1.5× to 2× higher than the calculated dissipation for thermal safety.
Does Ohm's Law apply to AC alternating current?
Yes, for purely resistive AC circuits. For circuits containing capacitors or inductors, resistance is replaced by impedance (Z = R + jX).
Related calculators
Engineering Computational Notebook
Interactive engineering computation notebook for evaluating algebra, trigonometry, calculus derivatives, matrices, units, and persistent variable assignments.
Resistive Voltage Divider Calculator
Calculate the output voltage (V_out) and current of a two-resistor voltage divider network for microcontroller ADC level shifting and sensor interfacing.
LC Resonant Frequency & Tank Circuit Calculator
Calculate the natural resonant frequency (f0) and angular velocity (ω0) of an LC tank circuit, radio tuner, or notch filter.
Electricity Bill & Appliance Power Cost Calculator
Calculate the power consumption and electricity running cost of home appliances (AC, geyser, refrigerator, heater) per day, month, and year.
Online Scientific Calculator
Free advanced online scientific calculator with full trigonometric, logarithmic, exponential, root, factorial, and hyperbolic functions.
Last reviewed: 2026-08-22.