How to Calculate Ohm's Law: Voltage, Current, Resistance & Watts
Master Ohm's Law (V = IR) and electrical power formulas (P = VI). Learn how to solve for voltage, current, resistance, and wattage with circuit examples.
How to Calculate Ohm's Law: Voltage, Current, Resistance & Watts
Ohm's Law is the most foundational mathematical relationship in electrical engineering and physics. Formulated by Georg Simon Ohm in 1827, it governs how electric potential difference (Voltage), electron flow rate (Current), and opposition to current (Resistance) interact in direct current (DC) and single-phase alternating current (AC) circuits.
This guide explains the Ohm's Law equations, the electrical power wheel (Watts), practical circuit examples, and connects to the CalculatorAll Ohm's Law Calculator.
The Core Ohm's Law Formulas (V = I × R)
- Voltage (V in Volts):
V = I × R - Current (I in Amperes):
I = V / R - Resistance (R in Ohms — Ω):
R = V / I
The Electrical Power Formulas (P = V × I)
Electrical power (measured in Watts) describes the rate at which electrical energy is converted into work, heat, or light:
- Power (P in Watts):
P = V × I - Power from Current & Resistance:
P = I² × R - Power from Voltage & Resistance:
P = V² / R
Worked Example: 120V Home Appliance
Suppose you plug a space heater with a 10 Ω internal resistance into a standard 120V household wall outlet:
- Calculate Circuit Current (I):
I = V / R = 120 V / 10 Ω = 12.0 Amps. - Calculate Power Dissipated (P):
P = V × I = 120 V × 12.0 A = 1,440 Watts (1.44 kW). - Safety Check: Since 12.0 Amps is below standard 15-Amp residential circuit breaker trip thresholds (80% continuous load rule = 12A), the appliance operates safely.
Ohm's Law & Power Equation Reference Matrix
| Known Variables | Calculate Voltage (V) | Calculate Current (I) | Calculate Resistance (R) | Calculate Power (P) |
|---|---|---|---|---|
| V and I | — | — | R = V / I | P = V × I |
| V and R | — | I = V / R | — | P = V² / R |
| I and R | V = I × R | — | — | P = I² × R |
| P and I | V = P / I | — | R = P / I² | — |
| P and V | — | I = P / V | R = V² / P | — |
| P and R | V = √(P × R) | I = √(P / R) | — | — |
Try the numbers with our calculator
Use your own assumptions instead of relying on a generic example.
Calculate Voltage, Current & Watts
