How to Calculate Using Ohm's Law
Ohm's Law describes the fundamental relationship between voltage, current, and resistance in electrical circuits. Published by Georg Ohm in 1827, it remains the cornerstone of basic circuit analysis.
The Formulas
V = I × R
I = V / R
R = V / I
Where:
- V = voltage (volts, V)
- I = current (amperes or amps, A)
- R = resistance (ohms, Ω)
Memory trick: The VIR triangle — cover the variable you want to find, the remaining two show the formula.
Step-by-Step Examples
Find current: A 9V battery connected to a 180Ω resistor. I = V / R = 9 / 180 = 0.05 A = 50 mA
Find resistance: A circuit has 12V and 2A flowing through it. R = V / I = 12 / 2 = 6 Ω
Find voltage: A 4Ω heater carries 5A of current. V = I × R = 5 × 4 = 20 V
Power Triangle
The power formulas extend Ohm's Law:
P = V × I = I² × R = V² / R
A 60W light bulb on a 120V circuit: I = P / V = 60 / 120 = 0.5 A R = V / I = 120 / 0.5 = 240 Ω
Series vs. Parallel Circuits
Series: Resistance adds up: Rtotal = R1 + R2 + R3 Parallel: Reciprocals add: 1/Rtotal = 1/R1 + 1/R2 + 1/R3
Two 100Ω resistors in parallel: 1/R = 1/100 + 1/100 = 2/100 → R = 50 Ω
Use our Ohm's Law calculator to solve for any circuit variable.