How to Calculate Ohm's Law (V, I, R)
Ohm’s law relates **voltage (V)**, **current (I)**, and **resistance (R)** in many DC and resistive AC circuits. Know two values to solve the third. **Power** in watts is P = V × I.
Core formula
V = I × R · P = V × I = I²RStep by step
1. Draw knowns with units
Volts (V), amperes (A), ohms (Ω). Convert mA → A (÷1000) before calculating.
2. Solve the missing variable
V = I×R, I = V/R, R = V/I. Rearrange only when R is approximately constant.
3. Check power & heat
P = V×I. Resistors dissipate heat; verify rating exceeds P.
Series vs parallel resistance
Ohm’s law applies to each element; combinations use equivalent resistance.
- Series: R_eq = R1 + R2 + …; same current through each.
- Parallel: 1/R_eq = 1/R1 + 1/R2 + …; same voltage across branches.
- AC circuits: Impedance Z replaces R for capacitors/inductors.
- Non-ohmic devices: LEDs/diodes need more than simple V=IR.
Common mistakes
- Mixing mA with A
- Applying DC formula to AC without impedance
- Ignoring power limits on components
FAQ
Does Ohm's law apply to everything?
No—only approximately for ohmic, linear resistors at steady state.
Why did my circuit differ?
Temperature changes R; contacts and measurement error matter.