SeriesCalc Logo

SeriesCalc

⚡ Ohm's Law Calculator

Enter any 2 values among Voltage (V), Current (I), Resistance (R), and Power (P) to calculate the remaining values.

⚡ Ohm's Law Calculator

Enter any 2 values among Voltage (V), Current (I), Resistance (R), and Power (P) to calculate the remaining values.

V
A
Ω
W

📊 📚 Ohm's Law Formulas & Theory

V
P
R
I
V = I × RV = P / IV = √(P × R)
P = V × IP = I² × RP = V² / R
R = V / IR = V² / PR = P / I²
I = V / RI = P / VI = √(P / R)

Calculator Description

Ohm's law states the fundamental linear relationship among the voltage across a conductor, the current flowing through it, and its resistance. It is the most basic law of electric circuits, established experimentally by Georg Simon Ohm in 1827.

What this calculator finds

Given any two of voltage (V), current (I) and resistance (R), it finds the third and then the power (P). It is the starting point for circuit design, load calculations and selecting protective devices (fuses, breakers).

Why it matters

  • Checking voltage, current and power applied to resistors, LEDs, motors and other components
  • Choosing correct wire gauge and fuse rating to prevent overload and fire
  • Analyzing voltage division and power consumption in series and parallel circuits

Formula

Ohm's Law

When a conductor's resistance is constant, the voltage across it is directly proportional to the current; that proportionality constant is the resistance R. Knowing two of the three quantities yields the third.

V=IRV = I R
I=VRR=VII = \dfrac{V}{R} \qquad R = \dfrac{V}{I}

Electric Power

P=VIP = V I
P=I2R=V2RP = I^{2} R = \dfrac{V^{2}}{R}
  • V — Voltage / potential difference [Volts, V]
  • I — Current [Amperes, A]
  • R — Resistance [Ohms, Ω]
  • P — Power (rate of energy use/supply) [Watts, W]

How the formula works

  • A larger resistance R gives less current at the same voltage (I = V/R).
  • Power is the product V × I, and dissipated in a resistance as heat P = I²R.
  • For the same power, lower voltage means higher current and greater conductor loss — which is why transmission uses high voltage.

Worked example

Connecting a 4 Ω resistor to a 12 V supply gives current I = 12 / 4 = 3 A, and the power dissipated is P = 12 × 3 = 36 W (equivalently P = 3² × 4 = 36 W).

Useful Tips

Practical tips

  • Keep the actual dissipated power comfortably below the resistor's rated power (e.g. 1/4 W).
  • For long wire runs where wire resistance is not negligible, calculate voltage drop separately.
  • For AC, use impedance Z instead of resistance and account for phase: real power is P = V·I·cosφ.

Limitations & cautions

  • Ohm's law holds strictly only for linear (ohmic) elements; diodes, LEDs and lamp filaments are non-linear.
  • Self-heating changes resistance, so computed and actual values can diverge.
  • AC inductive and capacitive effects are not captured by this calculation.