TAHA Engineering Toolsby TAHA Knowledge Lab

Electrical Engineering

Ohm’s Law Calculator

Enter any two of voltage, current, resistance and power to find the others.

Inputs

Calculate from
Circuit with a voltage source V, a resistor R and current I flowing round the loop, plus the Ohm’s law triangle V over I times R.+VRIVIR
A voltage V drives a current I through a resistance R. In the triangle, cover the quantity you want: V = I × R, I = V ÷ R, R = V ÷ I. Power P = V × I. Diagram not to scale.

Result

Current
2
Voltage
12
Resistance
6
Power
24

With 12 V across a resistance of 6 Ω, a current of 2 A flows and 24 W is turned into heat.

Educational estimate — check the assumptions below.

About this calculator

Ohm’s law links the voltage across a resistor, the current through it and its resistance. Combined with the power equation, it lets you find all four quantities when you know any two. It is the basic tool for checking resistor sizes, heater power, LED series resistors and simple DC circuits.

Formula

V = I × R     P = V × I

Rearranged: I = V ÷ R,  P = I²R = V² ÷ R,  I = √(P ÷ R),  V = √(P × R)

V in volts, I in amperes, R in ohms, P in watts.

Worked example

A 6 Ω resistor is connected to a 12 V supply.

  • Current: I = 12 ÷ 6 = 2 A
  • Power: P = 12² ÷ 6 = 24 W

A 100 W heater on 230 V: I = 100 ÷ 230 = 0.435 A, R = 230² ÷ 100 = 529 Ω (at operating temperature).

Assumptions and limitations

  • Valid for DC circuits, and for AC circuits with purely resistive loads using RMS values.
  • Not valid for motors, transformers, fluorescent drivers or other inductive/capacitive loads, where power factor matters.
  • Resistance is treated as constant. Real resistance changes with temperature (a heater element’s cold resistance is lower than its hot resistance).

Safety note

Mains voltages can kill. This calculator is for learning and simple checks — it is not a tool for sizing cables, fuses or protective devices. Electrical installation work must follow your local wiring regulations and be done by a qualified person.

References

  • Ohm’s law and Joule’s law, as taught in standard circuit-theory texts.