Ohm's Law & Power Calculator
Basic Electrical Math for Voltage, Current, Resistance & Power
I (Current): The rate of flow of electrical charge.
R (Resistance): The opposition to the flow of current.
P (Power): The rate at which electrical energy is transferred.
Understanding the Ohm's Law Wheel
Whether you are an electrical apprentice studying for your City & Guilds 2365, a DIY enthusiast sizing a fuse, or a qualified engineer checking circuit loads, Ohm's Law is the absolute foundation of all electrical theory.
Formulated by Georg Ohm in 1827, the law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance.
The Core Formulas (The Magic Triangles)
To use this calculator manually, electricians rely on two primary mathematical triangles. By covering up the value you want to find, the triangle reveals the required calculation.
The Ohm's Law Triangle (V, I, R)
- To find Voltage (V): Current (I) × Resistance (R)
- To find Current (I): Voltage (V) ÷ Resistance (R)
- To find Resistance (R): Voltage (V) ÷ Current (I)
The Power Triangle (P, V, I)
In DC circuits and purely resistive AC circuits, Power (measured in Watts) can be calculated using Joule's Law, which is commonly taught alongside Ohm's law.
- To find Power (P): Voltage (V) × Current (I)
- To find Current (I): Power (P) ÷ Voltage (V)
- To find Voltage (V): Power (P) ÷ Current (I)
Practical Applications for Electricians
This TMUK Group calculator combines all 12 permutations of the standard "Ohm's Law Wheel" into one seamless tool. It is commonly used in the field to:
- Determine Load: Calculate how many Amps an appliance will draw based on its Wattage rating, ensuring you do not overload a socket or distribution board.
- Size Protective Devices: Select the correct MCB, RCBO, or fuse rating by calculating the precise current demand of a fixed circuit.
- Calculate Heating Elements: Determine the exact Resistance (Ohms) required for an immersion heater or underfloor heating mat to achieve a specific Wattage output.