Ohm's Law Calculator
Ohm's Law Calculator — V, I, R & Power
Solve for Voltage · Current · Resistance · Power using V = IR and P = IV
| Voltage (V) | Volt | V |
| Current (I) | Ampere | A |
| Resistance (R) | Ohm | Ω |
| Power (P) | Watt | W |
Solve Any Variable
Enter any two known quantities to instantly compute the other two — all four values update together.
Includes Power
Power (P = V × I) is derived automatically alongside V, I, and R — covering AC & DC circuit basics.
Educational Guide
Scroll down for a complete plain-English explanation of Ohm's Law, the triangle method, and real-world uses.
Understanding Ohm's Law
What is Ohm's Law? A Complete Beginner-to-Advanced Guide
Master the foundation of all electrical circuit analysis — from LED resistors to industrial power systems
Ohm's Law is one of the most fundamental principles in electrical engineering and physics. Formulated by German physicist Georg Simon Ohm in 1827, it describes the relationship between three core electrical quantities: Voltage (V), Current (I), and Resistance (R). Simply put, Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.
This single law underpins the design of virtually every electronic device on the planet — from the charger on your phone to high-voltage industrial motors. Whether you are a student studying physics, an electronics hobbyist building circuits, or an engineer designing power systems, Ohm's Law is the starting point for all circuit analysis.
The core relationship can be expressed in three equivalent forms, each isolating a different unknown:
A helpful memory aid is the VIR Triangle (also called the Ohm's Law Magic Triangle): write V on top and I × R on the bottom. Cover the quantity you want to find and read the formula from the remaining two. Cover V → you get I × R. Cover I → you get V ÷ R. Cover R → you get V ÷ I.
Electrical Power (P) measures how much energy is transferred per second. It is measured in Watts (W) and connects to Ohm's Law through the following relationships:
Together, these twelve formulas (three for V, I, R and three more for P) form the complete Ohm's Law power wheel. This calculator handles all of them: enter any two values and it instantly resolves the remaining two.
If you know the current through a resistor and its resistance value, multiply them: V = I × R. Example: 3 A through a 10 Ω resistor → 30 V drop.
If you know the voltage source and resistance, I = V ÷ R. Example: a 9 V battery with a 100 Ω resistor draws 90 mA of current.
If you know voltage and current, R = V ÷ I. Useful when selecting a current-limiting resistor for an LED or measuring an unknown component.
LEDs require a current-limiting resistor. Use R = (V_supply − V_forward) ÷ I_LED to choose the right resistor value and protect the LED from burning out.
Charger engineers use Ohm's Law to calculate cable resistance losses, heat dissipation, and charging current limits to ensure safe and efficient designs.
Electricians calculate wire gauge and breaker ratings using I = P ÷ V to ensure wiring does not overheat under rated load conditions.
Amplifier output impedance must match speaker impedance for maximum power transfer — a direct application of Ohm's Law and the maximum power transfer theorem.
Ohm's Law applies perfectly to ohmic conductors — materials like metals whose resistance stays constant regardless of voltage or current. However, it does not apply directly to: