Ohm's Law Calculator
Ohm's Law is the foundation of all electrical theory. It defines the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Every electrician learns this formula early in their apprenticeship, and it remains one of the most frequently used calculations throughout their career. This calculator takes voltage and resistance as inputs and returns both the current in amps and the power in watts.
In practical terms, Ohm's Law helps electricians troubleshoot circuits, size conductors, verify that loads are operating correctly, and understand why certain components fail. If you measure 120 volts across a heating element with 10 ohms of resistance, you know it draws 12 amps and consumes 1,440 watts. If the element's resistance drops due to a fault, the current increases—potentially tripping the breaker or causing a fire hazard.
The power formula (P = V × I) extends directly from Ohm's Law. By substituting I = V/R, you get P = V²/R, which is useful when you know the voltage and resistance but not the current. These interrelated formulas form the Ohm's Law wheel that every electrician should know by heart.
Ohm's Law Formulas
I = V ÷ R
Current (amps) = Voltage (volts) / Resistance (ohms)
P = V × I
Power (watts) = Voltage (volts) × Current (amps)
Ohm's Law Reference Table
| To Find | Formula | You Need |
|---|---|---|
| Voltage (V) | V = I × R | Amps and Ohms |
| Current (I) | I = V / R | Volts and Ohms |
| Resistance (R) | R = V / I | Volts and Amps |
| Power (P) | P = V × I | Volts and Amps |
| Power (P) | P = V² / R | Volts and Ohms |
| Power (P) | P = I² × R | Amps and Ohms |
How to Use This Calculator
- Enter the voltage — the electrical potential in volts across the circuit or component.
- Enter the resistance — the opposition to current flow in ohms (Ω).
- Read the results — current in amps and power in watts are calculated instantly.
Frequently Asked Questions
What is Ohm's Law and how is it used in electrical work?
Ohm's Law states that current (I) equals voltage (V) divided by resistance (R): I = V/R. It is the foundational formula in electrical theory and is used by electricians to calculate current flow, determine resistance in circuits, size conductors, and troubleshoot electrical problems. From it, you can also derive power: P = V x I.
How do you calculate amps from voltage and resistance?
Divide the voltage by the resistance: Amps = Volts / Ohms. For example, a 120-volt circuit with 10 ohms of resistance draws 12 amps (120 / 10 = 12). This is the direct application of Ohm's Law: I = V/R.
How do you find watts using Ohm's Law?
Once you know voltage and current, multiply them together: Watts = Volts x Amps. You can also calculate power directly from voltage and resistance using P = V²/R, or from current and resistance using P = I²R. All three formulas give the same result.