Ohm's Law Calculator
An Ohm's Law calculator finds voltage, current, resistance, or power when you know any two of the four values. Enter two known electrical values in the panel and the calculator will instantly solve for the other two using V = IR and P = IV formulas.
What Is Ohm's Law?
Ohm's Law is a fundamental principle in electrical engineering and physics that describes the relationship between voltage, current, and resistance in an electrical circuit. Named after German physicist Georg Simon Ohm, who published his findings 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, provided the temperature and other physical conditions remain constant.
Ohm's Law applies to resistive conductors and is the foundation for circuit analysis, electronics design, electrical safety calculations, and virtually every field of electrical engineering. Understanding and applying Ohm's Law is a core skill for electricians, engineers, hobbyists, and anyone working with electrical systems.
Ohm's Law Formula
V = I × R
The primary Ohm's Law formula expresses voltage as the product of current and resistance:
- V — Voltage in volts (V). The electrical potential difference or "pressure" driving current through the circuit.
- I — Current in amperes (A). The rate of electron flow through the conductor.
- R — Resistance in ohms (Ω). The opposition to current flow provided by the conductor or component.
Rearranged Forms
The same formula can be rearranged to solve for any of the three variables:
The Power Wheel
Power Formula (P = IV)
Power is the rate at which electrical energy is consumed or produced in a circuit. The fundamental power formula combines with Ohm's Law to create the "Power Wheel" — a set of 12 formulas that allow you to calculate any one of the four electrical quantities (V, I, R, P) from any two others.
Power is measured in watts (W). One watt equals one volt multiplied by one ampere. A 60-watt light bulb connected to a 120-volt supply draws 0.5 amperes of current.
All 12 Formula Combinations
| Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P / I | V = √(P × R) |
| Current (I) | I = V / R | I = P / V | I = √(P / R) |
| Resistance (R) | R = V / I | R = V² / P | R = P / I² |
| Power (P) | P = V × I | P = I² × R | P = V² / R |
Ohm's Law Chart
The Ohm's Law wheel (also called the power wheel or PIE chart in some references) organizes all 12 formulas into four quadrants — one for each of the four electrical quantities. Each quadrant shows three ways to calculate that quantity from two other known values. Use the reference table above or the calculator on the left to solve any combination instantly.
How to Use Ohm's Law
Finding Voltage
To find voltage, you need to know the current and resistance, or the power and current, or the power and resistance. The most common approach is V = I × R. If 3 amperes flow through a 4-ohm resistor, the voltage across it is 3 × 4 = 12 volts. Use this when sizing power supplies or checking voltage drop across a component.
Finding Current (Amps)
Current is found using I = V / R when voltage and resistance are known. If a 24-volt battery is connected through a 6-ohm resistor, the current is 24 / 6 = 4 amps. Current calculations are critical for selecting the correct wire gauge, fuse ratings, and breaker sizes. Exceeding the rated current of a wire can cause overheating and fire. For motor applications, also see our motor FLA calculator to estimate full load amps.
Finding Resistance
Resistance is calculated using R = V / I. If 120 volts drives 2 amps through a component, its resistance is 120 / 2 = 60 ohms. Resistance calculations help identify component values needed in a circuit, check if a component is within specification, or diagnose circuit faults by comparing measured resistance to expected values. To identify resistor values from color bands, use our resistor color code calculator.
Finding Power (Watts)
Power is calculated most directly as P = V × I. A 120-volt appliance drawing 5 amps consumes 600 watts. Power calculations determine energy consumption, heat dissipation in resistors, and the wattage rating needed for components. Always select resistors with a wattage rating higher than the calculated power dissipation to prevent failure.
Ohm's Law Examples
Example 1 — LED Circuit (Voltage and Resistance Known)
A 9-volt battery powers an LED circuit with a 470-ohm current limiting resistor. Using I = V / R: current = 9 / 470 = 0.0191 A (19.1 mA). Power dissipated by the resistor: P = I² × R = (0.0191)² × 470 = 0.172 watts. A quarter-watt (0.25W) resistor is sufficient. The LED receives approximately 19 mA, which is within the typical 20 mA maximum rating for standard LEDs.
Example 2 — Home Appliance (Voltage and Power Known)
A household heater is rated 1500 watts on a 120-volt circuit. Current drawn: I = P / V = 1500 / 120 = 12.5 amps. Effective resistance: R = V / I = 120 / 12.5 = 9.6 ohms. This means the circuit requires a minimum 15-amp breaker and 14 AWG wire. Most electricians recommend a 20-amp circuit for heaters to provide a safety margin above the 12.5-amp draw.
Example 3 — Battery Discharge (Current and Resistance Known)
A 12-volt car battery is tested with an internal resistance of 0.05 ohms. A 200-amp starter motor load creates a voltage drop of V = I × R = 200 × 0.05 = 10 volts across the internal resistance, leaving only 2 volts at the terminals. This explains why battery voltage drops significantly during engine cranking and why battery internal resistance is a key measure of battery health.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that voltage equals current multiplied by resistance: V = I × R. It describes the relationship between the three fundamental electrical quantities in a resistive circuit. The law was formulated by Georg Simon Ohm in 1827 and remains a cornerstone of electrical engineering.
How do you calculate amps from volts and ohms?
To calculate amps from volts and ohms, divide voltage by resistance: I = V / R. For example, if you have 24 volts and 8 ohms of resistance, the current is 24 / 8 = 3 amps. Enter these values in the calculator above and it will solve instantly.
What is the relationship between volts, amps, and ohms?
Volts, amps, and ohms are related by Ohm's Law: V = I × R. Voltage (volts) is the electrical pressure. Current (amps) is the rate of electron flow. Resistance (ohms) is the opposition to that flow. Increasing voltage while keeping resistance constant increases current. Increasing resistance while keeping voltage constant decreases current.
How do you convert ohms to amps?
Ohms and amps measure different quantities, so they cannot be converted directly. To find amps from ohms, you also need to know the voltage: I = V / R. For example, with 12 volts and 4 ohms, the current is 12 / 4 = 3 amps. Use the calculator above by entering your voltage and resistance values.
What is the power formula in electricity?
The basic power formula is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amps. Combined with Ohm's Law, it becomes P = I² × R and P = V² / R. These three versions let you calculate power from any two known electrical values.