Ohm's Law Calculator

Ohm's Law is the foundation of every circuit — it defines the relationship between voltage, current, and resistance, and it's the first formula every engineer, electrician, and hobbyist learns. Use the Ohm's Law calculator below to instantly solve for voltage, current, resistance, or power by entering any two known values. Whether you're sizing a resistor, troubleshooting a circuit, or double-checking a power budget, this tool gives you accurate results in seconds — no manual math required.

Ohm's 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. It was first described by German physicist Georg Ohm, who published his findings in 1827 in Die galvanische Kette, mathematisch bearbeitet ("The Galvanic Circuit Investigated Mathematically") — work that remains the backbone of circuit design nearly 200 years later.

The core formula is simple:

Voltage (V) = Current (I) × Resistance (R)

From this single equation, you can derive every other relationship needed to solve a basic circuit, including power.

Where:

V = Voltage, measured in volts

I = Current, measured in amps

R = Resistance, measured in ohms (Ω)

P = Power, measured in watts

Getting a result takes just three steps:

1. Identify the two values you already know: for example, voltage and resistance.

2. Enter those values into the calculator fields above.

3. Read your result: the calculator automatically solves for the remaining variable(s), including current and power.

This makes it easy to reverse-engineer a circuit, verify a resistor's power rating, or plan a design before you ever pick up a multimeter.

Ohm's Law isn't just a classroom formula — it's a practical tool for choosing the right resistor for your application. Getting the math right up front helps you avoid underrated components, excess heat, and premature failure.

• Confirm power dissipation before you specify a part. Use P = I² × R to calculate how much heat a resistor will need to dissipate under load, then choose a resistor rated with adequate headroom above that figure.

• Size resistors for current-limiting applications. If you know your supply voltage and target current, R = V ÷ I tells you exactly what resistance value you need.

• Validate load calculations for high-power circuits. For applications like motor drives, power supplies, and braking systems, Ohm's Law helps confirm that a resistor can handle sustained current without exceeding its thermal limits.

• Troubleshoot unexpected circuit behavior. If measured voltage or current doesn't match your expected values, Ohm's Law is the fastest way to identify whether a component is out of spec.

Ohmite manufactures precision resistors engineered for exactly these scenarios — from current-sense and power resistors to specialty components built for demanding electrical loads. Once you've calculated the resistance and power values your application requires, our team can help you match those specs to the right part.

Running the numbers is the easy part — choosing a resistor that can reliably handle them is where experience matters. Explore Ohmite's full line of power, current-sense, and specialty resistors, or contact our engineering team to get expert guidance on the right component for your application.