Ohms Law Calculator

What this calculator does

The Ohms Law Calculator finds the two missing values when you know any two of voltage, current, resistance, and power. It combines Ohm's law, which relates voltage, current, and resistance, with Joule's law, which relates power to voltage and current. The result shows all four quantities in base units so you can see the full operating point of a circuit.

It is useful for checking a resistor value, estimating load current, finding component power, or confirming a supply can deliver what a device needs. The calculator accepts common prefixed units, including kilovolts, milliamperes, kilohms, megohms, kilowatts, megawatts, horsepower, and BTU per hour.

How to use this calculator

1. Enter exactly two of the four values and leave the other two fields empty.
2. Choose the matching unit for each value you entered.
3. Click Calculate.
4. Read all four results, including the values that were solved for.

The default example enters 110 V and 0.2 A. The calculator finds a resistance of 550 Ω and a power of 22 W. You can also enter 12 V and 4 Ω to find 3 A and 36 W, or enter 100 W and 50 V to find 2 A and 25 Ω.

Ohm's law and power formulas

V = I × R        I = V / R        R = V / I
P = V × I        P = V² / R       P = I² × R

V is voltage in volts, I is current in amperes, R is resistance in ohms, and P is power in watts. Only two independent values are needed because the equations link all four. For example, if voltage and resistance are known, current follows from I = V / R and power follows from P = V × I.

Each input is converted to base SI units before solving. Kilovolts become volts, milliamperes become amperes, kilohms become ohms, and horsepower becomes watts using the mechanical definition of 745.7 watts per horsepower. BTU per hour is converted using 0.29307107 watts per BTU/h.

What the results mean

Voltage is the electrical potential difference across the circuit. Current is the rate of charge flow. Resistance is the opposition to that flow. Power is the rate at which energy is transferred or dissipated. In a resistive circuit, the power result is also the heat that the component must safely dissipate.

A zero value can make a result zero or leave the calculation undefined. For example, 0 V across a known resistance produces 0 A, but solving for resistance from 0 V and nonzero current requires division by zero. The calculator reports an error instead of infinity.

Assumptions and limitations

• The equations assume ideal resistive behavior. They do not model capacitors, inductors, reactance, impedance, or frequency-dependent effects.
• The power result is the instantaneous or average real power for a resistive load, not apparent power or reactive power.
• The calculator uses positive values only. It does not handle signed current directions or complex impedance.
• Horsepower is converted as mechanical horsepower (745.7 W). Metric horsepower uses a different conversion.
• Actual components have tolerances, temperature drift, and voltage limits that the calculator does not capture.

Frequently asked questions

Can I enter all four values?

No. The calculation is overdetermined when more than two values are entered, and real measurements rarely agree exactly. Enter any two values and leave the other two empty so the calculator can solve for the rest.

Why do I get an error when I enter zero?

Zero is valid for voltage, current, or power, but it can make another equation undefined. For example, resistance cannot be found by dividing voltage by zero current. Change the inputs so the solved quantity has a nonzero denominator.

Is Ohm's law valid for AC circuits?

Ohm's law applies to AC circuits that are purely resistive. With capacitors or inductors, impedance replaces resistance and the phase relationship between voltage and current matters. This calculator does not model that case.

Reference table

Voltage unit conversions

Ohm's law: V = I × R, I = V ÷ R, and R = V ÷ I. Power: P = V × I.

Source

Reference: Calculator.net — Ohms Law Calculator.

Disclaimer

This calculator provides estimates for educational and planning purposes. Verify component ratings, tolerances, and circuit conditions with a datasheet and appropriate test equipment before building or repairing a circuit.