Ohm's Law Calculator

Solve for voltage, current, resistance, or power from any two known values using V = I × R and P = V × I.

This Ohm's Law calculator is for students, hobbyists, electricians, and engineers who need to relate voltage, current, resistance, and power quickly and reliably. It is the everyday tool for sizing a resistor, checking whether a component can handle the current, working out the wattage a device will draw, or teaching the fundamental relationship at the heart of electronics. Anyone who has ever stared at a breadboard wondering which resistor to use, or needed to confirm a power supply can deliver enough current, will find it faster than rearranging the formula by hand.

Ohm's Law states that voltage equals current times resistance, written V = I × R, and power is voltage times current, P = V × I. From any two of voltage, current, and resistance, the third follows, and power can then be derived. The calculator asks which quantity you want to solve for, takes the two values you know, and applies the appropriate rearrangement: voltage from current and resistance, current from voltage and resistance, or resistance from voltage and current. It then computes power so you can check component ratings. Because the relationships are exact, the results are precise for ideal direct-current conditions.

Suppose a circuit carries 2 amps through a 10-ohm resistor and you want the voltage. Multiplying 2 by 10 gives 20 volts. The power dissipated is voltage times current, 20 × 2, which is 40 watts — meaning that resistor needs to be rated for at least 40 watts to avoid overheating. Switch the problem around: with 12 volts across a 4-ohm load, current is 12 divided by 4, or 3 amps, and power is 12 × 3, or 36 watts. The calculator handles each rearrangement so you do not have to.

Physics does not change by region, so Ohm's Law is identical worldwide. What differs in practice is mains voltage and frequency — around 120 volts at 60 hertz in North America versus 230 volts at 50 hertz across much of Europe and beyond — and the wiring and safety standards built around them. This calculator deals with the underlying direct-current relationship and does not model alternating-current effects like impedance, reactance, or power factor, which matter for AC circuits and motors.

• Mixing up units — using milliamps where amps are expected, or kilohms where ohms are needed — which throws results off by factors of a thousand.
• Applying simple Ohm's Law to AC circuits with inductance or capacitance, where impedance, not plain resistance, governs the relationship.
• Ignoring the power rating. A resistor with the right resistance can still fail if it cannot dissipate the calculated wattage.
• Forgetting real components have tolerances, so measured values differ slightly from the ideal calculation.
• Dividing by zero — entering zero current when solving for resistance produces a meaningless result.

There are no deadlines in physics, but there is a safety point worth stressing: always confirm a component's voltage, current, and especially power rating before energizing a circuit. Undersized components overheat, and working on mains-voltage circuits without proper training or isolation is dangerous. Use the calculated power figure to choose parts with comfortable headroom rather than ones that sit right at their limit.

• Ohm's law - Reference (retrieved 2026-06-09)