Ohm's Law Calculator
Solve for voltage, current, resistance, or power in any DC circuit using Ohm's Law. Essential for electronics hobbyists, electricians, and engineering students troubleshooting circuits.
About this calculator
Ohm's Law defines the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. The core formula is V = I × R, meaning voltage equals current multiplied by resistance. Rearranged, you get I = V / R and R = V / I. Electrical power is calculated separately as P = V × I, which can also be expressed as P = I² × R or P = V² / R. These four quantities are interdependent — knowing any two lets you solve for the remaining ones. This calculator handles all four variants of the formula, making it useful whether you are sizing a resistor, checking wire gauge current capacity, or estimating heat dissipation in a component.
How to use
Suppose you have a 12 V battery connected to a 47 Ω resistor and want to find the current. Select 'Current' as the quantity to calculate, then enter Voltage = 12 V and Resistance = 47 Ω. The calculator applies I = V / R = 12 / 47 ≈ 0.255 A. To find the power dissipated, switch to 'Power' and enter Voltage = 12 V and Current = 0.255 A: P = V × I = 12 × 0.255 ≈ 3.06 W. That tells you the resistor must be rated for at least 3.06 W.
Frequently asked questions
What is Ohm's Law and why is it important in electrical circuits?
Ohm's Law states that the voltage across a conductor equals the current flowing through it multiplied by its resistance: V = I × R. It is the foundational principle for analyzing nearly every resistive circuit. Engineers and hobbyists use it to size components, calculate heat dissipation, and verify safe operating conditions. Without it, you cannot reliably design or troubleshoot any electrical system.
How do I calculate power using Ohm's Law?
Power is calculated as P = V × I, where V is voltage in volts and I is current in amperes. If you only know two of the three basic quantities, you can substitute: P = I² × R when you know current and resistance, or P = V² / R when you know voltage and resistance. Power is measured in watts and tells you how much energy per second a component consumes or dissipates as heat. Always ensure components are rated above the calculated power to avoid damage.
When should I use Ohm's Law versus Kirchhoff's Laws for circuit analysis?
Ohm's Law is ideal for simple series or parallel circuits where a single resistor or equivalent resistance is involved. Kirchhoff's Voltage Law (KVL) and Current Law (KCL) are needed for complex multi-loop networks where Ohm's Law alone cannot resolve all unknowns. In practice, Ohm's Law is applied at the component level within a Kirchhoff analysis. For basic troubleshooting — checking whether a resistor, fuse, or wire is correctly rated — Ohm's Law is almost always sufficient.