Ohm's Law & Power Calculator
Solve for voltage, current, resistance, or power in any DC circuit by entering just two known electrical values. Ideal for electronics hobbyists, electricians, and students checking circuit designs.
About this calculator
Ohm's Law defines the relationship between voltage (V), current (I), and resistance (R): V = I × R. From this single equation, all four fundamental electrical quantities can be derived. Power (P) ties into the system through P = V × I, which can be rewritten as P = I² × R or P = V² / R depending on which values are known. This calculator accepts any two of the four quantities and solves for the remaining two simultaneously. For example, if voltage and resistance are known, current is found via I = V / R and power via P = V² / R. These relationships are foundational to circuit analysis, component selection, and safety calculations in both low-voltage electronics and high-power electrical systems.
How to use
Suppose you know a resistor has 12 V across it and a resistance of 48 Ω. Enter 12 as the first value and select 'Voltage', then enter 48 as the second value and select 'Resistance'. The calculator finds current: I = V / R = 12 / 48 = 0.25 A. It then computes power: P = V² / R = 144 / 48 = 3 W. You now know the resistor must be rated for at least 3 W and the circuit carries 250 mA.
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 is directly proportional to the current flowing through it, with resistance as the constant of proportionality: V = I × R. It is the cornerstone of circuit analysis because it lets engineers and hobbyists predict how a circuit will behave before building it. Without it, selecting the correct resistor, fuse, or wire gauge would require trial and error. It applies to any purely resistive DC circuit and approximates behavior in many AC scenarios as well.
How do I calculate power when I only know current and resistance?
When voltage is unknown but current (I) and resistance (R) are available, power is calculated using the derived formula P = I² × R. For instance, 2 A flowing through a 10 Ω resistor dissipates P = 4 × 10 = 40 W. This form is especially useful when sizing resistors or checking whether a component will overheat. Always choose a resistor with a power rating at least 25% above the calculated dissipation for safety margin.
When should I use the V² / R form of the power formula instead of I × V?
Use P = V² / R when you know the supply voltage and the load resistance but haven't measured current directly — common when sizing heating elements or checking fixed-voltage loads. Use P = I × V when you have measured both current and voltage with a multimeter, as it requires no assumption about resistance linearity. Both formulas are mathematically equivalent for resistive loads, but choosing the right form reduces the number of intermediate calculations and the chance of compounding rounding errors.