Ohm's Law & Power Calculator
Instantly compute voltage, current, resistance, or power in any DC circuit by entering two known quantities. Ideal for electronics hobbyists, electricians, and engineering students.
About this calculator
Ohm's Law states that the voltage across a conductor equals the product of current and resistance: V = I × R. Rearranging gives I = V / R and R = V / I. Electrical power dissipated in a resistive element is P = V × I. Combining these two relationships yields additional forms: P = I²R and P = V²/R. These four quantities — voltage (V), current (A), resistance (Ω), and power (W) — are fully determined once any two are known. The calculator selects the correct formula based on which quantity you want to find and which two inputs you provide, covering all standard combinations used in circuit analysis and component sizing.
How to use
Imagine a 12 V LED driver circuit where the LED strip draws 2 A of current. To find the power consumed, select 'Power' as the calculation type, enter Voltage = 12 V and Current = 2 A. The calculator computes P = V × I = 12 × 2 = 24 W. Now suppose you want to know the effective resistance of the strip: select 'Resistance', keep the same inputs, and the result is R = V / I = 12 / 2 = 6 Ω. This tells you that a resistor or driver rated for at least 24 W and 6 Ω is needed.
Frequently asked questions
How do I use Ohm's Law to find current when voltage and resistance are known?
Rearrange V = IR to get I = V / R. For example, a 9 V battery connected to a 470 Ω resistor passes I = 9 / 470 ≈ 0.019 A, or 19 mA. This calculation is fundamental for selecting current-limiting resistors in LED circuits, ensuring the LED operates within its rated forward current. Always check the power dissipated (P = I²R) to confirm the resistor's wattage rating is not exceeded.
What is the difference between Ohm's Law and the electrical power formula?
Ohm's Law (V = IR) describes the relationship between voltage, current, and resistance in a conductor, and applies strictly to ohmic (linear) materials. The power formula (P = VI) is a broader energy relationship valid for any electrical component, ohmic or not. When combined, they produce P = I²R and P = V²/R, which are extremely useful for calculating heat dissipation in resistors without needing to measure both voltage and current simultaneously.
Why does doubling the current quadruple the power dissipated in a resistor?
Power dissipated in a resistor follows P = I²R, so current appears as a square term. If current doubles from I to 2I, power becomes (2I)²R = 4I²R — four times the original value. This quadratic relationship is why high-current circuits require careful thermal management: a relatively small increase in current produces a disproportionately large increase in heat. It is also why fuses and circuit breakers are sized conservatively to trip well before dangerous temperatures are reached.