Ohm's Law Calculator
This Ohm's law calculator solves electrical circuits in seconds. Pick what you want to find — voltage, current or resistance — enter the two known values, and the tool applies V = I × R to return the missing quantity along with the power dissipated. It is ideal for electronics projects, physics homework, and anyone sizing resistors, LEDs, or power supplies without reaching for a formula sheet.
Result
Power (W)
Ohm's law relates voltage, current and resistance: V = I × R. Power is given by P = V × I.
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How it works
Ohm's law states that the voltage V across a conductor equals the current I flowing through it multiplied by its resistance R: V = I × R. Rearranging the same equation lets you find any of the three quantities when the other two are known: current is I = V ÷ R and resistance is R = V ÷ I. Electrical power, the rate at which energy is used, follows from P = V × I, which can also be written as P = I² × R or P = V² ÷ R. For example, a current of 2 A through a 6 Ω resistor produces a voltage of 2 × 6 = 12 V and dissipates 12 × 2 = 24 W of power.
The Ohm's law formulas
Ohm's law can be rearranged to solve for any of its three quantities, and to that trio we add the power formula. These four relationships cover almost every calculation in a direct-current circuit. With just two known values you can always work out the rest: for example, if 0.5 A flows through a resistor at 9 V, the resistance is 9 ÷ 0.5 = 18 Ω and the power dissipated is 9 × 0.5 = 4.5 W.
| To find | Formula |
|---|---|
| Voltage (V) | V = I × R |
| Current (I) | I = V ÷ R |
| Resistance (R) | R = V ÷ I |
| Power (P) | P = V × I |
Example: same voltage, different resistance
To see how the three quantities relate, let's keep the voltage fixed at 12 V and vary the resistance. The larger the resistance, the smaller the current that flows (I = V ÷ R) and therefore the less power is dissipated (P = V × I). The table shows four cases at 12 V: going from 2 Ω to 12 Ω, the current drops from 6 A to 1 A and the power falls from 72 W to 12 W. This inverse relationship between resistance and current is the key to sizing, for instance, the current-limiting resistor for an LED.
| Resistance (Ω) | Current I = V ÷ R (A) | Power P = V × I (W) |
|---|---|---|
| 2 | 6 | 72 |
| 4 | 3 | 36 |
| 6 | 2 | 24 |
| 12 | 1 | 12 |
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Preguntas frecuentes
- What is Ohm's law?
- Ohm's law is a fundamental relationship in electronics stating that the voltage across a component equals the current through it times its resistance: V = I × R. Named after Georg Ohm, it lets you find any one of voltage, current or resistance when you know the other two, and it underpins almost every basic circuit calculation.
- How do I calculate current from voltage and resistance?
- Rearrange V = I × R to isolate current: I = V ÷ R. Divide the voltage in volts by the resistance in ohms and the result is the current in amperes. For example, 12 V across a 6 Ω resistor gives 12 ÷ 6 = 2 A.
- How do I find resistance from voltage and current?
- Use R = V ÷ I. Divide the voltage in volts by the current in amperes to get the resistance in ohms. For example, 12 V with a current of 2 A means the resistance is 12 ÷ 2 = 6 Ω.
- How is electrical power calculated?
- Power is the product of voltage and current: P = V × I, measured in watts. Using Ohm's law you can also write it as P = I² × R or P = V² ÷ R. A device at 12 V drawing 2 A uses 12 × 2 = 24 W.
- What are the units for voltage, current and resistance?
- Voltage is measured in volts (V), current in amperes or amps (A), and resistance in ohms (Ω). Power is measured in watts (W). Keeping all values in these base units ensures the formulas V = I × R and P = V × I give correct results.