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Ohm's law is the single most important relationship in electronics: the voltage across a resistor equals the current through it times its resistance, V = I × R. Know any two of the three and the third follows — and from them, the power dissipated.
Choose the quantity you want, enter the two you know, and get the result plus the power in watts.
How is it calculated?
The three forms of Ohm’s law
| Solve for | Formula |
|---|---|
| Voltage V | current × resistance |
| Current I | voltage ÷ resistance |
| Resistance R | voltage ÷ current |
Voltage is in volts (V), current in amperes (A) and resistance in ohms (Ω).
Power too
Once any two are known, the power dissipated follows from P = V × I (equivalently I²R or V²/R). This tool reports the power in watts alongside the missing quantity — useful for choosing a resistor's power rating so it doesn't overheat.
The water analogy
Voltage is like water pressure, current like the flow rate, and resistance like the narrowness of the pipe. Raise the pressure (voltage) and more flows; narrow the pipe (raise resistance) and less flows. It's an intuition, not a proof, but it gets the direction of every relationship right.
Where it helps
Sizing a series resistor for an LED, checking whether a component can handle its power, or working back to an unknown resistance from measured voltage and current. Real components drift with temperature, so treat the result as the ideal design value.
Worked example
An LED needs about 2 V and 20 mA (0.02 A) from a 5 V supply. The series resistor must drop 5 − 2 = 3 V at 0.02 A, so R = V ÷ I = 3 ÷ 0.02 = 150 Ω. The power it dissipates is P = V × I = 3 × 0.02 = 0.06 W, well within a standard ¼-watt resistor — so a 150 Ω, ¼ W resistor is safe.
FAQ
What is Ohm’s law?+
Ohm’s law states that voltage equals current times resistance: V = I × R. It describes how much current flows through a resistance for a given voltage.
How do I calculate current from voltage and resistance?+
Divide the voltage by the resistance: I = V ÷ R. A 12 V supply across a 4 Ω resistor drives 3 A. Pick "Current" mode and enter the two values.
How is power calculated?+
Power is voltage times current: P = V × I, which also equals I²R or V²/R. The calculator shows the wattage automatically so you can size a resistor’s power rating.
What units does the calculator use?+
Volts (V) for voltage, amperes (A) for current and ohms (Ω) for resistance, giving power in watts (W). Convert milliamps to amps first (20 mA = 0.02 A).
What resistor do I need for an LED?+
Subtract the LED’s forward voltage from the supply, then divide by the LED current: (5 V − 2 V) ÷ 0.02 A = 150 Ω. Check the power (here 0.06 W) is within the resistor’s rating.