Enter any two values (Voltage, Current, Resistance) and leave the third field blank — it will be calculated automatically. Power is calculated from whichever two of the three you provide.
What Ohm's Law Actually Tells You
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it, with resistance as the constant of proportionality: V = I × R. Rearranged, that also gives you I = V ÷ R and R = V ÷ I. It's one of the few laws in electrical engineering you'll use in almost every practical calculation, from checking whether a component can handle a circuit to sizing a resistor for an LED.
Where This Shows Up in Real Projects
The most common hobbyist use is picking a current-limiting resistor for an LED: subtract the LED's forward voltage from the supply voltage, then divide by the desired current to get the resistor value. On the safety side, Ohm's Law combined with Power (P = V × I) is how you check whether a wire or component will overheat — if the calculated power exceeds a resistor's rated wattage (commonly 1/4W or 1/2W for small resistors), it will run hot and can fail or start a fire in the worst case.
Series vs Parallel Circuits Change the Math
This calculator assumes a simple, single-component circuit. In a series circuit, the same current flows through every component, but voltage divides across each one based on its resistance. In a parallel circuit, every branch sees the same voltage, but current divides based on each branch's resistance, and total resistance is always lower than the smallest individual resistor. For multi-component circuits, calculate the equivalent resistance first, then apply Ohm's Law to that combined value.
Power, Voltage, Current — The Full Picture
| Known values | Formula to find the third |
|---|---|
| Voltage & Current | R = V ÷ I |
| Voltage & Resistance | I = V ÷ R |
| Current & Resistance | V = I × R |
| Any two of V, I, R | P = V × I (power in watts) |
Frequently Asked Questions
Why does the calculator need exactly two values?
Ohm's Law relates three quantities with one equation, so knowing any two lets you solve for the third. If you enter all three, the calculator will use whichever two make sense together and recompute the result to check consistency.
Does Ohm's Law apply to AC circuits too?
The basic V = IR relationship applies directly to resistive DC circuits. For AC circuits with inductors or capacitors, resistance is replaced by impedance, which also accounts for frequency-dependent effects — the same core relationship holds, but the math is more involved.
How do I size a resistor for an LED?
Subtract the LED's forward voltage (check its datasheet, commonly 1.8–3.3V) from your supply voltage, then divide by the LED's rated forward current (often 20mA = 0.02A). The result is the resistor value in ohms needed to limit current safely.
What happens if I exceed a resistor's power rating?
The resistor will run hotter than intended, which can shift its resistance value, degrade nearby components, or in extreme cases cause it to burn out or become a fire hazard. Always leave headroom above the calculated wattage — many designers pick a resistor rated for roughly double the expected power dissipation.