Ohm's law is the first equation of every electrical circuit. Choose the quantity you want, enter the other two, and get the answer along with the power dissipated in the component.
Choose what to solve for.
Enter the other two values.
Read V, I, R and power.
Ohm's law: V = I × R
Power: P = V × I = I²R = V² ÷ R
Ohm's law links the three basic quantities of an electric circuit: voltage V in volts, current I in amperes and resistance R in ohms, through V = I × R. This calculator lets you choose which one to find, takes the other two, and also returns the power P in watts dissipated in the component.
It is the first formula in the Class 10 electricity chapter and the one electronics hobbyists, electricians and engineering students use most. It tells you how much current a heater element draws, what resistor limits current to a safe level, or how much voltage drops across a long wire. The power figure matters just as much, because it decides whether a resistor will stay cool or burn.
1. Decide which quantity is unknown.
2. To find voltage: V = I × R.
3. To find current: I = V ÷ R.
4. To find resistance: R = V ÷ I.
5. Convert milliamps to amps (divide by 1000) and kilohms to ohms (multiply by 1000) before calculating.
6. Power: P = V × I, or equivalently I²R or V² ÷ R.
7. Pick a resistor whose power rating is comfortably above the calculated P, commonly about double.
In a metal wire, free electrons are pushed by the electric field set up by the voltage. They keep colliding with vibrating atoms, so instead of accelerating without limit they settle into a slow average drift. The drift speed, and hence the current, is proportional to the field, and therefore to the voltage. The constant of proportionality depends on the material and shape: R = ρL ÷ A, where ρ is resistivity, L length and A cross-sectional area. Longer, thinner wires have more resistance, which is why extension cords for heavy loads are thick.
Ohm's law is an empirical rule, not a law of nature. Metals at steady temperature obey it closely. But a bulb filament's resistance rises many times as it heats up, so its current is not proportional to voltage. Diodes and LEDs barely conduct until a threshold voltage and then conduct very steeply. Thermistors and LDRs change resistance with temperature or light. For such devices, V = IR still defines the resistance at one operating point, but that value changes as conditions change.
Voltage is energy per unit charge, joules per coulomb, and current is charge per second, coulombs per second. Multiplying them gives joules per second, which is watts. Substituting Ohm's law gives two more forms: P = I²R, useful when current is known, as in wiring losses; and P = V² ÷ R, useful when voltage is fixed, as with household appliances. The last form explains why a heater with a lower resistance is more powerful on the same 230 V supply.
Karthik is building a small circuit on a breadboard with a 9 V battery connected across a single 330 Ω resistor. He wants to know the current drawn and whether a standard quarter-watt resistor will cope.
Ohm's law: I = V ÷ R = 9 ÷ 330 = 0.027273 A
Power: P = V × I = 9 × 0.027273 = 0.245455 W
Answer: Voltage 9 V; Current 0.027273 A; Resistance 330 Ω
Mixing units, such as entering kilohms as ohms or milliamps as amps.
Applying Ohm's law to LEDs and diodes as if they had a fixed resistance.
Measuring the resistance of a bulb or heater when cold and expecting it to hold when hot.
Ignoring power and choosing a resistor rated below the heat it must dissipate.
Using the supply voltage when only part of it appears across the component in a larger circuit.
Choosing current-limiting and pull-up resistors in electronics projects.
Estimating current drawn by heaters, geysers and other resistive appliances.
Calculating voltage drop across long cables and wires.
Class 10 and Class 12 electricity numericals.
Fault-finding with a multimeter, where measured V and I reveal an unexpected R.
What resistor wattage should I buy?
Pick a rating at least double the calculated power for safety.
Does Ohm's law apply to LEDs?
Not directly; LEDs are non-linear. Use the LED Resistor Calculator instead.