Ohm's Law Calculator

Results

If this power is dissipated in a resistor, choose a rating of at least 1 W (twice the 306.4 mW dissipated) so it runs cool.

How to Use the Ohm's Law Calculator

1. 1. Choose the pair you know. Six pills cover every combination of two from voltage, current, resistance and power. Pick the one that matches your problem.
2. 2. Type the two values and set their units. Each box has its own unit menu — µV to MV, µA to kA, mΩ to GΩ, µW to MW — so 4.7 kΩ and 20 mA go in exactly as printed on the part or the worksheet.
3. 3. Read the other two quantities. They appear at once, rounded to the number of significant figures you choose, with the prefix that keeps each number readable (25.53 mA rather than 0.02553 A).
4. 4. Check the working. The box under the results writes each formula, puts your numbers in and gives the answer, so you can copy the steps into homework or a lab notebook.

Ohm's Law and the Power Law

Ohm's law says the current through a conductor is proportional to the voltage across it: V = I × R, where V is in volts, I in amperes and R in ohms. The constant of proportionality, the resistance, is the property of the component. Georg Ohm published the relationship in 1827, and a material or component that obeys it over a range of voltages is called ohmic.

Power is a separate relationship: the rate at which electrical energy is converted is P = V × I, in watts. Substituting Ohm's law into it gives two more forms, P = I² × R and P = V² ÷ R. Between them, V = IR and P = VI generate twelve formulas — three for each quantity — which textbooks draw as the power wheel. The calculator lists all twelve with your numbers and they always agree, because they are the same two laws rearranged.

Worked Example: A Resistor on a 12 V Supply

A 470 Ω resistor is connected across a 12 V supply. These are the calculator's starting values, using the pair voltage and resistance:

1. Current: I = V ÷ R = 12 V ÷ 470 Ω = 25.53 mA.
2. Power: P = V² ÷ R = (12 V)² ÷ 470 Ω = 306.4 mW.
3. Rating: a resistor should run at no more than about half its rated power, so twice 306.4 mW is 0.613 W, and the next standard size up is a 1 W resistor. A ¼ W part would be badly overloaded.

Try the preset LED resistor: 3 V at 10 mA for a design problem. If a 5 V supply drives a red LED that drops about 2 V, the resistor must drop the remaining 3 V while passing 10 mA: R = 3 V ÷ 10 mA = 300 Ω, dissipating 30 mW, so a ⅛ W part is ample. The LED's forward voltage comes from its data sheet — it is not given by Ohm's law, because an LED is not ohmic.

Limits to Keep in Mind

• Direct current, resistive loads. For AC circuits with coils or capacitors, V ÷ I is the impedance, and real power also depends on the phase angle. This calculator treats every value as a steady DC quantity.
• Non-ohmic parts. Diodes, LEDs, filament lamps and thermistors do not give a straight I–V line. Ohm's law still defines their resistance V ÷ I at one operating point, but that value changes with voltage and temperature.
• Resistance changes with temperature. A hot filament can have ten times the resistance of the same filament cold, so a lamp's rated power only holds at its rated voltage.
• Real supplies are not ideal. A battery's internal resistance lowers the voltage that reaches the load as the current rises.

Frequently Asked Questions

What is the formula for Ohm's law?

V = I × R: voltage in volts equals current in amperes times resistance in ohms. Rearranged, I = V ÷ R and R = V ÷ I. A triangle with V on top and I and R below is a common memory aid — cover the quantity you want and the other two show how to combine them.

How do I calculate power from voltage and current?

Multiply them: P = V × I, with P in watts. A 5 V charger delivering 2 A supplies 10 W. If you know the resistance instead, use P = I² × R or P = V² ÷ R.

What is the Ohm's law power wheel?

It is a diagram that arranges the twelve formulas linking V, I, R and P — three for each quantity — in a circle, so you can pick the one that uses the two values you already have. This calculator prints all twelve as a table with your numbers in.

Does Ohm's law work for AC?

For purely resistive loads such as heaters and incandescent lamps it works with RMS values. When inductors or capacitors are present, resistance is replaced by impedance and the power also depends on the phase difference between voltage and current, which this tool does not model.

What size resistor wattage do I need?

Calculate the power the resistor dissipates, P = I² × R, and pick a standard rating of at least twice that, so it runs cool and lasts. Common ratings are ⅛, ¼, ½, 1 and 2 W; the calculator suggests one for the power it finds.

Why is my measured current different from the calculated value?

Real resistors have a tolerance (often 1% or 5%), supplies sag under load because of their internal resistance, and wires and meter leads add a little resistance of their own. Resistance also drifts as a component warms up.