Ohm's Law Calculator

Enter any two of voltage, current, resistance and power to get the other two, with mA, kΩ and MW prefixes, the 12-formula wheel and a series/parallel helper.

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About this tool

What is the Ohm's Law Calculator?

Type any two of voltage (V), current (I), resistance (R) and power (P) and this Ohm's law calculator fills in the other two straight away. Each field has its own prefix selector, so you can enter 470 Ω and read the answer as 25.532 mA instead of 0.025532 A, or work in kilovolts and megawatts for power systems.

The two fields you edited most recently are treated as the inputs and the other two are calculated. Typing into a calculated field promotes it to an input, so you can swap from "voltage and resistance" to "resistance and power" without pressing anything. The Ohm's law wheel lights up the two formulas that produced the answer, and the steps panel shows the numbers substituted into each one.

Underneath is a series and parallel helper. List resistor values separated by commas to get both combined resistances, then send either total up to the main calculator as R.

  • Any two of V, I, R and P solve the other two
  • Prefixes from micro to mega on every field
  • Ohm's law wheel with the formulas used highlighted
  • Series and parallel resistor helper
  • Copy results and shareable link; runs entirely in your browser
Read the guide: Ohm's Law, Resistor Color Codes and Voltage Drop Explained

How to use the Ohm's Law Calculator

  1. Type a value into any two of the four fields: Voltage, Current, Resistance or Power.
  2. Pick the prefix for each field from its unit list, for example mA for current or kΩ for resistance. Calculated fields convert to the unit you pick.
  3. Read the two calculated values in the Result panel; the wheel highlights the formulas used and the steps show the arithmetic.
  4. To change which quantities are known, type into a calculated field. The two most recently typed fields always drive the result.
  5. For resistor networks, type the values into the series and parallel helper and use either total as R above.

Worked examples

Current through a resistor on a 12 V supply

Input

V = 12 V, R = 470 Ω

Output

I = 25.532 mA, P = 306.38 mW

I = V ÷ R and P = V² ÷ R. At 306 mW the resistor needs at least a half-watt rating, so a common quarter-watt part would run hot.

LED series resistor

Input

5 V supply, LED forward voltage 2 V, target 20 mA: enter V = 3 V and I = 20 mA

Output

R = 150 Ω, P = 60 mW

The resistor only sees the voltage left over after the LED, 5 − 2 = 3 V. 150 Ω is a standard E12 value and a quarter-watt part has plenty of margin.

A 2 kW kettle on a 230 V supply

Input

V = 230 V, P = 2 kW

Output

I = 8.6957 A, R = 26.45 Ω

I = P ÷ V and R = V² ÷ P. The element resistance is its hot value; measured cold with a meter it reads a little lower.

Series and parallel totals

Input

100, 220 and 330 Ω

Output

Series 650 Ω; parallel 56.897 Ω

Series resistances add. In parallel the total is always below the smallest resistor because each one adds another path for current.

Background and insight

Georg Ohm published the proportionality between voltage and current in 1827: for a conductor at constant temperature, current is voltage divided by resistance, I = V / R. Joule's first law adds power, P = V × I. Combining the two gives the twelve formulas on the familiar wheel, three for each quantity. With any two values known, two of those formulas give the remaining pair, and the SI units line up exactly: one volt across one ohm drives one ampere, and one volt times one ampere is one watt.

Ohm's law describes linear (ohmic) parts such as resistors, wire and heating elements. Diodes, LEDs, transistors and lamps are not ohmic: their resistance changes with voltage or temperature, so V / I gives you a resistance at one operating point only. A filament bulb, for example, has roughly ten times lower resistance cold than hot. For AC circuits with inductance or capacitance the same algebra works with impedance in place of resistance, and real power also depends on the power factor.

Power rating matters as much as resistance. A 1 kΩ resistor rated at 0.25 W reaches its limit at the square root of P × R, about 15.8 V and 15.8 mA. Designers usually pick a part rated for at least twice the dissipation it will see, which is why the calculator flags anything over a quarter watt when resistance is one of the inputs.

Common use cases

  • Choosing a resistor value

    Work out the resistance and power rating needed to drop a known voltage at a known current, such as an LED or a pull-up.

  • Checking a fuse or breaker

    Turn the wattage on an appliance label into the current it draws at your mains voltage.

  • Homework and lab reports

    Solve textbook circuit problems and copy the formula steps into your working.

  • Troubleshooting with a meter

    Compare a measured voltage and current with the expected resistance to spot a failed part or a bad connection.

Tips and limitations

  • Values must be positive. For a component with zero resistance or zero current the ratios are undefined, which the calculator reports instead of showing infinity.
  • For LEDs, enter only the voltage across the resistor (supply minus LED forward voltage), not the full supply voltage.
  • AC mains voltages such as 120 V or 230 V are RMS values, so the power you get is the average power for a resistive load.
  • Resistors drift with temperature. Use the Resistor Color Code Calculator to read the temperature coefficient band on precision parts.
  • Long cable runs add resistance of their own; the Voltage Drop Calculator estimates how much voltage is lost before the load.

Frequently asked questions

What is Ohm's law in simple terms?

Current equals voltage divided by resistance, I = V / R. Double the voltage across a resistor and the current doubles; double the resistance and the current halves. Rearranged, V = I × R and R = V / I.

How do you calculate power from voltage and resistance?

Use P = V² / R. For 12 V across 470 Ω that is 144 / 470 = 0.306 W. You can also find the current first (I = V / R) and multiply by the voltage.

What are the 12 formulas of the Ohm's law wheel?

Voltage: I × R, P / I, √(P × R). Current: V / R, P / V, √(P / R). Resistance: V / I, V² / P, P / I². Power: V × I, I² × R, V² / R. The calculator highlights the pair it used for your inputs.

How many amps does a 1500 W heater draw?

Divide watts by volts. At 120 V a 1500 W heater draws 12.5 A; at 230 V it draws about 6.5 A. Enter P = 1.5 kW and the voltage to get the exact figure and the element resistance.

Does Ohm's law apply to LEDs and diodes?

Not directly. Their current rises steeply once the forward voltage is reached, so they have no fixed resistance. Apply Ohm's law to the series resistor instead, using the supply voltage minus the LED forward voltage.

How do I combine resistors in parallel?

Add the reciprocals and take the reciprocal of the sum: 1 / (1/R1 + 1/R2 + ...). Two equal resistors in parallel give half the value. The helper below the calculator does this for any number of resistors.

What resistor wattage do I need?

Calculate the power the resistor will dissipate (P = I² × R or V² / R) and choose a part rated for at least twice that. A resistor dissipating 0.3 W should be a 1 W part, or at minimum 0.5 W with good airflow.

Is there an in-depth guide to the Ohm's Law Calculator?

Yes. “Ohm's Law, Resistor Color Codes and Voltage Drop Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.

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