Ohm's Law Calculator

Enter any two of voltage, current, resistance, or power to solve the other two, with full unit conversion for volts, amps, ohms, and watts

By Drew Budwin · Last updated July 2026 · Methodology

Fill in exactly two fields. The other two are calculated.

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How We Calculate This

Ohm's Law and Watt's Law

Ohm's law and Watt's law link four quantities: voltage (V, in volts), current (I, in amps), resistance (R, in ohms), and power (P, in watts). Give this calculator any two and it solves the other two. The core relationships are:

  • Ohm's law: V = I × R
  • Watt's law: P = V × I

Every other formula on the wheel is a rearrangement of these two, so you never have to solve the algebra yourself.

Choosing Units

Each field has a unit menu, so you can enter values at any scale and read the answer back the same way:

  • Voltage: µV, mV, V, kV, MV
  • Current: nA, µA, mA, A
  • Resistance: mΩ, Ω, kΩ, MΩ
  • Power: µW, mW, W, kW, MW, GW, TW, plus horsepower and BTU/h

Values are converted to volts, amps, ohms, and watts for the math, then shown in whatever unit each field is set to. 1 mechanical horsepower is 745.699872 W and 1 BTU per hour is 0.29307107 W.

Worked Example

A 12 V battery drives a 100 Ω resistor and you want the current and power. Divide voltage by resistance for current: I = 12 ÷ 100 = 0.12 A. Power follows from P = V² ÷ R = 144 ÷ 100 = 1.44 W. Enter 12 in Voltage and 100 in Resistance, leave Current and Power blank, and the calculator returns 0.12 A and 1.44 W.

The Ohm's Law Wheel

The Ohm's law wheel is a quick-reference chart of every way to combine V, I, R, and P. The center ring holds the four quantities; each surrounding segment is a formula for solving that quantity from the other two.

Ohm’s Law and Watt’s Law wheelA circular chart. The center is split into four quadrants for V (volts), I (amps), R (ohms), and P (watts). Each quantity is ringed by its three equivalent formulas for solving it from the other quantities.VRPRPVP × RPII × RVIPI²V²PV²RI² × RI × VVVoltsIAmpsOhmsRWattsP
  • Voltage: V = I × R, V = P ÷ I, V = √(P × R)
  • Current: I = V ÷ R, I = P ÷ V, I = √(P ÷ R)
  • Resistance: R = V ÷ I, R = V² ÷ P, R = P ÷ I²
  • Power: P = V × I, P = I² × R, P = V² ÷ R

Ohm's Law Calculator Formula

Solve for Voltage:

V = IR = P/I = √(PR)

Solve for Current:

I = V/R = P/V = √(P/R)

Solve for Resistance:

R = V/I = V²/P = P/I²

Solve for Power:

P = VI = I²R = V²/R

How to Use This Calculator

  1. Enter values for exactly two of the four fields: Voltage, Current, Resistance, or Power.
  2. Pick the unit for each value from its dropdown: volts, amps, ohms, watts, and their multiples.
  3. Click Calculate to solve the other two. The computed values are highlighted.
  4. Copy the URL to share your result. The link auto-fills the inputs and calculates on load.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality: V = I × R. Georg Ohm published the relationship in 1827, and it remains the starting point for nearly all DC circuit analysis.

How many values do I need to enter?

Exactly two. Enter any two of voltage, current, resistance, or power and the calculator solves the other two using Ohm's law and Watt's law. Leave the two you want to find blank.

What units does this calculator support?

Voltage in µV, mV, V, kV, or MV; current in nA, µA, mA, or A; resistance in mΩ, Ω, kΩ, or MΩ; and power in µW, mW, W, kW, MW, GW, TW, mechanical horsepower, or BTU/h. Pick a unit for each field, and each result is shown in that field's selected unit.

Why must all inputs be greater than zero?

Negative and zero values would produce physically meaningless or undefined results for basic DC circuit analysis. Convert any negative voltage or current to its magnitude before calculating.

How are results formatted?

Results are displayed with up to 6 significant figures. Very small values (below 0.001) use up to 4 significant figures so they don't round to zero.

What is electrical power and how is it calculated?

Electrical power (measured in watts) is the rate at which electrical energy is consumed or produced. It can be calculated as P = V × I, P = I²R, or P = V²/R, and all three give the same answer. Because power is a first-class input here, you can also work backward from watts to find voltage, current, or resistance.

How do I calculate current from power and voltage?

Divide power by voltage: I = P ÷ V. For example, 24 W at 12 V is 2 A. Enter power and voltage, leave current and resistance blank, and the calculator returns both.

Can this convert horsepower or BTU/h to watts?

Yes. Set the Power field's unit to horsepower or BTU/h and the value is converted for you: 1 mechanical horsepower is 745.699872 W and 1 BTU per hour is 0.29307107 W. You can also read a computed power result back in horsepower or BTU/h.

How do I calculate watts from volts and amps?

Multiply voltage by current: watts = volts × amps. For example, 12 V at 2 A is 24 W. If you know current and resistance instead, use P = I² × R; if you know voltage and resistance, use P = V² ÷ R. This calculator handles every case automatically.

How do I solve for resistance if I know voltage and current?

Divide voltage by current: R = V ÷ I. For 12 V and 0.12 A, resistance is 100 ohms. Enter voltage and current, leave the resistance and power fields blank, and the calculator solves them for you.

Further Reading

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