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Ohms Law Calculator

By · Updated Aug 2026 · V = I × R · P = V × I

Enter any two known values for voltage, current, resistance, or power. The calculator solves the remaining values and shows the formulas used.

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How to Use

  1. Enter any two known values - use any pair of voltage, current, resistance, and power.
  2. Choose the units beside each value - the calculator converts micro, milli, base, kilo, and mega units automatically.
  3. Leave unknown values blank - the missing values are calculated and highlighted in the result.
  4. Enter extra values to verify them - three or four entries are checked for consistency within a small rounding tolerance.
  5. Review the formula steps - the result shows the equations and substitutions used for the selected input pair.
Quick answer

Ohms Law ties four quantities together: voltage, current, resistance, and power. Enter any two and the calculator solves the remaining values. For example, 12 volts and 6 watts give 0.5 amps and 24 ohms. The core relationship is V = I × R, while power follows as P = V × I, P = I² × R, or P = V² ÷ R.

The Ohms Law Triangles

VIRPVIV = I × RP = V × ICover the unknown; the triangle shows the formula
Cover the quantity you want in the left triangle to read V = I × R, I = V ÷ R, or R = V ÷ I. The right triangle gives P = V × I. The calculator also uses the combined square and square-root forms when power and resistance are the known values.

What Is Ohm's Law?

Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. Current can be written as I = V ÷ R, and resistance as R = V ÷ I.

Power (P) relates through P = V × I. Combining the equations gives P = I² × R and P = V² ÷ R, so any two positive values are enough to solve the other two for a simple resistive circuit.

Voltage is normally written as V in English-language formulas. Some references use U or E instead, so URI, UIR, EIR, and VIR notation describe the same voltage-current-resistance relationship. To move from component formulas to a real panel estimate, total the loads with the electrical load calculator.

Ohm's Law Formula Wheel

All 12 useful forms of the Ohms Law and power relationships:

Solve forFormula 1Formula 2Formula 3
Voltage (V)V = I × RV = P ÷ IV = √(P × R)
Current (I)I = V ÷ RI = P ÷ VI = √(P ÷ R)
Resistance (R)R = V ÷ IR = V² ÷ PR = P ÷ I²
Power (P)P = V × IP = I² × RP = V² ÷ R

Enter any two values and the calculator selects the appropriate formulas. Extra entered values are checked against the solved circuit rather than silently overwritten.

Common Resistive-Circuit Examples

CircuitVoltageCurrentResistancePower
LED series resistor3.3V20mA165Ω66mW
60W incandescent bulb120V500mA240Ω60W
1500W resistive space heater120V12.5A9.6Ω1,500W
4500W water-heater element240V18.75A12.8Ω4,500W
12V resistive lamp12V4.17A2.88Ω50W
5V test resistor5V10mA500Ω50mW

Values are idealized or nominal. Ohm's Law applies directly to DC and purely resistive loads. Motors, transformers, capacitors, inductors, LEDs, and electronic power supplies may require impedance, nonlinear device data, efficiency, or power-factor calculations.

Core Formulas

The four essential electrical relationships:

Voltage = V = I × R (volts = amps × ohms)
Current = I = V ÷ R (amps = volts ÷ ohms)
Resistance = R = V ÷ I (ohms = volts ÷ amps)
Power = P = V × I (watts = volts × amps)
Power alt. = P = I² × R or P = V² ÷ R

Use the calculator for simple resistive circuits. In AC circuits with motors, capacitors, or inductors, impedance (Z) replaces resistance and phase relationships may also matter.

Next Steps

Ohms Law is the foundation. Put it to work:

  1. Convert watts, amps, voltsUse the practical one-device power converter when resistance is not part of the question.
  2. Check a wire runApply conductor resistance over distance to estimate voltage drop.
  3. Size the conductorUse the current result as an input for a planning-level conductor size.

Related Calculators

Electrical Load CalculatorReal-world load totals.Amp to Wire Size CalculatorWire gauge from amps.Breaker Size CalculatorBreaker for the calculated amps.Generator Size CalculatorGenerator watts for the load.

Build on the formula with the full electrical calculator collection for wire sizing, voltage drop, loads, and raceway planning.

Frequently Asked Questions

What is Ohm's Law in simple terms?
Ohms Law says voltage equals current times resistance: V = I × R. At the same resistance, more voltage produces more current. At the same voltage, more resistance reduces current.
How do I use this calculator?
Enter any two positive values for voltage, current, resistance, and power, choosing the correct unit beside each value. Leave unknown values blank. The calculator solves the remaining values and shows the equations used. If you enter three or four values, it checks whether they agree within a small rounding tolerance.
Can I enter milliamps or kilo-ohms?
Yes. Use the unit selectors for microamps, milliamps, amps, kiloamps, milliohms, ohms, kilo-ohms, megaohms, and the corresponding voltage and power units. Values are normalized before the formulas are applied.
Does Ohm's Law work for AC circuits?
The basic V = I × R relationship applies directly to DC and purely resistive AC loads. Motors, transformers, capacitors, and inductors require impedance and may involve phase angle or power factor.
What is the difference between resistance and impedance?
Resistance opposes current and dissipates real power. Impedance is the broader AC quantity that combines resistance with reactance from inductors and capacitors. Both use ohms, but impedance can include phase information.
What two values do I need to solve the rest?
Any two positive values are sufficient: V and I, V and R, I and R, V and P, I and P, or R and P.
How do I calculate resistor power in watts?
Use P = V × I when voltage and current are known. You can also use P = I² × R or P = V² ÷ R. The result is the ideal electrical power dissipated by the resistive load; component selection may require additional thermal and safety margin.
Updated Aug 2026 · See our Methodology
This calculator uses standard electrical formulas and published reference tables. Local codes and amendments vary. Use for planning estimates only. See our Data Sources and Methodology.