Ohm’s Law Calculator: Voltage, Current, Resistance and Power

Calculate voltage, current, resistance or power for a DC resistive circuit. Enter two positive values using volts, amperes, ohms and watts.

⚡ Ohm's Law Calculator

ℹ️ Instructions: Enter values into exactly two of the fields above. The calculator will automatically solve for the remaining two unknowns.
+ - 0 V Voltage 0 Ω Resistance 0 W Power 0 A Current Flow

How to use this calculator

  1. Choose two quantities you know and enter their values.
  2. Convert units first: 20 mA = 0.02 A, 4.7 kΩ = 4700 Ω, and 250 mW = 0.25 W.
  3. Read the other two quantities and check that the units and order of magnitude make sense.
  4. Clear the previous inputs before starting a calculation with a different pair of quantities. Reload the page if you need to reset all fields.

Formulas

For a resistor, V = I × R and P = V × I. Combining these gives P = I² × R = V² / R. Here V is the voltage across the resistor, I is the current through it, R is its resistance and P is the power it dissipates.

  • Voltage and current: R = V / I; P = V × I.
  • Voltage and resistance: I = V / R; P = V² / R.
  • Current and resistance: V = I × R; P = I² × R.
  • Voltage and power: I = P / V; R = V² / P.
  • Current and power: V = P / I; R = P / I².
  • Resistance and power: I = √(P / R); V = √(P × R).

Example: a 100 Ω resistor across 12 V

Enter 12 for voltage and 100 for resistance. The current is 12 / 100 = 0.12 A, or 120 mA. Power is 12 × 0.12 = 1.44 W. A 0.25 W resistor is unsuitable for this continuous load. Choose a part with adequate power margin after checking its datasheet, ambient temperature and mounting conditions.

If the resistance doubles to 200 Ω while the voltage stays at 12 V, the current becomes 0.06 A and power becomes 0.72 W. Both current and power halve. This conclusion depends on keeping voltage constant; at constant current, doubling resistance doubles the power.

Example: estimating an LED series resistor

Suppose a 5 V supply powers an LED with a 2 V forward drop at the desired 10 mA current. The resistor must drop 5 − 2 = 3 V. Enter 3 V and 0.01 A: R = 300 Ω and P = 0.03 W. Use the LED’s actual datasheet values and account for supply and component tolerances. The calculator models the resistor, not the LED’s nonlinear current-voltage behaviour.

Example: resistance and power are known

For a 100 Ω resistor dissipating 0.25 W, I = √(0.25 / 100) = 0.05 A and V = √(0.25 × 100) = 5 V. These values describe the specified operating point. A power rating is a limit under stated conditions, not a target that is automatically safe in every enclosure.

Limitations and common mistakes

  • Use voltage across the component, not necessarily the supply voltage. For a PCB trace, heat depends on the trace’s own voltage drop and resistance.
  • This calculation does not determine safe PCB trace width, wire size, insulation, cooling or fuse selection.
  • Zero inputs can make divisions undefined or leave multiple possible answers. Use positive values for the normal calculations described here.
  • For a purely resistive AC load, RMS voltage and current can be used. Capacitors, inductors and general AC loads require impedance and power-factor calculations.
  • Real resistance changes with temperature; tolerances and component ratings still matter.

For component identification, see the resistor code calculator. To report a calculation problem, send the input values and expected result.