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Free Ohm's Law Calculator

Enter any two of voltage, current, resistance, and power — the other two are solved instantly using Ohm's law (V=IR) and the power equation (P=VI).

Works offline once loaded
Enter any two values and the other two are solved automatically

💡 Ohm's law: V = I × R. Power adds P = V × I. Knowing any two lets you derive the rest — handy for sizing resistors, checking LED current, or working out heat dissipation.

Common uses

  • Size a Current-Limiting Resistor for an LEDWork out the resistor value an LED needs from supply voltage and current.
  • Calculate Power Dissipation (Watts)Find how many watts a component dissipates so you can size it safely.
  • Solve Ohm's Law HomeworkCheck circuit problems by entering any two known values.
  • Estimate Current Draw from a BatteryFind the current a load pulls so you can estimate battery runtime.
  • Calculate Voltage Across a ComponentFind the voltage drop across a resistor from current and resistance.

How It Works

This calculator runs entirely in your browser with no external libraries — it applies Ohm's law (V = IR) and the power identity (P = VI) algebraically, covering all six combinations of two-known/two-unknown pairs so you can enter any two quantities and instantly get the other two.

  1. 1
    Enter any two values
    Fill in any two of the four fields — voltage, current, resistance, or power. Each field has an SI prefix selector (mV/V/kV, µA/mA/A, Ω/kΩ/MΩ, mW/W/kW) so you work in the unit that matches your component, not base units.
  2. 2
    Prefix scaling and parsing
    As you type, the tool multiplies your number by the selected prefix factor (e.g. 4.7 kΩ becomes 4700 Ω internally). It then counts how many fields have valid finite values and passes the base-unit values to the solver.
  3. 3
    Algebraic solve across all six input pairs
    A hand-written solve() function checks which two quantities are known and picks the correct formula from six possible pairs: V+I, V+R, V+P, I+R, I+P, and R+P. The R+P case uses a square root (√(P×R)) to find voltage first, then derives current. Division-by-zero inputs such as zero resistance or zero voltage with nonzero power produce specific error messages instead of NaN.
  4. 4
    Formula breakdown and copy
    Results appear immediately in the two derived fields, each showing the exact formula used with your numbers substituted in (for example, V² / R = 12² / 470). A copy button writes all four results as plain text to the clipboard via the browser's Clipboard API.

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Frequently Asked Questions