Ohm's Law Calculator
Solve for voltage, current, resistance or power from any two known values.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Ohm's Law Calculator does
Ohm's law states that current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. Combined with the power relationship P = VI, any two of the four quantities determine the other two.
Formula
V = I × RI = V ÷ RR = V ÷ IP = V × I = I² × R = V² ÷ R
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Solve for | one of 4 options | Yes | — |
| Voltage | V | Optional | Accepts 0 or more. Shown in 3 of the 4 modes. |
| Current | A | Optional | Accepts 0 or more. Shown in 3 of the 4 modes. |
| Resistance | Ω | Optional | Accepts 0 or more. Shown in 3 of the 4 modes. |
| Power | W | Optional | Accepts 0 or more. Shown in 3 of the 4 modes. |
How to use it
- Choose Solve for.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
A 12 V supply driving a 4 Ω resistor — what current flows and what power is dissipated?
- Solve for
- Current
- Voltage
- 12 V
- Resistance
- 4 Ω
I = 12 ÷ 4 = 3 A, and P = 12 × 3 = 36 W.
Reading the result
- Voltage, current and resistance are linked by V = I x R: fix any two and the third follows. Power then follows from P = V x I.
- Doubling the voltage across a fixed resistance doubles the current but quadruples the power, because power scales with the square of either.
- The resistance figure is what the circuit presents at that moment. Filament lamps, motors and semiconductors all change resistance as they heat or as operating point shifts.
Assumptions and limitations
- Ohm's law applies to ohmic components at steady state. Diodes, transistors and gas discharge devices are non-ohmic and do not follow it.
- DC or purely resistive AC is assumed. With reactive AC loads you need impedance and a power factor, not resistance alone.
- Wiring resistance and source internal resistance are ignored unless included in the resistance you entered.
Common mistakes
- Mixing unit prefixes — entering kilo-ohms where ohms are expected, or milliamps where amps are expected.
- Applying the law across a whole circuit when the values belong to one component, or the reverse.
- Sizing a component on calculated power with no margin. Resistors are normally run well below their rated dissipation.
Frequently asked questions
Does Ohm's law apply to everything?
Only to ohmic materials, where resistance stays constant. Diodes, transistors and filament lamps are non-ohmic — their resistance changes with voltage or temperature.
Why does doubling voltage quadruple power?
Because P = V²/R. Doubling voltage doubles current as well, and power is the product of both.
Method and sources
Method. Ohm's law, V = I × R, combined with the power relationships P = V × I = I²R = V²/R, solving for whichever quantity is left blank.
Applies to. None — a physical law, not a regulated procedure
Assumptions
- The component is ohmic — resistance is constant and does not vary with the current through it or the voltage across it.
- Direct current, or an AC circuit purely resistive enough that reactance can be ignored.
Limitations
- Real components are only approximately ohmic. Resistance rises with temperature in most conductors, and semiconductors, lamps and motors depart from the law entirely.
- For AC circuits with inductance or capacitance, impedance rather than resistance governs, and this calculator does not model phase.
Sources
- Die galvanische Kette, mathematisch bearbeitet — T.H. Riemann, Berlin (Georg Simon Ohm), 1827. The proportionality between potential difference and current that the calculator solves.