Ohm's Law Calculator
V, I, R, P — pick two, get four.
Ohm’s Law Calculator
Enter any two of voltage, current, resistance, or power and solve the rest for a resistive circuit.
Assumes direct current and a purely resistive load. Alternating current with motors or capacitors needs impedance and power factor, which this does not model.
12V
Voltage across the circuit
- Current
- 2 A
- Resistance
- 6 Ω
- Power
- 24 W
Embed this calculator
Put this calculator on your own site with the snippet below. The frame contains the tool only, reports its own height, and carries a visible link back to the full page. Embedding is free; keep the attribution link as it is, and do not wrap the frame in a way that hides it.
Preparing the embed code for this page...
What This Calculator Does
The Ohm's Law Calculator solves a resistive circuit from any two known quantities. Choose which pair you have — voltage and current, voltage and resistance, and so on — and it returns voltage, current, resistance, and power with consistent units.
How It Works
Ohm's law relates voltage, current, and resistance as V = IR. Electrical power is P = VI, which combines with Ohm's law to give P = I²R and P = V²/R. Knowing any two values fixes the other two, so there is exactly one solution for a purely resistive load.
Example
A 12 V supply drives 2 A through a resistor. Resistance is 12 / 2 = 6 Ω and power is 12 × 2 = 24 W. The same resistor on 6 V would draw 1 A and dissipate 6 W, because power falls with the square of voltage.
Formula
V = I × RP = V × IP = I²R = V²/RUse the form that matches the two values you know.
Reading the Results
Voltage
The potential difference across the resistive element. It is the quantity a multimeter measures in parallel with the load.
Power
Energy converted per second, in watts. In a resistor this is heat, so the component must be rated for at least this dissipation.
Sources and References
ISO/IEC 80000-6 — Quantities and units: electromagnetism
Volt, ampere, ohm, and watt definitions
Defines the SI quantities and the relationships this calculator uses.
NIST — SI Brochure, 9th edition
Base unit definitions for the ampere
The ampere is defined from the elementary charge, which anchors the electrical units.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Which formula do I use for power?
Use P = VI when you know voltage and current, P = I²R when you know current and resistance, and P = V²/R when you know voltage and resistance. All three are the same relationship rearranged.
Does this work for AC circuits?
Only for purely resistive AC loads, where voltage and current are in phase. Motors, transformers, and capacitors need impedance and power factor, and the result will be wrong without them.
Why must both inputs be greater than zero?
A zero voltage or current makes resistance undefined or infinite, and a zero resistance would imply infinite current. Real circuits have limits, so the calculator asks for values that describe one.
How do I pick a resistor wattage?
Calculate the dissipation, then choose a resistor rated at roughly twice that figure for headroom. A 24 W load needs a 50 W part, not a 25 W one, because surface temperature and derating matter.