Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
This tool runs in your browser. Your calculator inputs and results are never transmitted to us or to ad/affiliate partners. Ads and sponsored links may set third-party cookies to serve and measure them, but they never receive your calculation values. If you explicitly save a scenario, its permitted fields stay in local browser storage until you clear them. See our Privacy Policy.
Use this result
Share the current inputs or ask ChatGPT to explain the calculation in context.
What does this calculator estimate?
An Ohm's law calculator solves for voltage, current, resistance, or power. Choose the target and enter the two known values to calculate.
- V = I × R
- P = V × I
- Current flows from higher to lower voltage
What Ohm's law describes
Ohm's law relates voltage, current, and resistance in an electrical circuit: the current through a resistor is proportional to the voltage across it. It's the first tool for sizing components, fuses, and power supplies.
Limitations to watch for
The law applies to ohmic materials (resistors, wires) at constant temperature — diodes, LEDs, and semiconductors don't follow it linearly. Power dissipation heats resistors, changing their resistance. AC circuits add impedance and phase considerations.
How to use it in practice
Use it to check that components won't exceed their ratings: P = V × I tells you heat dissipation (a 36 W load needs a resistor rated above that). For LED circuits, always use a series resistor to limit current.
['Choose the target: voltage, current, resistance, or power.', 'Enter the two known values.', 'The tool solves V = IR or P = VI.']
Ohm's law
Voltage = current × resistance: V = IR. A 12 V circuit with 4 Ω draws 3 A. The calculator solves for any variable from the other two.
The power connection
Power = V × I, also I²R and V²/R. A 12 V circuit at 3 A delivers 36 W. The calculator pairs with the power formula so load sizing is complete.
A worked example
A 120 V appliance drawing 10 A has 12 Ω resistance and consumes 1,200 W. Dropping the voltage to 12 V for the same load would need 100 A — the calculator shows why voltage matters for wiring.
Practical circuits
Resistor selection, LED current limiting, and wire sizing all use Ohm's law: choosing a resistor for an LED is (supply − LED voltage) ÷ current. The calculator handles the arithmetic.
AC and real components
For AC and components with reactance, impedance replaces resistance; for DC and resistive loads, Ohm's law is exact. The calculator covers the DC/resistive case cleanly.
How this calculator works
Formula
Ohm's law: V = I × R (voltage = current × resistance). Derived: I = V ÷ R, R = V ÷ I. Power: P = V × I (watts).
Worked example
A 12 V supply across a 4 Ω resistor: current I = 12 ÷ 4 = 3 A, and power P = 12 × 3 = 36 W.
Assumptions to verify
- The component is ohmic (linear resistance).
- DC or RMS AC values are used.
- Temperature effects on resistance are ignored.
Frequently asked questions
What is Ohm's law?
V = I × R: voltage equals current times resistance. It's the fundamental relationship in electrical circuits.
How do I calculate current?
I = V ÷ R. 12 V across 4 Ω gives 3 A.
What is power in a circuit?
P = V × I (watts). The same 12 V at 3 A dissipates 36 W — heat you must size the resistor for.
Does Ohm's law apply to everything?
No — it applies to ohmic components like resistors and wires. Diodes, LEDs, and semiconductors have nonlinear current-voltage behavior.
What happens if a resistor overheats?
Resistance changes with temperature, so the linear relationship drifts. That's why power ratings matter: an undersized resistor burns out.
What's the difference between AC and DC?
DC is steady; AC alternates. For AC with reactive components (capacitors, inductors), impedance replaces resistance and phase matters.
How do I size a series resistor for an LED?
R = (supply voltage − LED forward voltage) ÷ desired current. For a 5 V supply, 2 V LED, 20 mA: R = 3 ÷ 0.02 = 150 Ω.
How do I find current?
I = V ÷ R: 12 V ÷ 4 Ω = 3 A.
How is power related?
P = V × I, or I²R — the load's power draw.
Does it work for AC?
With impedance instead of resistance for reactive components.
Cite this tool
BoringToolsKit. “Ohm's Law Calculator.” boringtoolskit.com/ohms-law-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Any future sponsored recommendation or advertisement will be clearly labeled and kept separate from the calculation.