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Ohm's Law Calculator

Calculate voltage, current, resistance, or power from any two known values using Ohm's Law and the power equation.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Current

3 A (current)

High confidence
Power
36 W
Voltage
12 V
Resistance
4 Ω

With the figures above, the current comes to 3 A (current). Note that voltage (v) and resistance (ω) sit at the edge of the range this calculator was checked against, so treat the output as indicative rather than settled. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 3 A (current) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Ohm's Law: V = I x R. Power equation: P = V x I. Both are exact, universal physics relationships, not rules of thumb.

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

What if I know current and resistance instead of voltage?
Enter the voltage you'd calculate as V = I x R first (or use this tool iteratively) — this version solves from voltage and resistance for current and power, the most common starting point for wire/circuit planning.
Is this exact or an approximation?
Exact — Ohm's Law and the power equation are fundamental, universal physics relationships for resistive circuits, not empirical rules of thumb.
Does this work for AC circuits with reactive loads?
This simple form applies directly to purely resistive circuits (most household loads like heaters and incandescent bulbs). Circuits with significant inductance or capacitance (motors, some electronics) need impedance (Z) instead of plain resistance for accurate results.