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Choose a method

Method:

Tip: Use Ohm’s Law when you want current, voltage, or resistance directly from the other two. Use the P-forms when you’re solving power (or solving V/I/R from power).

We’ll disable options that aren’t solvable with your chosen method.

Resistance must be > 0 when used in division or square roots.

Options:

Result:

No results yet. Enter values and click Calculate.

How to use this calculator

  • Pick a method: P = V·I, P = I²R, P = V²/R, or V = I·R.
  • Choose what you want to solve for (P, V, I, or R).
  • Enter the other required values (units can differ; we convert internally).
  • Click Calculate to see the result, conversions, and steps.

How this calculator works

  • Convert inputs to consistent base units: W, V, A, Ω.
  • Solve for the selected variable using the chosen equation.
  • Display common unit equivalents (e.g., W ↔ kW ↔ hp, Ω ↔ kΩ).
  • Optionally show a mini power scale for intuition.

Formula & Equation Used

P = V·I

P = I²·R

P = V² / R

V = I·R

Identity: 1 W = 1 V·A

Example Problems & Step-by-Step Solutions

Example 1 — P = V·I

A charger outputs 5 V at 2 A.

  1. Use P = V·I
  2. P = 5 × 2 = 10 W

Example 2 — P = V²/R

A device runs at 12 V with 6 Ω resistance.

  1. Use P = V²/R
  2. P = 12² / 6 = 144 / 6 = 24 W

Example 3 — P = I²R

A current of 0.5 A flows through a 10 Ω resistor.

  1. Use P = I²R
  2. P = (0.5)² × 10 = 0.25 × 10 = 2.5 W

Frequently Asked Questions

Q: What is 1 watt?

1 watt means 1 joule per second. In circuit terms, 1 W = 1 V·A.

Q: Can I use this for AC circuits?

For basic use, treat inputs as DC or RMS values. Real AC power can depend on power factor (not included here).

Q: Why are there multiple formulas for power?

Because combining P = V·I with V = I·R gives P = I²R and P = V²/R.

Q: What if I enter kΩ or mA?

That’s fine — the calculator converts everything to base units internally.