Voltage Calculator
Work out voltage, current, resistance and power — from any two known values, with Ohm's law.
Ohm's law is the whole of basic circuit maths: voltage, current, resistance and power, any two giving the other two. This calculator is that law, in all four directions.
Pick what you know, enter the two values, and the other two follow.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Choose what you know (the two values you have).
- Enter the two values.
- Read the voltage, current, resistance and power.
Worked example
| Known | Current 2 A, resistance 120 Ω |
|---|---|
| Voltage | 2 × 120 = 240 V |
| Power | 240 × 2 = 480 W |
| Other direction | 2,000 W ÷ 240 V = 8.33 A, resistance 28.8 Ω |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What is Ohm's law?
Voltage = current × resistance (V = I × R). It is the basic law of electric circuits: given any two of voltage, current and resistance, the third follows — and power follows from P = V × I.
What do I enter for each mode?
Mode 1 (V = I × R): current then resistance. Mode 2 (V = P ÷ I): power then current. Mode 3 (I = P ÷ V): power then voltage. Mode 4 (P = V × I): voltage then current.
Does this work for AC circuits?
For resistive AC loads (heaters, incandescent lights) yes. For motors and other reactive loads, power factor matters — this calculator assumes a resistive load.
Is this a substitute for electrical design?
No. This is a calculation for planning. Circuit sizing and electrical safety require qualified professionals and local code compliance — see the safety notice.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.