Quick answer: use Ohm's Law (V = I × R) and the power formula (P = V × I) to convert between volts, amps, watts, and ohms. Try our free Voltage and Power Converter.
The Core Formulas
Ohm's Law: Voltage (V) = Current (I) × Resistance (R). Power: Watts (P) = Voltage (V) × Current (I). Rearranging these lets you solve for any missing value given the other two.
Practical Example
A 120V device drawing 5 amps uses 120 × 5 = 600 watts. This is how you'd check if a device fits within a circuit's power rating.
Frequently Asked Questions
How do I find watts if I only know volts and resistance?
First find current using I = V/R, then calculate power using P = V × I.
Is this safe to use for home electrical work?
This tool helps with calculations, but always consult a licensed electrician for actual home electrical work.
What's the difference between AC and DC calculations here?
These are basic DC formulas; AC circuits involve additional factors like power factor for more complex calculations.
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Open the Tool →Use the right electrical relationship
For a simple DC circuit, power in watts is voltage multiplied by current: P = V × I. Rearranging gives I = P ÷ V and V = P ÷ I. For example, a 60 W device operating at 12 V draws about 5 A under that simplified relationship. The values must refer to the same circuit and operating condition.
Ohm’s law connects voltage, current, and resistance: V = I × R. It is often used for resistive loads. Do not assume watts can always be converted to amps from voltage alone in an AC circuit: power factor and the type of load may matter, and three-phase systems require additional factors.
Check labels and safety limits
Use the manufacturer’s rated voltage and current, and check that wires, adapters, and protective devices are rated for the load. A calculation is not a substitute for electrical safety guidance. Mains electricity can cause serious injury; have a qualified electrician handle installations or uncertain wiring. For a quick estimate, record whether your inputs are DC or AC and use the formula that matches the equipment.
Example with a known current
If a 12 V DC circuit carries 2 A, the simplified power calculation is 12 × 2 = 24 W. If you know power and voltage, divide watts by volts to estimate current. Check that the power rating and voltage refer to the same device and operating mode. For AC equipment, the current may not equal watts divided by volts because power factor and phase matter. Never exceed a component’s rating; ask a qualified electrician if you are unsure about mains wiring.