Volt Watt Amp Calculator
Calculate watts, volts or amps from any two known values using P = V x I. Useful for power supplies, LED strips, appliance ratings and circuit planning.
Last updated: May 2026
Enter any two values to calculate the third.
Why this is valuable
Basic power math gets searched constantly because people need to compare adapters, appliances, LED loads, batteries and workshop electronics. Most visitors are not looking for theory. They want the number.
This version keeps it intentionally simple. It is useful for quick checks, but it is not a replacement for full AC power-factor calculations or local electrical code.
Typical use cases
- adapter checks
- device power draw
- LED and hobby electronics
- battery planning
Related tools and sections
Power math: Watts, volts and amps in practice
The simple formula Watts = Volts — Amps applies to DC circuits (batteries, LED systems) and resistive loads (heaters, incandescent bulbs) on AC. For motors, compressors, and reactive devices, the relationship is complicated by power factor, but this calculator assumes DC or purely resistive loads — the practical case for home electronics, hobby projects and quick checks.
Knowing power relationships matters for component selection: a 12V power supply rated for 5A can deliver 60W maximum. If your LED strip or device draws 10A, you need at least a 120W supply (12V — 10A). Undersizing a power supply causes voltage sag, heat buildup, and early failure.
Common V / A / W values
| Volts | Amps | Watts | Typical application |
|---|---|---|---|
| 5 V | 1 A | 5 W | USB phone charger |
| 5 V | 2 A | 10 W | Tablet or fast charger |
| 12 V | 1 A | 12 W | Small LED strip or router |
| 12 V | 5 A | 60 W | LED strip (5 m) or NAS drive |
| 19 V | 3.5 A | 66.5 W | Typical laptop adapter |
| 230 V | 0.5 A | 115 W | LED lamp or small appliance |
| 230 V | 10 A | 2,300 W | Kettle or hair dryer |
Frequently Asked Questions
Does this work for AC appliances like motors?
This simplified calculator assumes DC or purely resistive loads. Real AC motors, air compressors, and inductive devices have power factor below 1, so the actual watts are lower than V — A suggests. For motors, check the label rating or derate by 0.7–0.85 power factor.
What size power supply do I need for my device?
Calculate the watts your device needs (volts — amps). Then buy a power supply rated for at least that power, and add 20% headroom. A device needing 60W should use a 75W+ supply to avoid stress and heat.
Why does my power supply get hot?
Undersizing causes the supply to work at or near its limit, generating excess heat. If a 60W device is powered by a 60W supply, the supply has no headroom and will run hot (and fail prematurely). Use a 75W+ supply instead.