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.
Reading power adapter labels and checking compatibility
Most power bricks print all three values on the label. A typical 65-watt laptop adapter reads "19.5V, 3.33A" - enter either pair and this calculator confirms 65 W. When a label only shows two of the three values (common on older chargers or generic adapters), use this tool to find the missing one before connecting a device.
For LED strip planning: a 5-metre reel rated at 12 W per metre needs 60 W total. Divide by your supply voltage (12 V) to confirm the required current (5 A), then add 20% headroom and buy a 72 W or 8 A supply. Running a supply at its exact rated maximum causes early failure from sustained heat.
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.