Amp Calculator
Need to know the current draw of a device in amperes (amps)? An amp calculator takes the guesswork out of electrical math. Enter two known values – watts and volts, or volts and ohms – and get the amperage instantly. Use the calculator below for quick results, then dive into the formulas and examples that explain the numbers.
How to Calculate Amps from Watts and Volts
The most common way to find amps uses the relationship between power (watts) and voltage (volts). The formula is:
Amps = Watts ÷ Volts
This works for DC circuits and for AC circuits where the load is purely resistive or the power factor equals 1. For loads with a power factor less than 1 (motors, transformers, etc.), adjust the formula:
Amps = Watts ÷ (Volts × Power Factor)
Step‑by‑step example
- Locate the power rating in watts on the appliance label. If you have kilowatts, multiply by 1,000.
- Determine the supply voltage – typically 120 V or 240 V in North America, 230 V in many other regions.
- Divide the watts by the voltage.
- For AC inductive loads, also divide by the power factor (usually between 0.7 and 0.95).
Real numbers: A 1,500‑watt space heater on 120 V draws 1,500 ÷ 120 = 12.5 amps.
A 2‑horsepower motor (1 hp ≈ 746 W, so ≈ 1,492 W) at 240 V with a 0.85 power factor demands 1,492 ÷ (240 × 0.85) ≈ 7.3 amps.
Amps from Volts and Resistance – Ohm’s Law
When you know the voltage and the resistance (ohms), use Ohm’s Law:
Amps = Volts ÷ Ohms
This works for any resistive circuit. For instance, an electric heating element with a resistance of 12 ohms connected to a 120‑volt supply pulls 120 ÷ 12 = 10 amps.
How to Convert kW to Amps
Many industrial or large‑appliance ratings are in kilowatts (kW). Convert kW to watts first (1 kW = 1,000 W), then apply the same formula.
- Single‑phase AC: Amps = (kW × 1,000) ÷ (Volts × PF)
- Three‑phase AC: Amps = (kW × 1,000) ÷ (√3 × Line‑to‑Line Volts × PF)
For everyday household use, stick to the single‑phase formula. The calculator above handles the conversion automatically once you select the appropriate unit.
Common Examples of Amperage Draw
A quick reference for typical loads on a 120‑V circuit (power factor = 1 unless noted):
| Appliance | Power (W) | Current (A) |
|---|---|---|
| LED light bulb | 10 | 0.08 |
| Laptop charger | 65 | 0.54 |
| Refrigerator (running) | 150 | 1.25 |
| Microwave oven | 1,200 | 10 |
| Portable heater | 1,500 | 12.5 |
| Window AC unit | 900 | 7.5 |
| Central AC condenser (240 V) | 3,500 | 14.6 (single‑phase) |
For continuous loads (running for 3 hours or more), electrical codes often limit the current to 80% of the breaker rating. On a 15‑A breaker, that means a maximum continuous draw of 12 A.
Understanding Amps, Watts, and Volts
- Amps (A) – the flow of electric current; think of it as the volume of water moving through a pipe.
- Volts (V) – the electrical pressure; the “push” that drives the current.
- Watts (W) – the actual power consumed; the rate of energy use.
All three are linked: Watts = Volts × Amps (× power factor for AC). Knowing any two lets you find the third with a simple calculation.
Electrical work involves risks. The formulas and calculator provide theoretical values; always consult a certified electrician for design or safety‑related decisions.