Ohm's Law Calculator · Practical guide
How to Calculate Amps from Volts and Watts
Amps = watts ÷ volts. One division tells you the current any appliance draws.
At a glance
The Formula
To find current from power and voltage, use I = P / V — divide the wattage by the voltage. A 1,500 W space heater plugged into a standard 120 V US outlet draws 1500 ÷ 120 = 12.5 amps.
Interactive tool
Ohm's Law Calculator
Enter your own values, then compare the result with the examples below.
Ohm's Law Results
Common Appliances at 120 Volts
| Appliance | Watts | Amps (at 120 V) |
|---|---|---|
| LED bulb | 60 W | 0.5 A |
| Microwave | 1,000 W | 8.3 A |
| Space heater | 1,500 W | 12.5 A |
| Hair dryer | 1,800 W | 15 A |
Why This Matters for Circuit Breakers
Standard US household circuits are rated at 15 or 20 amps. A 1,800 W hair dryer alone pulls 15 A — the full capacity of a 15-amp circuit — which is why running a space heater and a hair dryer on the same circuit trips the breaker. Electricians also apply the 80% rule for continuous loads: plan to use no more than 12 A on a 15 A circuit, which is exactly 1,440 W at 120 V.
The Same Trick at Other Voltages
The formula is voltage-agnostic. On a 240 V circuit, a 4,800 W water heater draws 4800 ÷ 240 = 20 A. In a 12 V car system, a 120 W accessory draws 120 ÷ 12 = 10 A. Enter any wattage and voltage in the calculator above and it solves for amps — along with the circuit's resistance — instantly.
Frequently Asked Questions
How many watts can a 15 amp circuit handle?
At 120 volts, a 15 amp circuit supplies up to 1,800 watts at its limit (15 × 120), but the 80% continuous-load rule keeps planned usage at 1,440 watts or below.
Is the amps formula the same for DC and AC?
For DC and for resistive AC loads like heaters and incandescent bulbs, I = P / V applies directly. AC motors and other inductive loads also involve a power factor, so the real current is somewhat higher than watts divided by volts.
How do I convert amps back to watts?
Multiply amps by volts: P = V × I. A device drawing 2 amps on a 120 V outlet uses 240 watts.
Where do I find an appliance's wattage?
Check the label or plate on the back or bottom of the device — it lists watts, or volts and amps, from which you can compute the rest. Many plugs and power bricks list the rating as well.