Extension cord safety calculator
Enter what you are plugging in, the cord length, and the cord gauge. You get a safe or unsafe answer and the gauge you should use instead.
Enter the watts and the cord length
Method: voltage drop = 2 x 12.9 x amps x length / circular mils, limited to 5 percent of 120 volts. Amp limits also cap at the cord's ampacity and at 15 amps for a standard household plug. Results line up with published extension cord charts.
How this calculator works
Watts divided by 120 volts gives the amps your appliance draws. The calculator then works out the most current a cord of that gauge and length can carry before voltage drop exceeds 5 percent, using the standard formula drop = 2 x 12.9 x amps x length / circular mils. That limit is also capped by the cord's ampacity and by 15 amps for a standard household plug.
Longer cords and thinner wire (a higher gauge number) carry less current safely.
Frequently asked questions
What gauge extension cord do I need?
It depends on the load and the length. A 16 AWG cord safely carries about 6 amps at 100 feet. A 14 AWG cord carries about 10 amps at 100 feet. A 12 AWG cord carries about 10 amps at 150 feet.
Can I use an extension cord for a space heater?
It is not recommended. Space heaters draw about 12.5 amps at 1,500 watts, which is near the limit of most cords. Plug a heater directly into a wall outlet if you can.
What does the AWG number on a cord mean?
AWG is the wire gauge. A lower number means thicker wire, which carries more current and loses less voltage over distance. A 12 AWG cord is thicker than a 16 AWG cord.
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Last updated October 2026. Reviewed by the Fix or Hire editorial team. Estimates only, not a quote.