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Extension Cord Size Calculator: What Gauge Do I Need?

Find the extension cord gauge for a space heater or other appliance from its watts and the cord length. Shows amps and voltage drop.

How to use the Extension Cord Size Calculator

  1. Enter the appliance's watts, or its amps if you know them.
  2. Enter the length of the extension cord in feet.
  3. Choose the cord's gauge (AWG). It is printed on the cord, and a smaller number is thicker.
  4. Read the voltage drop and the heat in the cord. Optionally enter the amps printed on the cord to check it against the appliance.

How the extension cord calculator works

Electricity goes out along one wire in the cord and back along another, so the calculator counts twice the cord's length. The resistance of copper wire depends on its gauge, and the voltage lost is the current times the resistance.

Voltage drop = amps x resistance of the wire out and back

Heat in the cord = amps x amps x resistance of the wire out and back

The wire resistance is calculated from the American Wire Gauge definition and the resistivity of copper at 20 degrees C. Stranded cords run slightly higher, and warm cords higher still.

Gauge (AWG) Ohms per 1,000 ft of one wire
18 6.385
16 4.015
14 2.525
12 1.588
10 0.999
8 0.628

Voltage drop for a 1,500 watt heater (12.5 amps)

Cord length 18 gauge 16 gauge 14 gauge 12 gauge 10 gauge 8 gauge
10 ft 1.3% 0.8% 0.5% 0.3% 0.2% 0.1%
25 ft 3.3% 2.1% 1.3% 0.8% 0.5% 0.3%
50 ft 6.7% 4.2% 2.6% 1.7% 1.0% 0.7%
100 ft 13.3% 8.4% 5.3% 3.3% 2.1% 1.3%

Up to 3 percent is the usual design goal. Over 5 percent is poor. Notice how quickly a thin cord gets worse with length: an 18 gauge cord loses a large share of the voltage at 50 feet, while a 12 gauge cord stays low.

Voltage drop is not the same as safe current

This tool does not say how much current a cord can safely carry. That is set by the rating printed on the cord, and it must be at least the appliance's amps. A heater runs for hours, which warms the cord, so choose heavy duty, keep it short, and check that the cord and plug stay cool.

Space heaters and extension cords

Fire safety agencies, including the US Fire Administration, advise plugging space heaters directly into a wall outlet and not sharing that outlet with other devices. If a heater keeps tripping the breaker, see why a space heater trips the breaker. To check what else is on the circuit, use the circuit load calculator.

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Frequently asked questions

What gauge extension cord do I need for a space heater?

Fire agencies advise plugging a space heater straight into a wall outlet, not an extension cord. If you must use one, many guides recommend 12 gauge or heavier for a 1,500 watt heater, as short as possible, and the amp rating printed on the cord must be at least the heater's 12.5 amps. Retail guidance varies, so check the cord's label.

What does the voltage drop mean?

The cord's wire has resistance, so some voltage is lost along its length and the appliance gets less than 120 volts. A drop under 3 percent is the usual goal. A bigger drop wastes power as heat in the cord.

Why does my extension cord get warm?

The watts shown as heat in the cord are lost in the wire. A heavier, shorter cord loses less. A cord that is hot to the touch, or has a warm plug, should be unplugged and replaced.

Does a longer cord need a thicker wire?

Yes. Resistance grows with length, so the same current causes a larger drop on a longer cord. Doubling the length doubles the drop. A thicker gauge brings it back down.

Is this the same as how many amps a cord can carry?

No. This tool measures voltage drop and heat. How many amps a cord may carry is set by the rating printed on the cord, so always check that the rating is at least the appliance's amps.

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