WeekendCalc

What Size Space Heater for 100 Square Feet?

A 100 sq ft room with average insulation and an 8 ft ceiling needs about 1,000 W of heat (3,412 BTU per hour), which is one 1,500 W heater. Running it flat out costs $0.17 an hour at $0.17 per kWh.

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Example job shown. Change the numbers to match yours.

Heater size needed
1,000 watts
In BTU per hour
3,412 BTU/hr
1,500 W heaters needed
1 heaters
Room area
100 ft²

Running cost at full output

Cost per hour
$0.17
Cost per 8 hours
$1.36
  • A thermostat cycles the heater on and off, so real running cost is usually lower than full output. Plug 1,500 W heaters straight into a wall outlet, not an extension cord, and keep them clear of anything flammable.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Heater watts for 100 sq ft, by insulation and ceiling height

A 100 sq ft room is a small bedroom, a home office or a bathroom-sized space. The rule of thumb is about 10 W per square foot for an average room, 8 W for a well-insulated one and 12 W for a drafty one, scaled up for ceilings over 8 ft.

Insulation8 ft ceiling9 ft ceiling10 ft ceiling1,500 W heaters (8 ft)
Well insulated800 W900 W1,000 W1
Average1,000 W1,125 W1,250 W1
Poor or drafty1,200 W1,350 W1,500 W1

What it costs to run, at $0.17 per kWh

The cost of 1,000 W for 8 hours a day, 30 days a month. Change the price to your own rate in the electricity cost calculator.

How it runsPer hourPer 8-hour dayPer month
Flat out (no thermostat)$0.17$1.36$40.80
Thermostat, cold room$0.13$1.02$30.60
Thermostat, cycling about half the time$0.09$0.68$20.40

One heater or more for 100 sq ft?

One standard 1,500 W heater covers 100 sq ft at average insulation, since the room needs 1,000 W. It draws 8.3 A on a 120 V outlet, so plug it straight into the wall and keep other big loads off that circuit.

Choose a heater with a thermostat, tip-over and overheat protection, and keep it 3 ft from bedding and curtains. The space heater sizing guide covers the types of heater, and the running cost guide compares prices.

Want to change anything else? The calculator above is the full Space Heater Size Calculator: edit any number and the results update instantly.

Tools & materials for this project

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Ceramic space heaterOil-filled radiatorDraft stopperWindow insulation filmSmart thermostat

Frequently asked questions

What size space heater do I need for 100 square feet?

About 1,000 W (3,412 BTU per hour) for a room with average insulation and an 8 ft ceiling. A well-insulated room needs 800 W, and a drafty one 1,200 W.

How many 1500 watt heaters for 100 sq ft?

1 at average insulation. In a drafty room with 10 ft ceilings it is 1.

How much does it cost to heat 100 sq ft with a space heater?

Running 1,000 W flat out at $0.17 per kWh costs $0.17 an hour, $1.36 for an 8-hour day and $40.80 a month. With a thermostat cycling about half the time it is closer to $20.40 a month.

Related sizes and guides

150 sq ft room1,500 W200 sq ft room2,000 WBaseboard Heater Size CalculatorWatts, feet and breakerWhat size space heater do I need?Guide

Related sizes

300 sq ft room3,000 W500 sq ft room5,000 W250 sq ft room2,500 W400 sq ft room4,000 W

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.