Heater Room Size Calculator: What Size Room a Heater Heat?

How big a room can you heat with your gas heater or electric heater? Oil? Baseboard?

This Heater Room Size Calculator gives you a quick, accurate answer to common questions like – what size room will a 1500 watt heater heat?

Choose electricity Watts or BTU for your heater when using the calculator below.

Heater Room Size Calculator

Below the calculator are instructions on how to use it, Tables with common BTU and Watt heater sizes and the size of the rooms they will heat.

The Tables can also be used if you know the room size and want to know what size heater is needed.

You will also find examples to show you exactly how these ratings are determined.

OK – Here is the Pick HVAC Room Size Calculator for Heater units of various sizes.

Heater Room Size Calculator

Heater Room Size by BTU

This BTU heater room size table can be used whether you have a heater and want to know how much space it will heat – or if you know the square feet of the space and need to know what heater to buy.

Heater types that are rated by BTU include a gas heater and oil heater (kerosene/home heater oil).

Heater Capacity (BTU):Room Size:
4,000 BTU130 sq. ft.
4,600 BTU150 sq. ft.
5,000 BTU160 sq. ft.
5,100 BTU170 sq. ft.
5,200 BTU170 sq. ft.
6,000 BTU200 sq. ft.
9,000 BTU300 sq. ft.
10,000 BTU330 sq. ft.
12,000 BTU400 sq. ft.
13,000 BTU430 sq. ft.
15,000 BTU500 sq. ft.
16,000 BTU530 sq. ft.
20,000 BTU660 sq. ft.

For example, the 5200 BTU heater room size is 170 square feet. 5200 BTU is equivalent to 1500 watts of electricity, a very common heater size.

Another example shows that a 12,000 BTU heater, one 1 ton heater, serves up to 400 square feet.

Heater Room Size by Watts

This is your Table for heaters powered by electricity as their only source – not a gas heater, for example, with a fan. Types commonly rated in watts are electric heater, radiator heater, baseboard heater and an oil heater if by that you mean it is one of those radiant-heat space heaters with oil inside instead of water that retains heat after the power is shut off.

Question: How many watts per square foot are needed to heat a room?

Answer: 10 watts of heating power per square foot is recommended for electric heaters. So, a 150 square foot room needs a heater with about 1,500 watts of heating power.

Heater Power (Watt):Room Size:
500 watt50 sq. ft.
1,000 watt100 sq. ft.
1,500 watt150 sq. ft.
3,000 watt300 sq. ft.
4,500 watt450 sq. ft. 

Example 1: What size room will a 1500 watt heater heat?

Check the Table, and you’ll see that a 1500 watt heater heats a room up to 150 square feet.

You can use this Table to ask the question a different way.

Example 2: How big of a heater needed for 300 square feet?

Find 300 square feet in the Room Size Column, and you’ll see that a 3000 watt heater is the right size for a 300 square foot room.

How Many Watts Per Square Foot?

The rule is 10 watts of heating power per square foot is required for indoor comfort. This is true for insulated space. If you’re trying to heat a garage or other poorly insulated or non-insulated space with an electric heater, 10 watts of heating power per square foot is not going to do the job.

Factors Affecting Heater Room Size Calculator

As noted just above, the Heater Room Size by BTU and Heater Room Size by Watts Tables are designed for indoor space, not unheated garages or outbuildings or tents or RVs, etc.

There are other factors that will affect, for example, 5200 BTU heater room size.

Ceiling Height: Rooms with ceilings higher than 8 feet might need a little more heater power. Go up one size. For example, in a 200 square foot bedroom, a 6,000 BTU heater would normally be sufficient. If the ceiling is 10 feet high, choose an 8,000 or 9,000 BTU heater instead.

Or you can increase size by 1.25 times. So, if the Table says you need 6,000 BTU, the equation is 6,000 x 1.25 = 7,560, or rounded up to 8,000.

You can do the same for watts. 1,000 x 1.25 = 1,250 watts, for example.

Drafty Rooms: Even if the house has good insulation, if the windows let in air, the room will feel chilly. So, how big of a room will a 1500 watt heater heat? In this case, you might have to say a 100 square foot room instead of the 150 square feet listed in the Table.

Heat Needed: The average of 10 watts of heating power per square foot is for a decently insulated room in an average climate during winter. If it is super-cold outside, you might need more watts, like 15 watts of heating power per square foot.

The Heater Room Size by BTU Table is based on 30 BTUs of heating power per square foot, approximately. On a sub-freezing day, you might need 40 BTUs per square foot, so choose a bigger heater. On the other hand, if it’s a chilly day in Missouri and you want a space heater, then you might need only 20 BTU per square foot.

Heater Room Size Calculator and Other Calculators from Pick HVAC

We trust this page is helpful, so feel free to share it with others. And you’ll see a whole list of exclusive, useful conversion calculators making it easy to “do the math” when comparing equipment, fuels, heat and ratings or determining what size heater or AC you need for your space.

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