How big of a heater do I need for 800 square feet?

Quick Answer

For an 800 square foot space, you will likely need a heater that provides between 20,000 and 30,000 BTUs of heating power. A good rule of thumb is to plan for 25 BTUs per square foot. So for 800 square feet, you would need a heater with around 20,000 BTUs (800 x 25 = 20,000). Going a bit larger, like a 30,000 BTU heater, gives you some extra power to account for factors like high ceilings, poor insulation, or especially cold weather.

What Size Heater Do You Need?

Choosing the proper size heater for an 800 square foot space involves considering several factors:

1. Room Size

The first factor is simply calculating the square footage of the space you need to heat. 800 square feet is a medium sized room. Here are some heater guidelines for different room sizes:

– 100-300 square feet – 5,000-9,000 BTUs
– 350-600 square feet – 10,000-18,000 BTUs
– 650-1000 square feet – 18,000-30,000 BTUs

So for 800 square feet, you’ll need a heater somewhere in the 20,000-30,000 BTU range.

2. Room Height

The height of the room also impacts heating needs. Heat rises, so it takes more energy to heat taller rooms. For standard 8-10 foot ceilings, the 25 BTU per square foot guideline works well. But for spaces with cathedral ceilings over 10 feet high, increase the BTU requirements by 10% for each additional foot of ceiling height.

3. Insulation Level

Poorly insulated spaces require more heating power than well-insulated ones. Older homes with minimal insulation may need 30-40 BTUs per square foot. Well-insulated modern spaces may only need 20 BTUs. Consider your insulation situation and adjust BTU needs up or down.

4. Number of Windows

Lots of windows means more heat loss. Increase heater capacity around 10% if your space has an abundant amount of large windows.

5. Local Climate

BTU guidelines assume average winter temperatures around 30-50 degrees Fahrenheit. In colder climates, increase capacity 10% per 10 degree drop in average outdoor temperature.

6. Supplemental Heating Sources

If you’ll be using other heat sources like a wood stove or radiant floor heating, you may be able to reduce the heater size since it won’t be the sole heat source.

Heater Types

There are several types of heaters suitable for 800 square feet:

Electric Heater

– Type: Electric powered heater with heating elements and a fan to circulate warm air.

– BTUs: Offered in models from 5,000-10,000 BTUs typically

– Benefits: Affordable purchase price, easy to install and operate

– Drawbacks: More expensive to operate than gas, not practical for larger areas

Propane Heater

– Type: Heater that runs on propane fuel, creates heat from burning. Include self-contained portable versions.

– BTUs: Can provide up to 50,000 BTUs.

– Benefits: Don’t need electric hook-up, versatile, high heating capacity.

– Drawbacks: Need to have propane fuel delivered and refilled regularly.

Natural Gas Heater

– Type: Connects to natural gas fuel line to heat via gas burners. Include wall furnaces, floor furnaces, or central forced air furnaces.

– BTUs: Up to 100,000 BTUs for home furnaces. Wall heaters up to 40,000 BTUs.

– Benefits: Efficient to operate, provide whole home heating, long lasting.

– Drawbacks: Higher upfront installation cost, require professional installation.

Oil Filled Radiator Heater

– Type: Electric heater containing heated oil that slowly radiates warmth. Quiet operation. Include oscillating models.

– BTUs: Smaller units from 1,500-5,000 BTUs typically.

– Benefits: Compact and portable, good for small to medium rooms, inexpensive purchase price.

– Drawbacks: Take longer to heat up than forced air options, exposed heating elements can be safety hazard.

How Many BTUs Per Square Foot?

When sizing a heater, a good rule of thumb is to plan for 25 BTUs of heating power per square foot of space. This covers average room height, insulation level, and window amount for most homes.

Here is a BTU per square foot table:

Square Footage BTUs Needed
100 sq ft 2,500 BTUs
200 sq ft 5,000 BTUs
300 sq ft 7,500 BTUs
400 sq ft 10,000 BTUs
500 sq ft 12,500 BTUs
600 sq ft 15,000 BTUs
700 sq ft 17,500 BTUs
800 sq ft 20,000 BTUs
900 sq ft 22,500 BTUs
1000 sq ft 25,000 BTUs

So for an 800 square foot room, take 800 x 25 = 20,000 BTUs for your heater size.

You can adjust up or down from the 25 BTU per square foot baseline as needed based on room layout, insulation, and your local climate. But it provides a good starting point for your heater selection.

What Size Heater for 800 sq ft Room?

Given the 25 BTUs per square foot guideline, here are some recommended heater capacities for an 800 square foot room depending on your exact needs:

– 20,000 BTUs – This heater size is suitable for well insulated 800 sq ft room with 8 foot ceilings and average window amount.

– 25,000 BTUs – Provides an extra buffer for an 800 sq ft room that is moderately insulated or has lots of windows.

– 30,000 BTUs – Best for an 800 sq ft room with high ceilings over 10 feet, poor insulation, or very cold climate conditions.

– 40,000 BTUs – Only needed if the room has multiple heat loss factors like high ceilings, lots of windows, and poor insulation.

Going too large on your heater won’t cause issues other than being more expensive upfront and wasting a bit of energy. But an undersized heating unit will struggle to warm the space effectively.

Electric Heater Size for 800 sq ft

For electric heaters, you’ll need multiple units to meet the heating needs of 800 square feet since individual electric heaters max out at 10,000 BTUs typically.

Here are electric heater configurations for 800 square feet:

– 3 x 10,000 BTU electric heaters – Provides 30,000 BTUs total.

– 4 x 7,500 BTU electric heaters – Also provides 30,000 total BTUs.

– 2 x 10,000 BTU + 1 x 5,000 BTu heaters – Offers 25,000 total BTUs.

– 5 x 5,000 BTU heaters – Delivers 25,000 BTUs for well insulated smaller rooms.

Electric heaters are best for warming smaller spaces. For larger applications, a central gas or propane furnace is usually a better heating option.

Propane Heater Size for 800 sq ft

Standalone propane heaters can provide sufficient heating capacity for 800 square feet with a single unit. Good propane heater options include:

– 30,000 BTU propane convection heater
– 35,000 BTU propane forced air heater
– 50,000 BTU portable propane shop heater

Propane heating does require regularly refiling propane tanks and so isn’t as convenient as natural gas or electric. But propane heaters provide powerful space heating and the flexibility of not needing ductwork or electrical hook-ups.

Natural Gas Heater Size for 800 sq ft

For heating an 800 square foot open space, consider these natural gas heater sizes:

– 30,000 BTU natural gas wall furnace
– 30,000 BTU natural gas floor furnace
– 45,000 BTU natural gas central furnace

Natural gas furnaces require professional installation and gas line hook-up. But they provide economical whole home heating for larger areas.

You also get options like central heating and air conditioning, programmable thermostats, and zoning control with a ducted gas furnace system. This makes natural gas ideal for heating 800+ square foot spaces.

Oil Filled Radiator Heater Size

Oil filled radiator heaters use less energy and are quieter than forced air electric heaters. But they have lower BTU outputs best suited for small to mid-sized rooms.

For an 800 square foot space, you would need multiple oil radiator heaters like:

– 6 x 1,500 BTU oil filled radiators = 9,000 BTUs
– 8 x 1,500 BTU oil filled radiators = 12,000 BTUs
– 3 x 5,000 BTU oil filled radiators = 15,000 BTUs

While this can provide supplemental warmth, oil filled radiators may not be able to meet all your heating needs for larger areas. They work better for offices, bedrooms, and other small to medium rooms up to 500 square feet.

Selecting a Properly Sized Heater

Choosing the ideal heater size involves:

1. Calculating your total square footage.

2. Multiplying by 25 BTUs per square foot for a baseline.

3. Adjusting up or down based on room layout, insulation, windows, and local climate.

4. Selecting a heater type and model that matches your BTU requirements.

Always better to oversize your heater capacity slightly rather than go too small. And consult with a qualified HVAC professional if unsure about properly heating and cooling your particular space.


For an 800 square foot room, a good rule of thumb is to plan for a heater sized between 20,000-30,000 BTUs. This factors in the 25 BTUs per square foot guideline along with extra capacity adjustments for higher ceilings, lots of windows, or poor insulation.

Electric heaters can provide supplemental warmth, but may fall short of fully heating 800 square feet without multiple units. Propane and natural gas heaters in the 30,000-50,000 BTU range offer sufficient power. Work with an HVAC professional to determine your exact BTU requirements and choose the right heating system for your space. Properly heating an 800 square foot room provides cozy and comfortable warmth for you to enjoy.

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