Sizing a heater for a specific space can be a lot like buying clothes on the Internet. You can do the numbers based on manufacturers' sizing charts, but there's no guarantee the actual size will be just right, and you won't know for sure until you buy the item and test it out. Fortunately, with heating there are a few more ways to minimize your risk, and factoring in some measurable variables will give you the benefit of an educated guess. Like clothes, heaters have to be the right size, but they're more like socks than shoes: close is usually good enough. The best approach is to learn as much as you can about your needs and goals, then talk to a heating supplier or manufacturer for specific recommendations.

What the Pros Do

When sizing heating systems for entire houses, HVAC pros perform complex "load" calculations that include local climate data, the home's layout and construction type, insulation levels, windows and doors…and possibly a little alchemy. To figure out what size heater you need for just a room or other specific space, you'll consider some of the same factors but at a much simpler level. The point is, you can do better than simply sizing a heater based on square footage alone.

Measure the Space

Answering the question What size heater do I need? starts with some basic load calculations and applying rules of thumb. The first step is to measure the area of the room by multiplying the length by the width. For example, 15 x 20-foot room has an area of 300 square feet. Many heater manufacturers estimate heater size based on room area alone, but it's more accurate to use the room volume, measured in cubic feet. Simply multiply the area by the ceiling height; for example, if the 300-square-foot room has a 9-foot ceiling, the volume of the space is 2,700 cubic feet.

Heat Values and Basic Sizing

Electric heaters often are sized, or rated, by wattage. Gas-fueled heaters (including propane, natural gas and kerosene types) are rated by the BTU, or British thermal unit. Technically speaking, a BTU is the heat energy required to raise the temperature of 1 pound of water by 1 degree F.

Of course, when you're figuring what size heater you need, thinking in terms of water weight is like cooking with a plumb bob. So let's put it into perspective. According to a very basic industry rule of thumb, you need 10 watts of electric heat for each square foot of room area. Therefore, that 300-square-foot rooms needs a 3,000-watt electric heater. You can easily convert watts to BTUs by multiplying by 3.41: 3,000 x 3.41 = 10,230 BTUs.

Other Factors

The calculations shown above give you a starting point for determining what size heater you need. There a few other factors that will give a more accurate estimate. A knowledgeable sales rep or installer can use this additional info in making size recommendations, and some manufacturers have sizing charts that require more than square footage data.

One simplified load calculation involves finding the temperature difference between the indoor and outdoor areas, known as the heat rise. To get a rough estimate, find the difference between the desired room temperature (say, 72 degrees F) and the coldest average temperature for your area (just enter your city on any weather site; for example, in Kansas City the average coldest day is about 22 degrees). Subtract 22 from 72 to get 50 degrees. This is the heat rise you'll need to keep your room warm on the coldest day of the year.

Additional factors to consider include:

• ceiling height (which you used in calculating the room's volume)
• number and type of windows and doors in the space (and how often the doors are opened)
• room insulation levels.

In simple terms, more windows and doors means you need a bigger heater. Same goes if the room is poorly insulated or not insulated at all. Even the best windows have very low insulation value compared to a standard wall assembly, so be sure to discuss the window size and placement with your heater supplier. This can affect everything from the recommended type of heater to the size, placement and number of heaters needed.

Primary or Supplemental?

One final consideration is the question of primary vs. supplemental heat. Primary is the main (and often only) source of heat for a space. Supplemental is a secondary heat source to help raise the temp when the primary heat isn't quite enough. When sizing supplemental heating, look at how warm the room gets with the primary heat and decide how much warmer you want it to be. If, during the coldest time of year, the main heat keeps the temp at 65 degrees, you might want to boost that another 12 degrees, using a supplemental heater. You still factor in room size and sources of heat loss (windows, doors, poor insulation, etc.), but be sure to discuss your temperature goals (the 12-degree heat rise) when you call a heating contractor.

Philip Schmidt writes for Networx.com.