How to Calculate BTUs for Fireplaces

If you're about to install a gas fireplace in your house, knowing how many BTUs, or British thermal units, is appropriate for the room will save you money by getting the most efficient unit possible -- one that provides enough but not too many BTUs. If you already have a gas fireplace in your house, you can find out if it is appropriate for the room in which it is installed, even if you don't know its BTU rating.

Calculating Appropriate BTUs for a Room

Step 1

Measure the length of the room with the measuring tape.

Step 2

Measure the width of the room.

Step 3

Measure the height of the ceiling.

Step 4

Multiply all three figures. For instance, if your room is 10 feet wide, 12 feet wide and 10 feet high, the result would be 1,200 cubic feet. The maximum number of BTUs per 50 cubic feet is 1,000, so the maximum BTU rating for your room would be 24,000.

Calculating BTUs of Existing Gas Fireplace

Step 5

Read your monthly gas bill to find gas usage in cubic feet per hour.

Step 6

Divide the total cubic feet of gas by number of hours of monthly operation to calculate the CFH. If necessary, track your usage for a month to arrive at a monthly total.

Step 7

Multiply the resulting figure by 1,000 to determine the BTU rating of your fireplace.