How to Calculate HVAC Unit Size

Instructions

• 1

  Figure out the square footage of space that you need to heat and cool. Measure the room's length and width. Multiply these together to get the square footage.

• 2

  Continue measuring each room and add all the square footages together. As a general rule, you'll want one CFM (cubic feet per minute) per square foot. The exception to this is if you have an area with a lot of windows. You'll then want two CFM per square foot.

• 3

  Convert the CFMs into tons by dividing by 400. For example, say your total square footage is 2,000, and you determined you need 2,000 CFM. Take 2,000 and divide by 400. For this example you need a five-ton air-conditioning unit.

• 4

  Calculate the BTU (British Thermal Units) needed for your furnace. This is done by taking your CFM and dividing by 150 and then multiplying that number by 10,000. For instance, from the example in step 3, you had 2,000 CFM. Two thousand divided by 150 is 13 1/3. This number multiplied by 10,000 is 133,333 BTU. You would install a furnace close to that output of BTU.

• 5

  Purchase your calculated furnace and air conditioner from your local heating and air-conditioning retailer, and install it according to the manufacturer's specifications.