How to use the btu calculator
- Enter the room area in square feet and the ceiling height in feet.
- Pick the climate and sun exposure that best describe the room. Each choice applies a labeled rule-of-thumb percentage.
- Enter the number of people who usually use the room, and tick the kitchen box if the room has a lot of heat-producing equipment.
- Read the BTU per hour and tons. Open More options to change the base BTU per square foot or other rule-of-thumb figures.
Formula
Base load = room area x base BTU per square foot (20 by default)
Height adjustment = base load x ceiling height / reference height (8 ft)
Climate and sun: multiply by (1 + climate percent + sun percent)
Add 600 BTU per hour for each person beyond two
Add 4,000 BTU per hour for a kitchen
Tons = BTU per hour / 12,000
One ton of cooling is 12,000 BTU per hour. Every number here except that conversion is a rule of thumb that you can edit, not a measured property of your home.
Worked example: a bedroom and a sunny kitchen-living room
A 300 sq ft room with 8 ft ceilings, a moderate climate, normal sun and two people: 300 x 20 = 6,000 BTU/hr, which is 0.5 tons.
A 400 sq ft kitchen-living room with 9 ft ceilings in a warm climate with strong sun and four people: 400 x 20 = 8,000, times 9 / 8 gives 9,000.
Warm (+10%) and very sunny (+10%) together multiply it by 1.2, giving 10,800. Two extra people add 2 x 600 = 1,200, and the kitchen adds 4,000.
The total is 16,000 BTU/hr, or 1.33 tons.
This is not a Manual J calculation
Professionals size heating and cooling equipment with a detailed load calculation, commonly called Manual J. It accounts for insulation, window area and type, orientation, air leakage, ducts, local design temperatures and humidity. This calculator uses none of that. It applies a simple rule of thumb per square foot with a few labeled adjustments, so it can be far off for a particular home. Use it to get a ballpark and to compare options, then consult an HVAC professional before buying or installing equipment.
Oversizing and undersizing
A unit that is too small may never reach the temperature you want on the hottest days, while a unit that is too large can switch on and off in short cycles and handle humidity poorly. That is another reason to treat the number as a starting point. Window air conditioners and portable units are labeled in BTU per hour, so you can compare this estimate with product labels, but check each label for the conditions it assumes.
Running costs
Once you have a size in mind, you can estimate what it costs to run. Look up the unit power draw in watts on its label, then enter it in the electricity cost calculator with your own rate and the hours you expect to use it. For irregular rooms, find the area with the square footage calculator first.
Assumptions and limits
- The base of 20 BTU per square foot is a commonly quoted rule of thumb, not a standard. You can edit it under More options.
- Climate, sun, people and kitchen adjustments are rule-of-thumb amounts shown in their labels, and they are not derived from your home.
- The result is for cooling. Heating loads are different and are not calculated.
- Insulation, windows, air leakage, ducts, humidity and local design temperatures are not considered.
- This is not a Manual J load calculation. Consult an HVAC professional before choosing equipment.
Frequently asked questions
How many BTU do I need for a 300 square foot room?
With the common 20 BTU per square foot rule and no adjustments it is 6,000 BTU per hour. Sun, climate, people and ceiling height can raise or lower that, and this tool shows each step.
How many BTU are in a ton of cooling?
One ton equals 12,000 BTU per hour. Divide BTU per hour by 12,000 to get tons, as the tool does.
Is this the same as a Manual J calculation?
No. Manual J is a detailed method that models your actual building. This tool is a quick rule-of-thumb estimate that cannot account for insulation, windows or air leakage.
Why does the kitchen add BTU?
Cooking appliances give off heat that the cooling equipment has to remove. The extra amount is a rule of thumb that you can edit, and it is not measured from your appliances.
Can I use this to size a furnace?
No. The rule-of-thumb adjustments here are for cooling, and heating loads depend on different factors. Ask an HVAC professional to size heating equipment.