Cooling BTU: The estimated cooling capacity needed to keep this room comfortable in summer. Match this number to a window AC unit or mini-split rating — a 12,000 BTU unit equals 1 ton of cooling capacity.
Heating BTU: The estimated heating capacity, set at 65% of the cooling figure. Heating demand is generally lower because you are raising indoor temperature by a smaller delta than you are lowering it in summer.
Room area: The floor area used as the starting point. The standard rule of thumb is 20 BTU per square foot for cooling, then adjusted for insulation, sun exposure, and occupants.
Insulation and sun adjustments: Poor insulation adds 25% to the base load; good insulation subtracts 15%. High sun exposure adds 15%; low sun subtracts 10%. These factors stack multiplicatively.
Occupant load: Each person in the room adds roughly 150 BTU of body heat. For spaces with more than 4–5 occupants, this becomes a significant factor.
How This Calculator Works
You enter room length, width, and ceiling height in feet, then select insulation quality, sun exposure level, and number of occupants. The tool computes floor area, multiplies by 20 BTU/sq ft as a baseline cooling load, applies insulation and sun multipliers, and adds 150 BTU per occupant. Heating capacity is estimated at 65% of the cooling result. Ceiling height is collected for reference but the formula uses floor area only, consistent with most rule-of-thumb HVAC sizing guides.
Quick Questions
What does BTU stand for?
BTU stands for British Thermal Unit — the amount of energy needed to raise one pound of water by one degree Fahrenheit. In HVAC, BTU/hr measures how much heat an air conditioner can remove (cooling) or a heater can produce (heating) per hour.
How do I convert BTU to tons of cooling?
One ton of cooling equals 12,000 BTU/hr. So a 24,000 BTU result means you need a 2-ton unit. Most residential central AC systems range from 1.5 to 5 tons.
Should I round up or down when buying a unit?
Slightly oversized is generally better than undersized — an undersized unit runs constantly without reaching the target temperature. However, grossly oversized units short-cycle, waste energy, and dehumidify poorly. Aim for a unit within 10–15% of the calculated BTU.
Does this include kitchen or bathroom heat loads?
No. Kitchens, server rooms, and spaces with heat-generating equipment need additional BTU allowances. A kitchen typically adds 1,000–4,000 BTU depending on appliance use. Consult an HVAC professional for rooms with unusual heat loads.
Why is the heating number lower than cooling?
In most climates, the temperature difference between indoor and outdoor is smaller in winter heating mode than in summer cooling mode. The 65% factor is a common residential rule of thumb — actual heating loads depend on your climate zone and insulation.