Heat Pump Room Sizing Calculator
Whole-house calculators give you one number for the whole building, but installers need one per room. Add each room with its size and insulation level to get a heating load and recommended unit size for every room. Nothing uploaded.
Results
| Room | BTU/hr | kW | Recommended unit |
|---|---|---|---|
| Total | - | - | - |
Notes
Estimates are based on simple area, insulation, ceiling height, and window-area inputs. Actual heat pump sizing should also account for climate zone, air leakage, and manual J-style load calculations for a final installation decision.
Learn more: sizing a mini-split without overshooting
How the room load is estimated
The load starts with floor area times a rate for insulation quality: 20 BTU per square foot for good or modern, 30 for average and 40 for poor or older. That is multiplied by the ceiling height divided by 8 feet, since a taller room has more air to condition. Windows add 50 BTU per square foot of glass.
A 200 square foot room with average insulation, an 8 foot ceiling and 20 square feet of window comes to 6,000 + 1,000 = 7,000 BTU/hr, or 2.05 kW. The calculator picks the smallest standard indoor unit at or above the load, which is 9,000 BTU/hr. The standard sizes are 9,000, 12,000, 18,000, 24,000, 30,000 and 36,000, and a load above 36,000 is marked as needing more than one unit.
A 350 square foot room with poor insulation, a 9 foot ceiling and 40 square feet of window works out to 15,750 + 2,000 = 17,750 BTU/hr, so an 18,000 BTU/hr unit. The insulation rate alone can double the result between a modern room and a drafty one.
Why not just round up
ACCA's article on sizing variable-capacity heat pumps says oversized equipment starts and stops far more often than right-sized equipment, and that where summers are humid, oversizing often compromises comfort in the milder, wetter shoulder seasons. It also says sizing should start from a rigorous Manual J load calculation.
That is the limit of this tool. It is a rule-of-thumb estimate for one room at a time and has no climate, orientation or air leakage input. Use it to get a sense of the size, and have the final choice checked with a proper load calculation.
FAQ
Why does ceiling height matter?
A taller room holds more air for the same floor area, so the base load is scaled by the ceiling height divided by 8 feet.
Should I round up to the next size to be safe?
Generally not. ACCA says oversized equipment starts and stops more often and can hurt comfort. Pick the smallest standard size that covers the load, and confirm it with a load calculation.
Do older rooms need a much bigger unit?
Often. The calculator's insulation rate doubles from 20 to 40 BTU per square foot between a good and a poor room, so an older drafty room can need about twice the capacity of a similar well-insulated one.