BTU calculator for mini-splits and room units
Enter the space, the sun it gets, and your winter low. You get a cooling size, a heating estimate, the nearest standard unit size, and a warning when a 115V unit will come up short.
| Cooling load (ENERGY STAR chart) | 10,000 BTU/h |
| Heating load at 10°F (estimate) | 8,400 BTU/h |
| Size to the larger load | 10,000 BTU/h |
| Nearest standard size | 12,000 BTU |
| 115V unit enough? | Likely |
Cooling: ENERGY STAR room sizing chart and adjustments; ceilings over 8 ft scaled by height. Heating: a rule-of-thumb heat-loss estimate, not a Manual J load calculation. For whole homes, get a Manual J.
How the cooling number is worked out
Cooling uses the ENERGY STAR room air conditioner sizing chart, which sets capacity by floor area at an 8 ft ceiling, from 5,000 BTU for a small room up to 34,000 BTU for 2,500 sq ft. We apply the four adjustments ENERGY STAR publishes: take off 10 percent for a heavily shaded room, add 10 percent for a very sunny one, add 600 BTU for each person beyond two, and add 4,000 BTU for a kitchen.
Ceilings above 8 ft are scaled by height. That part is our assumption, since the chart stops at 8 ft.
Why heating is the number that bites
A heat pump's nameplate heat is rated at 47°F. As it gets colder outside, output drops, and many standard units deliver well under their rating by 5°F. The heating line here is a rule-of-thumb heat-loss estimate (floor area, ceiling height, insulation and the gap between 70°F and your winter low). It is meant to flag when heating, not cooling, should set the size.
When it does, compare units on their published capacity at low temperatures, not the size on the box. For a whole house, a Manual J load calculation is the right tool.
Bigger is not better
An oversized unit short-cycles: it hits the setpoint fast, shuts off, and leaves the room clammy because it never runs long enough to pull moisture out. Size to the load, then round up to the nearest standard size, not two sizes up.