How big a mini-split does this room need?
Pre-loaded for a 400 square foot room with a standard ceiling, average light and two people in it. Change anything and the capacity rebuilds. You will not be asked for a tonnage, because working out the tonnage is the entire job.
Capacity this room needs
10,000 BTU/h
Nearest head anyone actually sells: 12,000 BTU/h, which overshoots the requirement by 20.0 percent.
Measure the room, then adjust these
Length times width. Do not include rooms behind a closed door.
A tall ceiling is more air to cool, in direct proportion.
Bodies are heat. Only the ones past two count.
Our own labelled adjustment, not a published multiplier.
The table this is built on, and exactly how far it goes
The spine of this tool is the room air conditioner sizing table ENERGY STAR publishes, reproduced below without alteration. It is the only government-backed area-to-capacity table we could find that also publishes its own adjustment rules, which is what makes it usable rather than merely quotable. It starts at 100 square feet and stops at 2,500. Ask for a room outside that and this site returns no number at all — extrapolating past the end of a source’s own table is how invented figures get born.
| Floor area, square feet | Capacity, BTU/h | Implied BTU per square foot at the low end |
|---|---|---|
| 100 up to 150 | 5,000 | 50.0 |
| 150 up to 250 | 6,000 | 40.0 |
| 250 up to 300 | 7,000 | 28.0 |
| 300 up to 350 | 8,000 | 26.7 |
| 350 up to 400 | 9,000 | 25.7 |
| 400 up to 450 | 10,000 | 25.0 |
| 450 up to 550 | 12,000 | 26.7 |
| 550 up to 700 | 14,000 | 25.5 |
| 700 up to 1,000 | 18,000 | 25.7 |
| 1,000 up to 1,200 | 21,000 | 21.0 |
| 1,200 up to 1,400 | 23,000 | 19.2 |
| 1,400 up to 1,500 | 24,000 | 17.1 |
| 1,500 up to 2,000 | 30,000 | 20.0 |
| 2,000 up to 2,500 | 34,000 | 17.0 |
Read the third column before anyone sells you a rule of thumb. At the bottom of the table a room needs 50.0 BTU per square foot; at the top it needs 17.0. Small rooms lose proportionally more heat through their walls than large ones, so a single multiplier applied across the whole range oversizes the big rooms and undersizes the small ones.
The four adjustments, and the one we made up
Four of the five things this tool asks about come with published adjustment rules attached to that same table. Very sunny adds 10 percent[2]; heavy shade takes 10 percent off [3]; each person past two adds 600 BTU per hour [4]; and a kitchen adds a flat 4,000[5] for the cooking. Those are the source’s numbers, not ours.
The fifth is ours and we say so on the control itself. There is no insulation term anywhere in the ENERGY STAR table, and we could not obtain a published residential envelope multiplier we were willing to cite. So the three envelope settings — 0.85 for tight, 1.00 for average and 1.2 for leaky [7] — carry an assumed label everywhere they appear. Our own labeled adjustment. The ENERGY STAR table has no insulation term; a room whose envelope is much better or worse than typical will sit outside these multipliers. If a contractor has run a Manual J on your house, that number beats this one.
Ceiling height sits in between. The source states the eight-foot baseline and the direction to move, but gives no formula, so scaling in proportion to room volume is our arithmetic on a sourced baseline. That makes it derived: the input is published, the operation is ours, and the operation is printed on the methodology page where you can disagree with it.
- Why the bigger unit is worseWhat an oversized head does to humidity, run time and the compressor.
- What it delivers at 5 FThis capacity is a 47 F rating. Cold mornings are a different, smaller number.
- Price the whole systemTurn the capacity into a head count and an installed cost band.
Questions people actually ask about this
How many BTU do I need per square foot?
There is no single answer, which is exactly why a per-square-foot rule of thumb misleads. Worked from the published table itself, the implied rate runs from 50.0 BTU per square foot at the smallest rooms down to 17.0 at the largest — a small room loses proportionally far more heat through its walls than a big one.
Does a sunny room really need a bigger unit?
10 percent bigger, per ENERGY STAR's own adjustment rule, and a heavily shaded room goes 10 percent smaller. West-facing glass in the late afternoon is the case this rule was written for.
What about a tall ceiling?
The published table assumes 8 feet and says taller ceilings warrant stepping up, without giving a formula. We scale in proportion to volume, publish that arithmetic, and never scale downward — a low ceiling is not a discount the source ever offered.
Why is the recommended head bigger than the number you calculated?
Because the market does not sell arbitrary sizes. Heads come in 8 standard capacities from 6,000 BTU/h up, so we round up to the smallest one that covers your load and tell you by how much it overshoots.
Where each figure on this page came from
- ENERGY STAR (U.S. EPA), Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners (retrieved 2026-08-06)
- ENERGY STAR (U.S. EPA), Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners (retrieved 2026-08-06)
- ENERGY STAR (U.S. EPA), Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners (retrieved 2026-08-06)
- ENERGY STAR (U.S. EPA), Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners (retrieved 2026-08-06)
- ENERGY STAR (U.S. EPA), Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners (retrieved 2026-08-06)
- ENERGY STAR (U.S. EPA), Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners (retrieved 2026-08-06)
- minisplitcostcalculator.com — our own labelled adjustment, disclosed on the methodology page — https://minisplitcostcalculator.com/methodology/ (retrieved 2026-08-06) [our assumption — no direct source]