Room BTU Calculator — Air Conditioner Sizing by Room
Who this is for: For homeowners sizing one room — a sun-baked bedroom, a converted garage, a home office — without buying a whole-house-sized unit for a 200 sqft space.
Room size, sun, occupants and kitchen use in — get the BTU and the mini-split or window unit size to buy.
Quick answer: Room BTU = area × 20–30 BTU/sqft (by climate), then +6% per foot of ceiling above 8, +10% if sunny, +600 per extra person, +4,000 for a kitchen, +15% if poorly insulated. A 200 sqft warm-climate bedroom for two lands at ≈5,500 BTU — a standard 5,000-class window unit.
sqft
ft
Summer climate
Sun exposure
Kitchen?
Insulation
BTU needed
5,500 BTU/h
rounded to 500
Window / portable AC
6000 BTU class
6000 if between sizes
Mini-split head
6k class
9k/12k/18k/24k are common
Between two sizes, take the smaller: an oversized room unit chills fast and shuts off before drying the air — cold and clammy beats warm but only just. Sunny garages and top-floor rooms are the ones that deserve the step up.
Room-level rule of thumb (20–30 BTU/sqft by climate + adjustments). Whole-house sizing belongs in the load calculator; permits may require a certified Manual J.
Core facts
Base
20–30 BTU/h per sqft by climate
Ceiling
+6% per ft above 8 ft
People / kitchen
+600 per extra occupant · +4,000 kitchen
Sun / insulation
+10% sunny · +15% poor insulation
Example
200 sqft warm, 2 people → ≈5,500 BTU
Status
Estimating tool. Compiled September 2026.
How many BTU does my room need?
Start from the room's area and climate: about 20–30 BTU/h per square foot depending on how hot your summers run. Then adjust: ceilings above 8 feet add roughly 6% per extra foot, a sunny room adds 10%, each person beyond the first adds 600 BTU/h, a kitchen adds 4,000, and poor insulation adds 15%. The result rounds to the nearest 500 BTU so you can match real units — a 200 sqft bedroom in a warm climate with two occupants lands near 5,500 BTU, which is a standard '5,000 BTU window unit' size. Slightly under is fine; oversizing a room unit makes it chill fast and shut off before dehumidifying, which is why the honest number beats the 'bigger is safer' instinct.
Common uses
Sizing a window or portable AC for one bedroom or office
Picking a mini-split head size for a converted garage or sunroom
Checking whether two small units beat one big one across two rooms
Talking a family member out of a 12,000 BTU unit for a 150 sqft room
Frequently Asked Questions
What size AC for a 200 sq ft room?
About 5,000–5,500 BTU/h in a warm climate with normal occupancy — the classic 5,000 BTU window unit. Hot-climate or sunny exposures push toward 6,000. Bigger is not better: an oversized unit short-cycles and leaves the room clammy.
Do I add BTU for people and kitchens?
Yes: 600 BTU/h per occupant beyond the first, and about 4,000 BTU/h if cooling a kitchen (the original room-BTU rule; appliance-heavy kitchens run higher). A media room with four gamers counts as three extra people.
How do ceiling height and sun change it?
Above-8-foot ceilings add ~6% per extra foot (you're cooling volume, not area). Direct-sun rooms add 10%; heavily shaded rooms can drop 10%. Poor insulation adds 15% — attic-above rooms often need it.
Can I use this for a mini-split?
Yes — mini-splits come in 6k/9k/12k/18k/24k BTU classes (a '9k' is 9,000 BTU/h). Match your calculated BTU to the nearest class, rounding down when between two. One head per room beats one oversized head for two rooms.
Does heating need the same BTU?
Heat pumps sized for cooling sometimes under-deliver on the coldest nights (that's what resistance backup is for). In cold climates, heating load — driven by design temperature difference — can exceed cooling; this calculator leans cooling-first, like most room sizing.