BTU Calculator
Find how many BTUs you need to cool a room — enter the size, ceiling height, sun exposure and climate to get the cooling BTUs and a recommended air conditioner size in BTU and tons.
This is a quick rule-of-thumb estimate to get you in the right size range. For a permanent installation, a contractor’s Manual J load calculation is the accurate way to size equipment.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
base load = room area (sq ft) × 22 BTU/hr
× (ceiling height ÷ 8) × sun factor × climate factor
+ 600 BTU/hr per occupant above two + 4,000 BTU/hr if it is a kitchen
recommended unit = the smallest standard capacity at or above the load
Assumptions & limits
- The base rate is 22 BTU/hr per square foot, mid-range for the 20–25 rule of thumb used for room air conditioners.
- Ceiling height scales the load linearly against an 8 ft baseline, which is a volume proxy rather than a true heat-gain model.
- Sun factors are 0.9 shaded, 1.0 average, 1.1 very sunny. Climate factors are 0.9 mild, 1.0 moderate, 1.15 hot.
- Standard capacities are 5,000 · 6,000 · 8,000 · 10,000 · 12,000 · 14,000 · 18,000 · 21,000 · 24,000 · 28,000 · 36,000 · 48,000 BTU/hr, and the result rounds up to the next one.
- This is a rule of thumb, and it is explicitly not a load calculation. It ignores insulation levels, window area, glazing type, air-tightness, duct losses and orientation — the variables that dominate a real load.
- For anything permanently installed, an ACCA Manual J load calculation is the accurate method, and oversizing is a real fault: an oversized unit short-cycles and fails to dehumidify, leaving a room cold and clammy.
Worked example
A 300 sq ft very sunny room, 9 ft ceilings, hot climate, three occupants
- Base = 300 × 22 = 6,600 BTU/hr
- Ceiling = × 9 ÷ 8 = 7,425 · sun = × 1.1 = 8,168 · climate = × 1.15 = 9,393
- Occupants = one above two × 600 = +600 → 9,993 BTU/hr
- Smallest standard size at or above 9,993 = 10,000 BTU/hr
Result:A 10,000 BTU/hr unit, about 0.83 tons — then confirm with Manual J before buying a fixed system.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 17, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
How many BTUs do I need to cool a room?
The starting point for cooling is roughly 20–25 BTUs per square foot of floor area. From there you adjust for the things that change the heat load: tall ceilings, strong sun, a hot climate, the number of people, and kitchen appliances. The calculator above applies these factors and rounds the result up to the nearest standard air-conditioner size.
BTU ≈ area × 22 × (ceiling ÷ 8) × sun × climate + people + kitchen
AC size by room area
| Room area | Cooling BTUs | Tons |
|---|---|---|
| 150 sq ft | 5,000–6,000 | 0.5 |
| 300 sq ft | 7,000–8,000 | 0.6 |
| 500 sq ft | 10,000–12,000 | 1.0 |
| 700 sq ft | 14,000–18,000 | 1.5 |
| 1,000 sq ft | 18,000–24,000 | 2.0 |
Ranges for average conditions — sun, high ceilings and hot climates push toward the top.
Looking at a ductless system? The mini split sizing calculator rounds to standard mini-split sizes. New to the unit? See what a BTU is and convert to kW or tons.
AC sizes and the rooms they fit
| Capacity | Room size (avg. conditions) | Typical unit |
|---|---|---|
| 5,000 BTU | up to ~230 sq ft | Small window unit |
| 8,000 BTU | ~340–360 sq ft | Window / portable |
| 10,000 BTU | ~450 sq ft | Large window unit |
| 12,000 BTU | ~500–550 sq ft | Window / mini-split (1 ton) |
| 18,000 BTU | ~800 sq ft | Mini-split (1.5 tons) |
| 24,000 BTU | ~1,050 sq ft | Mini-split / central zone (2 tons) |
Derived from the same ~22 BTU per sq ft baseline the calculator uses; sun, ceilings and climate shift every row.
Common mistakes
- Oversizing “to be safe”. An oversized AC cools fast, shuts off before it dehumidifies, and leaves the room cold and clammy — then repeats. Right-sized beats bigger.
- Sizing a vaulted room by floor area. A 14 ft peak holds far more air than the 8 ft baseline; the ceiling factor exists for a reason.
- Ignoring the kitchen. An open kitchen adds roughly 4,000 BTU/hr of appliance and cooking heat — the single biggest adjustment in the model.
- One unit for connected rooms. A window unit conditions the room it sits in; air does not turn corners through doorways well. Size each enclosed space on its own.
- Portable = window in specs. Portables lose real capacity through the exhaust hose; expect performance a size class under their label.
For a fixed installation, treat this result as the shortlist and let a contractor run a Manual J load calculation before buying. Sizing a ductless system instead? The mini-split calculator rounds to head sizes, and the BTU converter translates between BTU, kW and tons. Before buying capacity, it is worth checking the attic: the blown-in insulation calculator works out how many inches you are short of the code R-value, and the cheapest BTU is the one you never have to produce.
Frequently asked questions
How many BTUs do I need to cool a room?
As a rule of thumb, plan on about 20–25 BTUs per square foot of room area for cooling, then adjust for ceiling height, sun, climate, occupants and any kitchen heat. A 300 sq ft room needs roughly 6,000–7,500 BTUs. The calculator above applies these adjustments and rounds to a standard unit size.
What size air conditioner do I need for a 12×12 room?
A 12×12 room is 144 square feet, which needs about 5,000–6,000 BTUs to cool under average conditions. A sunny room, high ceilings or a hot climate push it toward the higher end. Enter your details above for a tailored size.
How many BTUs are in a ton of cooling?
One ton of air-conditioning equals 12,000 BTUs per hour. So a 24,000 BTU system is 2 tons, and 36,000 BTUs is 3 tons. The calculator shows both the BTU figure and the equivalent in tons.
Is it bad to oversize an air conditioner?
Yes. An oversized AC cools the air quickly but short-cycles — turning on and off before it removes humidity — which leaves the room cold and clammy and wears the equipment. Right-sizing matters more than simply buying the biggest unit, which is why the adjustments in this calculator are worth getting right.
How many square feet does a 12,000 BTU air conditioner cool?
About 500-550 square feet under average conditions, using the ~22 BTU per square foot rule this calculator runs on. A very sunny room, high ceilings or a hot climate pull that down toward 450 sq ft. As quick reference points: 5,000 BTU covers about 230 sq ft, 8,000 about 360, 10,000 about 450, and 18,000 about 800 - always under the same average-conditions caveat.
How many BTUs do I need to cool a 20x20 room?
A 20x20 room is 400 square feet. At the ~22 BTU per square foot baseline that is an 8,800 BTU/hr load, so the next standard size up - a 10,000 BTU unit - is the usual answer. With standard 8 ft ceilings, average sun and a moderate climate some rooms squeak by with 8,000 BTU; make it sunny or hot and 10,000 is the safe call. Enter the exact conditions above to see which side your room falls on.