BTU Calculator
Estimate how many BTU/h your room needs for cooling, based on floor area with documented adjustments for climate, insulation, sun, people and appliances.
Open calculatorLast updated: 2026-09-15
How the calculation works
- The baseline is 25 BTU/h per square foot — a widely used rule of thumb for rooms with standard 8 ft ceilings.
- Multipliers adjust for hot or cool climates, insulation quality and sun exposure, each ±10–15%.
- 600 BTU/h is added per person (sensible + latent) and 3.412 BTU/h per watt of equipment, since all appliance power ends up as heat.
Worked example
Interpreting the result
Round up to the nearest standard equipment size, but do not grossly oversize: an oversized AC short-cycles, dehumidifies poorly and wears out faster. If the result sits between sizes, prefer the smaller one in humid climates. The CFM figure (~400 per ton) is the airflow the equipment should deliver.
Assumptions
- Standard 8 ft (2.4 m) ceilings — add ~10% per extra foot of ceiling height.
- Typical residential envelope; commercial spaces with glass curtain walls need real load calculations.
- The 25 BTU/h·ft² baseline assumes reasonably airtight construction.
Limitations
- A rule-of-thumb estimate — not a substitute for ACCA Manual J for equipment selection.
- Does not model duct losses, orientation per wall, window areas individually or internal latent loads like cooking.
- Kitchens with heavy cooking loads need dedicated ventilation sizing.
Why oversizing is worse than undersizing
An oversized AC satisfies the thermostat before it has run long enough to dehumidify — the room reaches temperature while still feeling clammy, and the short cycling wears the compressor and increases demand peaks. A slightly undersized unit runs longer cycles: it dehumidifies properly, holds steadier comfort and typically outlives the oversized neighbor. That asymmetry is why professional sizing errs toward the smaller end when a result falls between two standard sizes, and why multiplying every margin conservatively in this calculator's inputs is the wrong instinct.
When a rule of thumb is not enough
- Rooms with large west- or south-facing glass, vaulted ceilings or kitchens need a room-by-room Manual J calculation — the screening model here averages what those loads separate.
- Ducted systems add duct losses the room estimate doesn't include; the duct sizing tool gives the airflow side of that problem.
- Mini-split indoor units come in fixed sizes (9k, 12k, 18k BTU/h); pick the nearest size below the result, not above.
Frequently asked questions
How many BTU do I need per square foot?
About 20–30 BTU/h per ft² for typical residential rooms — this calculator uses 25 as the baseline and adjusts from there. Hot climates, poor insulation and sunny rooms push it toward 30+.
How many BTU is a ton of cooling?
One ton of refrigeration = 12,000 BTU/h. A 2-ton system delivers 24,000 BTU/h.
Is bigger better for AC?
No. Oversized ACs cool the air quickly but shut off before dehumidifying it, leaving rooms cold-clammy. Slightly undersized runs longer, dehumidifies better and maintains steadier comfort.