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Blueprint Calc

BTU / AC Sizing Calculator

How many BTUs do I need? Size an air conditioner or heater for any room or whole house. Accounts for climate, insulation, occupancy, and solar exposure — a rule-of-thumb good to within ±20% of a Manual J calculation.

How many BTUs do I need?

Default: a 500 sq ft room in climate zone 4 with average insulation and 2 occupants.

You need
  • BTU capacity needed
    12,500 BTU/hr
    500 sq ft × 25 BTU/sq ft × 1 construction factor + occupancy and solar adjustments
  • Equivalent tons
    1.04 tons
    1 ton of cooling = 12,000 BTU/hr
  • Recommended AC unit size
    15,000 BTU
    Closest standard unit ≥ calculated BTU needs. Oversizing causes short-cycling — avoid going bigger.

This is a rule-of-thumb estimate good to ±20%. For a new whole-house system, have an HVAC contractor run a Manual J load calculation — it accounts for window orientation, duct losses, and infiltration rates that this simple formula does not.

How we calculated it

This calculator uses a climate × insulation × occupancy multiplier model that approximates Manual J (the ACCA residential load calculation) within ±20% for standard rooms:

BTU/hr = sqft × climate_btu × construction_factor + occupancy_add + kitchen_add

Climate factors range from 17 BTU/sq ft in deep-northern Alaska to 30 BTU/sq ft in south Florida. Construction factor multiplies from 0.75× (Energy Star spray-foam new build) to 1.3× (pre-1980 single-pane home with minimal insulation).

Limitations: this is a room-by-room or single-zone rule of thumb. For whole-home central AC sizing, duct leakage and orientation matter enormously and only a Manual J can capture them. Above ~1,500 sq ft, treat this calculator as a sanity check on contractor bids, not a design tool.

Frequently asked questions

How many BTUs do I need for a room?
Rule of thumb: 20 BTU per sq ft for a typical insulated room in the mid-US. Hot climates need 25-30 BTU/sq ft, cold climates 15-20. A 500 sq ft room needs roughly 10,000-15,000 BTU of cooling capacity.
How big an AC unit do I need for my house?
Whole-home central air: roughly 1 ton (12,000 BTU) per 500-700 sq ft in moderate climates. A 2,000 sq ft house typically needs 3-4 tons (36,000-48,000 BTU). Oversizing causes short-cycling and high humidity — undersizing can't keep up on hot days.
What is a ton of air conditioning?
One ton of cooling = 12,000 BTU/hr. The term comes from the energy needed to melt one ton of ice in 24 hours. Residential AC units are sold in half-ton increments: 1.5, 2, 2.5, 3, 3.5, 4, 5 tons.
What is Manual J and do I need one?
Manual J is the ACCA (Air Conditioning Contractors of America) standard load calculation — the proper way to size residential HVAC. It accounts for window orientation, duct losses, infiltration, and specific construction details. For a new whole-home system, yes — require your contractor to provide a Manual J. For a single-room mini-split, a rule-of-thumb BTU estimate is usually fine.
Why does oversizing AC make it worse?
An oversized AC cools the air quickly but doesn't run long enough to pull humidity out. Result: cold, clammy rooms that feel uncomfortable at 72°F. It also cycles on and off 2-3× more often, wearing out the compressor years earlier.
Do I add BTUs for kitchens?
Yes, add ~1,200 BTU if the room is a kitchen — oven + cooktop + dishwasher + refrigerator all dump heat into the room. If you're also cooking regularly for a family, some HVAC guides say add 4,000 BTU.
How do sun exposure and shade affect BTU needs?
South- and west-facing rooms with lots of windows add 10-20% to cooling load. Heavily shaded rooms subtract 10%. Sun-bright sunrooms may need 30%+ more than the square-footage rule suggests.
BTU vs SEER vs kW — what do they mean?
BTU/hr is the capacity (how much heat it moves). SEER is the efficiency (capacity ÷ electrical energy over a season). A higher SEER (say 18+) uses less electricity for the same cooling. kW is electrical power draw — useful for sizing generators or solar. A 3-ton (36,000 BTU) 16-SEER unit draws roughly 2.3 kW at full load.
Can I use a portable or window AC instead of central?
For single rooms up to 400 sq ft, window units ($200-400) are cheaper and fine. Above ~500 sq ft per zone, central air or mini-split systems are more efficient and quieter. Portable ACs are the weakest option — they're loud, use the air they're cooling as their own exhaust, and rarely hit rated capacity.
Should I upgrade insulation before replacing HVAC?
Yes, if your insulation is poor. Dumping $8,000 into a bigger AC to compensate for a leaky envelope is the worst possible use of that budget. Insulate, air-seal, and weatherstrip first — then size the HVAC to the newly efficient house. This often drops the required tonnage by a full ton, saving the upgrade cost.

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See also

Related terms

Plain-English definitions for the terms used in this calculator.

R-value
A measure of thermal resistance — higher R = better insulation. Fiberglass batt R-13 for 2x4 walls, R-19 for 2x6; blown cellulose R-19 to R-60 for attics. Building code minimums vary by climate zone.
linear foot vs square foot vs cubic foot
Linear foot = 12 inches in a single dimension (trim, pipe, lumber length). Square foot = a 12×12" area (tile, flooring, drywall, paint). Cubic foot = a 12×12×12" volume (mulch, gravel, concrete).