BTU / HVAC Sizing Calculator
Free BTU and HVAC sizing calculator. Find the right air conditioner or heater capacity for any room from square footage, climate, and insulation level.
Quick answer
Rough rule: 20 BTU per square foot of conditioned space, adjusted for climate, insulation, and sun exposure. A 600 sqft room in a moderate climate needs about 12,000 BTU (1 ton) of cooling. Oversizing wastes energy and creates short-cycling humidity issues.
BTU / HVAC Sizing Calculator
Recommended size
How it works
Starts with the standard 20 BTU/sq ft cooling baseline, then adjusts for climate (±20%), sun exposure (±10%), occupants (+600 BTU each beyond 2), and kitchen heat load (+4,000). Heating uses 25-40 BTU/sq ft depending on climate. 1 ton of AC = 12,000 BTU/hr.
When to use it
Sizing a window AC, a mini-split for a single room, a portable heater, or sanity-checking a contractor's whole-house Manual J load calculation.
Common mistakes
Oversizing AC. A unit that's too big short-cycles, never runs long enough to dehumidify, and leaves the room clammy. For a whole-house system, always use a Manual J load calculation — these rules of thumb are for single rooms only.
How the BTU calculator works
The basic formula is sqft × 20 BTU. The calculator then adjusts for climate (cold climates need 25 BTU/sqft for heating; hot/humid climates need 25 for cooling), insulation quality (poor insulation +20%, good insulation -10%), sun exposure (south/west-facing rooms +10%), ceiling height (over 8 feet, scale up proportionally), and number of occupants (each adds ~600 BTU). The output is total BTU needed plus the equivalent in tons (12,000 BTU = 1 ton) — useful for shopping AC units that quote tons.
When to use it
Sizing a window AC for a single room. Sizing a mini-split for an addition or finished basement. Verifying a contractor's central HVAC bid is appropriately sized (HVAC techs sometimes upsize aggressively). Comparing portable AC units with different BTU ratings.
Common mistakes
- Oversizing 'just to be safe.' An oversized AC cools the air fast but doesn't run long enough to dehumidify. The room feels cool but clammy. Size correctly.
- Confusing heating BTU with cooling BTU. Furnaces are sized by output BTU/hour; AC is sized by cooling BTU/hour. Some climate zones need much more heating than cooling capacity (or vice versa).
- Ignoring duct losses. Central systems lose 10-30% of capacity to leaky ducts in unconditioned spaces. Mini-splits avoid this entirely. The BTU number from this calculator assumes the conditioned space is the only load.
Frequently asked questions
What size AC do I need for a 1,500 sqft house?
A rough estimate is 30,000 BTU (2.5 tons) for moderate climates, 36,000 BTU (3 tons) for hot or humid climates, possibly less for well-insulated homes. Run a full Manual J calculation for accurate sizing — the rule-of-thumb is just a starting point.
Can I size my AC bigger to cool faster?
It cools faster but runs less, which means it doesn't dehumidify properly. The room feels cool but clammy. Oversizing also short-cycles the compressor, shortening equipment life. Size correctly — bigger is not better.
How many BTUs do I need per square foot?
Roughly 20 BTU/sqft for moderate climates, 25 for hot or poorly insulated spaces, 15 for well-insulated cool climates. The calculator above adjusts for those factors automatically.