AC BTU SIZING
About this calculator
This BTU calculator gives a quick rule-of-thumb cooling load for sizing an air conditioner. The base rule is 20 BTU per square foot of room area, adjusted for sun exposure (±10%) and adding 600 BTU per occupant above two. For tight specs in commercial or unusual residential spaces, an HVAC contractor should run a Manual J load calculation. 12,000 BTU/hr equals one ton of cooling. For a proper sizing instead of a rule-of-thumb estimate, do a load calc per ACCA Manual J.
How to use this calculator
Enter the room's square footage (length × width). Pick the sun exposure: heavily shaded rooms (north-facing, blocked by trees, lower floor) need 10% less cooling; very sunny rooms (south- or west-facing, large windows, top floor) need 10% more. Enter the typical number of occupants — every person above two adds 600 BTU/hr to the cooling load.
The result is the BTU/hr cooling capacity needed plus the equivalent in tons (1 ton = 12,000 BTU) and a recommended unit size. Use this for window units, mini-splits, or sizing portable AC. For whole-house central air, get a Manual J load calculation from an HVAC contractor — the rule-of-thumb method is fine for individual rooms but undersizes whole houses by 15–30%.
Worked example
For a 200 ft² bedroom with normal sun and 2 occupants:
Base load = 200 × 20 = 4,000 BTU/hr. Sun adjustment: none. Occupant load: 0 (only counts above 2). Total = 4,000 BTU/hr = 0.33 tons.
Suggested unit: 5,000 BTU window unit (next size up; window units come in 5K, 8K, 10K, 12K, 15K BTU sizes).
For a 350 ft² south-facing living room with 4 occupants:
Base = 350 × 20 = 7,000. Sunny adjustment: × 1.10 = 7,700. Occupant load: 2 × 600 = 1,200. Total = 8,900 BTU/hr = 0.74 tons.
Suggested unit: 10,000 BTU window unit, or a 1-ton mini-split for quieter operation. A 5,000 BTU unit would run continuously and never cool the room.
Common mistakes & waste factors
Oversizing. Bigger isn't better — an oversized AC short-cycles, never running long enough to dehumidify the room. The result is a cool but clammy space. Right-size it.
Undersizing for sun. A south- or west-facing room with floor-to-ceiling windows can need 30–50% more BTU than the rule-of-thumb suggests. Bump sun exposure to "sunny" and add another 1,000 BTU as a buffer.
Forgetting the kitchen. Kitchens generate 4,000+ BTU of waste heat from cooking, refrigeration, and dishwashing. Add 4,000 BTU to a kitchen's calculated load.
Ignoring duct losses. For ducted central AC, ducts that run through unconditioned attics or crawlspaces lose 20–40% of the cool air. The rule-of-thumb method assumes ductless or short ducts.
Rules of thumb
Base: 20 BTU/hr per ft². 12,000 BTU/hr = 1 ton of cooling.
Sun: ±10% based on heavy shade vs heavy sun. Top-floor west-facing rooms can need +20%.
Occupants: +600 BTU/hr per person above 2.
Kitchens: add 4,000 BTU for cooking equipment.
Whole-house systems: rule of thumb only as a sanity check. Get a Manual J for accurate sizing — costs $200–$500 and prevents oversized systems that short-cycle.