Question 1

What is BTU/hr?

Accepted Answer

BTU per hour is the rate of heat transfer your HVAC system needs to add or remove. A 12,000 BTU/hr (1-ton) unit moves 12,000 BTU of heat every hour at rated conditions. BTU/hr is the unit equipment is rated and sized in.

Question 2

How many BTU per square foot?

Accepted Answer

For cooling, roughly 35 BTU/sqft in very hot zones (Miami, Phoenix) and 20 BTU/sqft in cool zones (Seattle, Boston). For heating, the relationship inverts — 50+ BTU/sqft in cold zones, 18–22 BTU/sqft in warm ones. This calculator uses zone-specific rates plus envelope adjusters.

Question 3

How many BTU do I need to cool a 1,000 square foot room?

Accepted Answer

In an average climate (zone 4), about 22,000–28,000 BTU/hr of cooling, depending on ceiling height, sun exposure, and insulation. With excellent insulation and shade, the lower end; with vaulted ceilings and west-facing glass, the higher end.

Question 4

How many BTU for a 12x12 room?

Accepted Answer

A 12×12 room is 144 sq ft. At ~25 BTU/sqft in temperate climates with average envelope, that's ~3,600 BTU/hr. Round up to 5,000 BTU/hr for window units (the smallest common size) — most rooms below 200 sq ft fit a 5,000 BTU/hr window AC.

Question 5

Why does sun exposure matter?

Accepted Answer

South- and west-facing rooms with lots of glass can pick up 10–25% more solar heat gain in summer. Solar gain through a single sunny window can rival the conduction load through 50 sqft of insulated wall. The calculator multiplies the cooling base load by a sun factor to account for this.

Question 6

Do I add BTU for occupants?

Accepted Answer

Yes — for cooling. Each person sensible-radiates ~230 BTU/hr at rest and exhales ~200 BTU/hr in latent (humidity) load. A family of 4 adds ~1,700 BTU/hr to the cooling load. Heating loads ignore occupants because people offset some of the heating need rather than adding to it.

Question 7

Does BTU change with ceiling height?

Accepted Answer

Yes — taller ceilings mean more air volume to condition. The calculator multiplies the base load by ceiling/8. A 10-foot ceiling adds ~25%, a 12-foot ceiling adds ~50%. Vaulted ceilings often need explicit volume calculation rather than rule of thumb.

Question 8

What's the difference between cooling BTU and heating BTU?

Accepted Answer

Cooling BTU is the heat removed per hour at the indoor design temperature (typically 75°F) and outdoor design temperature (typically 90–95°F). Heating BTU is the heat added per hour at indoor 70°F vs outdoor design (typically 5–25°F depending on zone). Cold climates have larger heating loads; hot climates larger cooling loads.

Space	Typical area	Cooling BTU/hr	Notes
Bedroom	120–180 sq ft	5,000–6,000	Low occupancy, low gain
Living room	300–500 sq ft	8,000–14,000	Higher occupancy + glass
Kitchen	150–250 sq ft	8,000–14,000	Cooking adds ~4,000
Master suite	300–400 sq ft	9,000–12,000	Includes bath
Whole apartment	600–900 sq ft	18,000–24,000	1.5–2 tons
Whole house (mid)	1,500–2,000 sq ft	30,000–48,000	2.5–4 tons
Whole house (large)	2,500–3,500 sq ft	48,000–60,000	4–5 tons

BTU Calculator

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Where your BTU/hr comes from

Where the BTU/hr comes from

The BTU/hr formula

BTU/hr by room and space type

When the BTU rule of thumb breaks down

BTU FAQ