CalcMyHVAC

How many BTU do you need?

Size the cooling and heating for a room or a whole house. Add your ZIP code and the numbers adjust to your climate as you type.

Size of the space
sq ft
Location

Enter a ZIP code and we set the climate zone for you.

The space itself
ft

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How the BTU calculator works

BTU (British Thermal Unit) measures how much heat an air conditioner removes, or a heater adds, in an hour. The calculator multiplies your floor area by a BTU-per-square-foot figure for your climate zone, then adjusts for the things that change the load the most:

  • Climate zone. Hot southern homes need more cooling and less heating than northern ones. Your ZIP code sets this.
  • Ceiling height. The base figures assume 8-foot ceilings. Taller rooms hold more air, so the result scales up.
  • Insulation. A drafty older home needs about 15% more; a well-sealed newer one about 10% less.
  • Sun. A very sunny room needs about 10% more cooling; a shaded one about 10% less.
  • People and kitchens. Each person beyond two adds 600 BTU, and a kitchen adds 4,000 BTU of cooling.

Cooling BTU = sq ft × zone factor × (ceiling ÷ 8) × insulation × sun + people + kitchen

Every factor is listed on how we calculate.

BTU chart by square footage

Cooling BTU for an average home with 8-foot ceilings.

AreaHot climateMild climateCold climate
150 sq ft4,5003,7503,000
250 sq ft7,5006,2505,000
350 sq ft10,5008,7507,000
450 sq ft13,50011,2509,000
550 sq ft16,50013,75011,000
700 sq ft21,00017,50014,000
1,000 sq ft30,00025,00020,000
1,200 sq ft36,00030,00024,000
1,500 sq ft45,00037,50030,000
2,000 sq ft60,00050,00040,000
2,500 sq ft75,00062,50050,000

Converting BTU to tons

Central air conditioners are sold in tons. One ton equals 12,000 BTU per hour, so divide your cooling BTU by 12,000. Common sizes are 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons. For whole-home sizing, use the AC size calculator.

Common questions

How many BTU do I need per square foot?

For cooling, most US homes need about 20 to 30 BTU per square foot: around 20 in cold northern climates and up to 30 in hot southern ones. For heating, plan on roughly 30 to 60 BTU per square foot, with colder climates at the high end.

How many BTU is a ton of air conditioning?

One ton of cooling equals 12,000 BTU per hour. A 3-ton central air conditioner removes 36,000 BTU of heat per hour.

Is a bigger air conditioner better?

No. An oversized unit cools the air quickly and shuts off before it removes humidity, so the home feels clammy, and the frequent on-off cycling wears the system out faster. Aim for the size that matches your load.

How accurate is this BTU calculator?

It uses the same rule-of-thumb method HVAC pros use for quick estimates, adjusted for climate, ceiling height, insulation and sun. It is a good starting point. Before buying central equipment, ask a contractor for a Manual J load calculation.

How many BTU do I need for a 1,500 sq ft house?

In a mild climate, about 37,500 BTU of cooling, which is a 3-ton air conditioner. In a hot climate it rises to about 45,000 BTU (3.5 to 4 tons), and in a cold northern climate it drops to about 30,000 BTU (2.5 tons).