Heating BTU Calculator: Furnace & Heater Size

Rule of thumb

Heating BTU per square foot by climate zone

Output needed with average insulation and 8 ft ceilings. Use the calculator above to adjust for your insulation and ceiling height.

Zone	Example cities	BTU / sq ft	500 sq ft	1,000 sq ft	2,000 sq ft
Zone 1 · Hot	Miami, Houston, Phoenix	30–35	16,250	32,500	65,000
Zone 2 · Warm	Los Angeles, Dallas, Atlanta	35–40	18,750	37,500	75,000
Zone 3 · Mild	San Francisco, Charlotte, Oklahoma City	40–45	21,250	42,500	85,000
Zone 4 · Cool	New York, Chicago, Denver	45–50	23,750	47,500	95,000
Zone 5 · Cold	Minneapolis, Buffalo, Fargo	50–60	27,500	55,000	110,000

Totals use the middle of each range. These are contractor rules of thumb for a first estimate, not a substitute for a Manual J load calculation.

How the heating estimate works

Heat loss grows with floor area and with how cold it gets outside. The quick method HVAC contractors use is a number of BTU per square foot for your climate, then a correction for how well the house holds heat.

Climate zone: from about 30 BTU per sq ft in hot climates to 60 in cold ones.
Insulation: about 25% more for a drafty older home, about 15% less for a new, tight one.
Ceiling height: taller rooms hold more air, so we scale the result above 8 ft.

Furnaces: input vs output

Furnace labels show input BTU, the fuel burned. What heats your home is the output: input multiplied by the efficiency rating (AFUE). A 60,000 BTU furnace at 80% AFUE delivers 48,000 BTU per hour; at 96% it delivers 57,600. The calculator shows the input you would need from a 95% furnace.

Space heaters: watts to BTU

Every watt of electric heat gives 3.412 BTU per hour. A standard 1,500 W plug-in heater puts out about 5,118 BTU per hour. If your room needs 10,000 BTU, one heater will not keep up.

Heat pumps

Heat pumps are sized in tons like air conditioners: one ton is 12,000 BTU per hour. In cold climates, check the unit's capacity at 5°F, not only its rated capacity at 47°F, because output drops as it gets colder.

Need to cool the same room? Switch the calculator to Cooling, or use the main BTU calculator with the ENERGY STAR air conditioner chart.

FAQ

Heating BTU questions

How many BTU do I need to heat a room?

Multiply the floor area by the BTU per square foot for your climate: about 30 to 35 in hot climates up to 50 to 60 in cold ones. A 12 x 15 ft bedroom (180 sq ft) in a cool climate like Chicago needs roughly 180 x 47.5 = 8,550 BTU per hour with average insulation.

What size furnace do I need for 1,500 square feet?

With the rule of thumb and average insulation, about 48,750 BTU per hour of output in a hot climate, 71,250 in a cool climate and 82,500 in a cold one. Furnaces are labeled by input, so divide the output you need by the efficiency (AFUE): 71,250 / 0.95 is about 75,000 BTU input. Confirm with a Manual J load calculation before buying.

How many square feet will a 1,500 watt space heater heat?

A 1,500 W heater puts out about 5,118 BTU per hour. That covers roughly 150 sq ft in a hot climate and about 100 sq ft in a cold one with average insulation. That is why one plug-in heater rarely keeps a large room warm.

What is the difference between furnace input and output BTU?

Input is the fuel the furnace burns; output is the heat that reaches your home. A 60,000 BTU furnace at 80% AFUE delivers 48,000 BTU per hour; the same input at 96% AFUE delivers 57,600.

Does insulation change how many BTU I need?

Yes. A drafty older home can need about 25% more heat than average, and a new, well-insulated home about 15% less. Choose the insulation level in the calculator to apply it.

Is this a Manual J calculation?

No. It is a fast estimate. A Manual J load calculation by an HVAC professional looks at windows, walls, air leakage and ductwork, and it is the right way to size a furnace or heat pump for a whole house.