How many BTU per square foot do you need?

Last updated: October 7, 2026

This calculator uses 25 BTU/hr per square foot, then ceiling, sun, insulation, and windows.

The rule, and why it's a range

Per-square-foot rules are a planning tool, not a Manual J replacement. They work because the error bars are known and adjustable — which is exactly what the next section does.

The adjustments

Finish with a 10% safety margin and round up to the nearest 500 BTU. That margin is already the safety buffer — adding "one more size up" on top of it is the oversizing trap in action.

The percentages, the +600, and the 10 percent margin are the factors in the table above.

Square footage alone does not settle the result. The 1,000 sq ft page walks the same factors. A room air conditioner on the ENERGY STAR chart is a different table: 400 sq ft is one example.

Heating uses different math

The cooling factors above are not a heating load. Electric baseboard on this site is watts = BTU/hr ÷ 3.41, on the baseboard watts guide. A heat pump still needs a heating load beside the cooling load: heat pump sizing.

Where this calculator is the wrong tool

• Kitchens and gyms: internal heat is not an input in this calculator.
• Extreme climates and unusual envelopes: use a Manual J.
• Multi-room zoned systems: see the mini-split guide.

As an Amazon Associate I earn from qualifying purchases. The Amazon links below are affiliate links; they don't change the math.

BTU vs tons vs watts: the units, translated

DOE’s heating and cooling guide defines one ton of cooling as 12,000 Btu. DOE’s forced-air guide converts resistance heat as Btu/h ÷ 3,412 = kW. This calculator’s baseboard function divides BTU/hr by 3.41. Dividing a cooling tonnage by either constant is not a furnace size. The baseboard page is that conversion.

Ceiling height deserves its own warning

This calculator multiplies by ceiling height ÷ 8. A 16 ft ceiling is 16 ÷ 8 = 2, so 500 sq ft × 25 × 2 = 25,000 BTU/hr before windows, sun, insulation, and the 10% margin. The 8 ft figure for the same 500 sq ft is 500 × 25 = 12,500 BTU/hr before those same steps. Measure the ceiling and use the multiplier.

Worked example: the same factors at five sizes

Here is the calculator run at five floor areas with an 8 ft ceiling, average sun, average insulation, and a window count that grows with the space. The last two columns are ENERGY STAR's room air conditioner chart for the same area, divided back into BTU/hr per sq ft. Each result links to that example in the calculator.

The calculator's per-square-foot figure stays between about 32 and 35 in these examples, because 25 BTU/hr per sq ft is a constant and the window count and 10% margin grow with the space. ENERGY STAR's room chart does not stay constant. It works out to 30 BTU/hr per sq ft at 200 sq ft and falls to 16 to 20 at 1,500 sq ft. So no single per-square-foot number describes the chart. For whole houses, DOE describes one ton (12,000 Btu) per 400 to 500 sq ft as an old estimate, which is 24 to 30 BTU/hr per sq ft. DOE also says that in a very efficient house one ton can condition 800 to 1,000 sq ft, which is 12 to 15 BTU/hr per sq ft. This calculator sits above the old estimate because it adds windows and a margin, so treat its result as a planning ceiling, not a purchase size.

How each factor changes 25 BTU/hr per sq ft

Windows are not a rate. Each one is a flat 600 BTU/hr, so a window matters more in a small room: two windows add 6 BTU/hr per sq ft to a 200 sq ft room and less than 1 to a 1,500 sq ft space. ENERGY STAR's occupant and kitchen adjustments work the same way, as fixed amounts. 600 BTU/hr per person beyond two is 3 BTU/hr per sq ft at 200 sq ft. The 4,000 BTU/hr kitchen adder is 20 BTU/hr per sq ft at 200 sq ft and about 2.7 at 1,500 sq ft. This calculator has no occupant or cooking input, so add those yourself when they apply.

Climate is not an input here or on the ENERGY STAR chart. DOE says the ton-per-area estimate "does not take into account local climate and how well a house is sealed and insulated" (DOE guide). Manual J takes climate from outdoor design conditions in Manual J Tables 1A/1B or ASHRAE weather data (ACCA). DOE adds that correct sizing matters most in humid climates, where short-cycling leads to poor humidity control (DOE fact sheet).

Common mistakes with BTU per square foot

• Treating 25 as a standard. It is this calculator's constant. The nearest published figure is DOE's old one ton per 400 to 500 sq ft estimate, and DOE says that estimate ignores climate and how well the house is sealed and insulated.
• Using one rate at every size. ENERGY STAR's room chart is 30 BTU/hr per sq ft at 200 sq ft and 16 to 20 at 1,500 sq ft. A rate that fits a bedroom oversizes a large open space.
• Applying a rate to a kitchen. Cooking heat is not in the rate. ENERGY STAR adds a flat 4,000 BTU/hr for a unit used in a kitchen.
• Ignoring a vaulted ceiling. This calculator multiplies by height ÷ 8, so a 12 ft ceiling is × 1.5. ENERGY STAR's chart assumes 8-foot ceilings and says to consider a unit with a higher CADR above that.
• Sizing central equipment from any per-square-foot rule. DOE says to insist on documented ACCA Manual J calculations and to select equipment with Manual S (DOE fact sheet).

Sizes worked out on this site

• 150 sq ft room
• 200 sq ft room
• 250 sq ft room
• 300 sq ft room
• 400 sq ft room
• 500 sq ft room
• 600 sq ft
• 700 sq ft
• 800 sq ft
• 1,000 sq ft
• 1,200 sq ft
• 1,500 sq ft
• 2,000 sq ft
• Dehumidifier for 2,000 sq ft
• HVAC load calculator

Run your exact rooms through the HVAC load calculator — every factor above is a live input.

The unit itself is what BTU stands for. The refrigeration loop is how an air conditioner works. Resistance heat in kilowatts, which is not this cooling rule, is BTU to kW.

Smaller room? See what size air conditioner for a 10x10 room.