ComfortCalc

HVAC Sizing Calculator

A quick rule-of-thumb estimate — a real HVAC installer uses a full Manual J load calculation for precision.

Home area (sq ft)

BTU per sq ft (climate factor)

System size

36,000 BTU/hr

20 BTU/sq ft is a moderate-climate baseline — hotter climates and poorly insulated homes need more, cooler climates need less. A professional load calculation (Manual J) is more precise.

Tons of cooling: 3

What you entered

Home area (sq ft): 1800BTU per sq ft (climate factor): 20

1
Apply the BTU-per-square-foot rule of thumb
1800 × 20= 36000 BTU/hr
2
Convert to tons of cooling (1 ton = 12,000 BTU/hr)
36000 ÷ 12,000= 3 tons

Result

System size: 36,000 BTU/hr

A 1800 sq ft home needs a roughly 36000 BTU/hr system, or about 3 tons.

HVAC capacity by floor area

A rule-of-thumb sizing at BTU per square foot. One ton of cooling is 12,000 BTU per hour.

Floor area	BTU/sq ft	Capacity (BTU/hr)	Tons
500 sq ft	20	10,000	0.83
800 sq ft	20	16,000	1.33
1,000 sq ft	20	20,000	1.67
1,500 sq ft	20	30,000	2.50
2,000 sq ft	20	40,000	3.33
2,500 sq ft	20	50,000	4.17
1,000 sq ft	25	25,000	2.08
1,000 sq ft	30	30,000	2.50

20 BTU per square foot suits a reasonably insulated home in a temperate climate; hot climates, poor insulation, and lots of glazing push it to 25 or 30. This is a sanity check, not a design. Oversizing is the common mistake: an oversized unit short-cycles, never runs long enough to remove humidity, and leaves the house cold and clammy. A proper Manual J load calculation is what an installer should do.

This is a rule of thumb, not a load calculation

Actual HVAC sizing (a 'Manual J' calculation) accounts for insulation quality, window count and orientation, ceiling height, local climate data, and more — this calculator's BTU-per-square-foot shortcut is useful for a ballpark estimate, not for finalizing an install.

Oversizing is a real problem

A bigger system isn't simply 'safer' — an oversized AC unit cycles on and off too quickly, cooling the air but not properly removing humidity, leading to a clammy feeling despite a comfortable temperature. Right-sizing matters as much as raw capacity.

Frequently asked questions

My house is 2,000 sq ft — what size AC unit do I need?

Roughly 2.5-3.5 tons (30,000-42,000 BTU/hr), depending on your climate, insulation quality, and window exposure. Hot, humid climates need the higher end; mild climates and well-insulated homes need less. This is a rule-of-thumb estimate — a proper Manual J load calculation gives a precise answer.

What's the difference between BTU and tons for HVAC?

1 ton of cooling = 12,000 BTU/hr. A '3-ton' AC unit produces 36,000 BTU/hr of cooling capacity. The term comes from the amount of heat needed to melt one ton of ice in 24 hours.

Why is a bigger AC unit not always better?

An oversized unit cools the air quickly but shuts off before properly dehumidifying. The result is a cold but clammy house, plus more wear from frequent on/off cycling. Proper sizing removes humidity and temperature together.

How does this differ from the BTU calculator?

This HVAC calculator sizes a whole-home central system from total square footage. The BTU calculator sizes a single room unit (window AC, portable, or space heater) from individual room dimensions.