HVAC Tonnage & BTU Calculator (AC Size by Square Foot)
Enter the square footage and pick a climate zone to get a quick BTU/hr and tonnage estimate for sizing central air conditioning. This is a rule-of-thumb starting point — the math updates as you type.
HVAC Tonnage Calculator
Pick a climate zone and enter square footage. 1 ton of cooling = 12,000 BTU/hr.
This is a rule-of-thumb square-footage estimate, not a substitute for a Manual J load calculation, which accounts for insulation, window area/orientation, ceiling height, ductwork, and local climate data. Have an HVAC contractor run a Manual J before buying equipment.
How the tonnage estimate works
Central air is rated in tons, where 1 ton removes 12,000 BTU of heat per hour. A quick square-footage estimate multiplies your home’s area by a BTU-per-square-foot factor for your climate, then divides by 12,000 and rounds to the nearest standard half-ton size.
| Step | Example: 1,500 sq ft, moderate climate |
|---|---|
| BTU/hr (sq ft × factor) | 1,500 × 20 = 30,000 BTU/hr |
| Tons (BTU ÷ 12,000) | 30,000 ÷ 12,000 = 2.50 tons |
| Rounded to nearest 0.5 ton | 2.5‑ton unit |
Climate factor guide: 18 BTU/sq ft for cool northern climates, 20 for moderate, 22 for warm, and 25 for hot southern/desert climates with heavy sun load.
Contractor Estimate Calculator (Excel & Google Sheets)
Turn the tonnage estimate into a priced, itemized proposal — equipment, refrigerant line set, labor, and markup, auto-totaled. The estimate you hand the client after the sizing calculator tells you the unit size.
Why square footage alone isn’t enough
Two homes with identical square footage can need very different equipment. Insulation quality, window area and orientation, ceiling height, duct condition, and the number of occupants all change the real load. A square-footage estimate is a fast starting point for budgeting; a Manual J calculation is the industry-standard method that gets the size right and avoids an oversized unit that short-cycles or an undersized one that never keeps up.
Once the unit is sized, send a clean invoice
The calculator gives you a starting tonnage; your bid turns it into a price. When the job is done, bill it with an itemized HVAC invoice that separates equipment, refrigerant, and labor — and quote new installs from an HVAC estimate template so your margin is in the number before the truck rolls.
Frequently asked questions
Is square-footage sizing accurate enough to buy equipment?
No. It is a useful rule-of-thumb for budgeting and ballpark quotes, but a Manual J load calculation is the correct method for actually sizing and purchasing equipment, since it accounts for insulation, windows, ductwork, and orientation.
What happens if the AC is oversized?
An oversized unit cools the air quickly but shuts off before it properly removes humidity, leading to a clammy feel, short-cycling, and extra wear on the compressor.
How many BTU per square foot should I use?
Roughly 18 for cool climates, 20 for moderate, 22 for warm, and 25 for hot, high-sun-load climates. These are rule-of-thumb figures, not a substitute for a load calculation.
Is this HVAC tonnage calculator free?
Yes — free, no sign-up. Pick a climate zone, enter square footage, and read the estimated BTU/hr and tonnage. Estimates only; confirm with a Manual J calculation.