Enter conditioned floor area and an assumed cooling-load intensity in Btu/h per square foot. The options 20, 25, 30 and 35 are scenarios, not official climate zones or surveyed requirements. This multiplies a stated intensity by area and converts Btu/h to refrigeration tons.
It does not calculate an actual building load.
Rules of thumb can be off by a ton or more, in either direction. Newer, tight, well-insulated homes need far less capacity than square footage suggests. Sizing must also consider the ductwork, since duct losses reduce delivered capacity.
Heating loads are separate from cooling loads.
Estimated cooling load (BTU/hr) from conditioned square footage
| Conditioned square footage | 1,800 |
|---|---|
| Assumed load intensity | Assumed 25 Btu/h per sq ft |
| Estimated cooling load (BTU/hr) | 45,000 |
| Estimated tonnage | 3.8 |
Common error: Sizing up to be safe. An oversized system short-cycles, leaves the air clammy and wears out faster than a right-sized one.
HVAC BTU/Tonnage Rough Sizing: formula
HVAC BTU/Tonnage Rough Sizing: changing Conditioned square footage
| Conditioned square footage | Estimated cooling load (BTU/hr) | Estimated tonnage |
|---|---|---|
| 1,440 | 36,000 | 3 |
| 1,620 | 40,500 | 3.4 |
| 1,800 (example) | 45,000 | 3.8 |
| 1,980 | 49,500 | 4.1 |
| 2,160 | 54,000 | 4.5 |
HVAC BTU/Tonnage Rough Sizing: compare input ranges
| Input | Tested from | To | Estimated cooling load (BTU/hr): low input | Estimated cooling load (BTU/hr): high input | Swing |
|---|---|---|---|---|---|
| Assumed load intensity | Assumed 20 Btu/h per sq ft | Assumed 35 Btu/h per sq ft | 36,000 | 63,000 | 27,000 (60%) |
| Conditioned square footage | 1,620 | 1,980 | 40,500 | 49,500 | 9,000 (20%) |
HVAC BTU/Tonnage Rough Sizing: individual input comparisons
| Input | Value | Estimated cooling load (BTU/hr) | Estimated tonnage |
|---|---|---|---|
| Conditioned square footage | 1,440 | 36,000 | 3 |
| 1,620 | 40,500 | 3.4 | |
| 1,800 | 45,000 | 3.8 | |
| 1,980 | 49,500 | 4.1 | |
| 2,160 | 54,000 | 4.5 | |
| Assumed load intensity | Assumed 20 Btu/h per sq ft | 36,000 | 3 |
| Assumed 25 Btu/h per sq ft | 45,000 | 3.8 | |
| Assumed 30 Btu/h per sq ft | 54,000 | 4.5 | |
| Assumed 35 Btu/h per sq ft | 63,000 | 5.3 |
HVAC BTU/Tonnage Rough Sizing: values and definitions
| Quantity and definition | Value used |
|---|---|
| Conditioned square footage — Total conditioned floor area, in square feet. | 1,800 |
| Assumed load intensity — Rough BTU-per-square-foot cooling factor for your climate zone. | Assumed 25 Btu/h per sq ft |
| Estimated cooling load (BTU/hr) | 45,000 |
| Estimated tonnage | 3.8 |
The tonnage rule turns floor area into a rough cooling load, and it is a starting point, not a design. Multiplying 1,800 square feet by a factor of 25 gives about 45,000 BTU per hour, or 3.75 tons at 12,000 BTU per ton. Real loads depend on insulation, windows, sun exposure, air leakage and climate, and two houses of the same size can differ by a ton or more.
Oversized equipment cycles on and off, which cools poorly and removes less moisture.
Ask a contractor for a full load calculation, commonly a Manual J, before you buy equipment. The rule of thumb is useful to see whether a quote is in the right range.
Estimate savings from a better unit on the SEER upgrade calculator, size ducts on the duct sizing page and check airflow on the CFM page. The static pressure calculator checks the system's health, and the degree days calculator shows local climate load.
Are these official climate-zone factors?
No. They are editable assumed load intensities. Climate alone does not determine load per square foot.
Does 3.75 tons establish which system to buy?
No. It is the conversion of the assumed 45,000-Btu/h load. Building heat gain, design conditions and equipment performance require a real load calculation.
What is one refrigeration ton?
It is a cooling-capacity unit equal to 12,000 Btu/h in this conversion; it is not the mass of the equipment.