BTU to Tonnage Conversion Guide for HVAC Systems
BTU to Tonnage Conversion Formula
The conversion is straightforward:
Tons = BTU/hr ÷ 12,000
One ton of cooling capacity equals 12,000 BTU per hour. This unit comes from the amount of energy needed to melt one ton (2,000 pounds) of ice in 24 hours.
Quick Conversion Table
| BTU/hr | Tonnage | Typical Home Size | Total CFM |
|---|---|---|---|
| 12,000 | 1.0 ton | 400 to 600 sq ft | 400 CFM |
| 18,000 | 1.5 tons | 600 to 1,000 sq ft | 600 CFM |
| 24,000 | 2.0 tons | 1,000 to 1,300 sq ft | 800 CFM |
| 30,000 | 2.5 tons | 1,300 to 1,600 sq ft | 1,000 CFM |
| 36,000 | 3.0 tons | 1,600 to 2,000 sq ft | 1,200 CFM |
| 42,000 | 3.5 tons | 2,000 to 2,400 sq ft | 1,400 CFM |
| 48,000 | 4.0 tons | 2,400 to 2,800 sq ft | 1,600 CFM |
| 60,000 | 5.0 tons | 2,800 to 3,500 sq ft | 2,000 CFM |
Why Tonnage Matters for Duct Sizing
Tonnage determines the total system airflow. Each ton requires approximately 400 CFM of airflow over the evaporator coil. This total CFM must be distributed through the duct system to every room.
CFM = Tons × 400
A 3 ton system needs 1,200 CFM total. If the trunk duct cannot carry 1,200 CFM, the entire system is restricted and underperforms.
Connecting Tonnage to Duct Size
Main Trunk Sizing by Tonnage
| System Size | Total CFM | Minimum Trunk Size |
|---|---|---|
| 1.5 tons | 600 CFM | 12 inch round |
| 2.0 tons | 800 CFM | 14 inch round |
| 2.5 tons | 1,000 CFM | 16 inch round |
| 3.0 tons | 1,200 CFM | 16 inch round |
| 3.5 tons | 1,400 CFM | 18 inch round |
| 4.0 tons | 1,600 CFM | 18 inch round |
| 5.0 tons | 2,000 CFM | 20 inch round |
These are approximate sizes at 700 FPM velocity. Use our HVAC Duct Calculator for exact sizing with your specific friction rate and TEL.
Tonnage to BTU (Reverse Conversion)
BTU/hr = Tons × 12,000
This is useful when you know the equipment tonnage and need to verify the heating/cooling capacity.
Common Misconceptions
”Bigger is better”
A common mistake is installing an oversized system for “extra cooling power.” An oversized system:
- Short cycles (runs for short bursts instead of steady operation)
- Cannot dehumidify effectively because it cools the air too quickly
- Wastes energy with frequent start/stop cycles
- Costs more to purchase and install
”One ton per 500 sq ft is always right”
This rough rule only applies to average homes in moderate climates. Actual sizing depends on insulation, windows, climate zone, and building orientation. A well insulated new home in a moderate climate might need only 1 ton per 800 sq ft, while a poorly insulated older home in a hot climate might need 1 ton per 400 sq ft.
BTU Estimation by Square Footage
For a rough estimate of cooling BTU:
Cooling BTU = Square Feet × 20 to 30
| Home Size | Low Estimate (20 BTU/sqft) | High Estimate (30 BTU/sqft) | Tonnage Range |
|---|---|---|---|
| 1,000 sq ft | 20,000 BTU | 30,000 BTU | 1.7 to 2.5 tons |
| 1,500 sq ft | 30,000 BTU | 45,000 BTU | 2.5 to 3.8 tons |
| 2,000 sq ft | 40,000 BTU | 60,000 BTU | 3.3 to 5.0 tons |
| 2,500 sq ft | 50,000 BTU | 75,000 BTU | 4.2 to 6.3 tons |
| 3,000 sq ft | 60,000 BTU | 90,000 BTU | 5.0 to 7.5 tons |
For accurate sizing, always use a Manual J load calculation.