Heat Gain Calculator Formula
Understand the math behind the heat gain calculator. Each variable explained with a worked example.
Formulas Used
Total Heat Gain
total_btu = envelope_gain + people_gain + appliance_gainCooling Tons
tons_cooling = (envelope_gain + people_gain + appliance_gain) / 12000Envelope Component
envelope_component = envelope_gainVariables
| Variable | Description | Default |
|---|---|---|
floor_area | Floor Area(sq ft) | 1500 |
btu_per_sqft | Heat Gain Factor(BTU/sqft) | 30 |
occupants | Number of Occupants(people) | 4 |
appliance_watts | Appliance Heat Load(W) | 1000 |
envelope_gain | Derived value= floor_area * btu_per_sqft | calculated |
people_gain | Derived value= occupants * 600 | calculated |
appliance_gain | Derived value= appliance_watts * 3.412 | calculated |
How It Works
Cooling Load (Heat Gain)
Total = Envelope Gain + People Gain + Appliance Gain
Envelope gain uses a factor of 20 to 40 BTU per sq ft depending on climate and insulation. Each person adds about 600 BTU/hr. Appliances convert at 3.412 BTU per watt. One ton of cooling = 12,000 BTU/hr.
Worked Example
1,500 sq ft, 30 BTU/sqft factor, 4 people, 1000W appliances.
- 01Envelope = 1500 x 30 = 45,000 BTU/hr
- 02People = 4 x 600 = 2,400 BTU/hr
- 03Appliances = 1000 x 3.412 = 3,412 BTU/hr
- 04Total = 50,812 BTU/hr = 4.2 tons
Frequently Asked Questions
What BTU per sqft factor should I use?
Hot/sunny climate: 30-40. Moderate climate: 20-30. Cool climate with good insulation: 15-25. South-facing rooms with large windows use higher values.
How much heat does a person generate?
A sedentary person generates about 400 BTU/hr sensible and 200 BTU/hr latent (moisture). Active occupants generate more. The 600 BTU/hr figure is a common combined estimate.
Learn More
Guide
Understanding HVAC Sizing: How to Calculate Heating and Cooling Loads
Learn how to size HVAC systems using Manual J load calculations. Covers BTU requirements, climate factors, insulation values, ductwork sizing, and equipment selection.
Ready to run the numbers?
Open Heat Gain Calculator