How to use the Heating and Cooling Load Calculator
Start with the space dimensions, then set the climate, envelope, window, occupancy, and equipment values. Select Calculate to see heating and cooling estimates in BTU/hr and watts, plus cooling in tons. The result includes the formulas, component values, assumptions, and a ±15% range around the calculated load.
- Enter Length, Width, Ceiling height (per floor), and Number of stories. Choose ft or m beside each dimension.
- Choose a Climate preset and Temperature unit. Check Winter outdoor design temperature, Summer outdoor design temperature, Indoor heating setpoint, and Indoor cooling setpoint.
- Choose Insulation level (walls, roof, floor), Window / glazing type, and Air-sealing / infiltration. Enter Total window area and choose Predominant window orientation.
- Enter Number of occupants and Appliance / equipment load, choose a Safety factor, and select Calculate.
How the heating and cooling estimate is calculated
The calculator adds conduction through walls, windows, the roof, and the floor to infiltration. Conduction uses U-value x Area x ΔT. Infiltration uses 1.08 x CFM x ΔT, with CFM = ACH x Volume / 60.
Cooling also includes window solar gain, about 400 BTU/hr per occupant, and appliance heat from Watts x 3.412. Climate preset fills the outdoor temperatures, while Insulation level, Window / glazing type, and Air-sealing / infiltration choose the calculation values. Predominant window orientation changes the solar-gain component. The selected Safety factor is applied to each total before the ±15% range is shown.
Input: 40 ft x 30 ft, 9 ft ceiling, 1 story, 300 ft2 windows
Mixed climate, average insulation, 2 occupants, 500 W appliance load
Output: heating 26,480-35,827 BTU/hr; cooling 22,744-30,771 BTU/hr
Cooling range: 1.90-2.56 tonsHeating and cooling load calculator limits
Length, Width, and Ceiling height (per floor) accept entered values from 1 to 1000. Winter outdoor design temperature, Summer outdoor design temperature, Indoor heating setpoint, and Indoor cooling setpoint accept -60 to 130. Number of stories must be a whole number from 1 to 10. Number of occupants must be a whole number from 0 to 200.
Total window area accepts 0 to 100000 in its selected unit and cannot exceed the calculated gross wall area. Appliance / equipment load accepts 0 to 1000000 in its selected unit. The checks use entered values before conversion, so changing units does not widen the raw ranges. An invalid value stops Calculate and shows an error.
For multiple stories, only the top floor exchanges heat through the roof, and only the bottom floor exchanges heat with the floor or ground. A negative seasonal temperature difference contributes zero to that component. The presets are generic assumptions. The estimate does not size ducts, select equipment, check code compliance, or replace an ACCA Manual J calculation. The calculation script does not upload entered values.
What the result section shows
After Calculate, the result section shows heating load, cooling load, cooling load in tons, and footprint area. Formula used prints the calculation lines, while Component breakdown & assumptions lists converted dimensions, temperature differences, selected factors, and each heating or cooling component.
Use Copy result for plain text or Download result (.txt) for a text file. Reset clears the entered dimensions, restores the defaults, and hides the result.
When another construction calculator fits better
Use Construction Calculators for Materials, Dimensions, and Loads to choose among the site's construction estimates. Use the Concrete Calculator for concrete volume and bag count, the Construction Materials Calculator for its 23 material and project modes, or the Solar Panel Calculator for panel count, system size, production, battery contribution, and payback estimates.
Frequently asked questions
How do I calculate a heating and cooling load?
Enter the dimensions, climate temperatures, envelope choices, occupants, and Appliance / equipment load, then select Calculate. The result combines conduction and infiltration, adds cooling-only gains, applies the selected Safety factor, and shows a ±15% range.
What does a BTU/hr and watts load estimate include?
It includes wall, window, roof, floor, and infiltration loads for heating and cooling. Cooling also includes window solar gain, occupant heat at about 400 BTU/hr per person, and appliance or equipment heat calculated from watts.
Why does the cooling load show tons and a range?
The calculator converts cooling to tons as well as watts. It applies a ±15% band around the final load because the form uses generic preset assumptions.
Can this calculator size HVAC equipment or ducts?
It estimates a whole-space load only. It does not size ducts, select equipment, check local code compliance, or produce a certified Manual J, S, or D calculation.