HVAC Load Calculator
Free simplified HVAC cooling load calculator: BTU per hour and tons of cooling from floor area, climate, insulation and windows.
What does the HVAC Load Calculator do?
It gives a quick cooling load estimate in BTU per hour and tons of refrigeration from the conditioned floor area, the climate, the insulation quality and the number of windows, as a first pass before a full Manual-J calculation.
- Inputs: conditioned floor area, climate, insulation quality, window count.
- Output: total cooling load in BTU per hour and tons.
- Method: a base load per square foot, adjusted for insulation, plus a window load allowance.
Quick answer
A 1,500 square foot home in a moderate climate with average insulation and ten windows comes out near 47,500 BTU per hour, about 4.0 tons of cooling. Colder climates and better insulation lower the figure; a hot climate or poor insulation raises it.
What This Calculator Really Does
An air conditioner has to remove the heat that flows into a home through its surfaces and the heat generated inside it. Sizing the equipment means estimating that total heat gain, called the cooling load, in BTU per hour. The professional method is Manual-J, which measures each wall, window and appliance. This calculator is the quick estimate that contractors and homeowners use to sanity check a number before the detailed work: a base of 20 to 30 BTU per square foot depending on climate, adjusted up or down for insulation, plus a flat allowance for each window because glass admits far more heat than an insulated wall.
The rules it applies
- Base load: 30 BTU per square foot in a hot climate, 25 in a moderate one, and 20 in a cold one.
- Insulation adjustment: good insulation multiplies by 0.85, average by 1.0, and poor insulation by 1.2.
- Window allowance: 1,000 BTU per hour added for each window, a rough figure that stands in for solar gain.
- Tons: one ton of cooling equals 12,000 BTU per hour.
- Not modelled: ceiling height, orientation, appliance and occupant heat, duct losses and ventilation. Use a full Manual-J for a purchase decision.
Worked example with real numbers
A 1,500 square foot home, moderate climate, average insulation and ten windows. The area load is 1,500 x 25 x 1.0 = 37,500 BTU per hour. The window load is 10 x 1,000 = 10,000 BTU per hour. Total = 47,500 BTU per hour, which is 47,500 / 12,000 = 4.0 tons of cooling. Improving the insulation to good would knock the area load down to about 31,875 and the total to roughly 41,875, just under 3.5 tons.
Reading the result
Treat the number as a starting bracket, not a specification. Oversizing an air conditioner is a real mistake: it cools the air quickly but does not run long enough to remove humidity, leaving a cold and clammy house, and it cycles more often, which wears the compressor. Undersizing means it never reaches the setpoint on the hottest days. If your figure sits near a size boundary, or the home has large west-facing glass, high ceilings or an uninsulated attic, pay for a proper room-by-room Manual-J before buying. Improving insulation and shading is usually cheaper than the larger unit it replaces.
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Frequently Asked Questions
How is the HVAC cooling load calculated?
The conditioned floor area is multiplied by a climate base load and an insulation factor, then a flat allowance is added for each window, giving BTU per hour that is divided by 12,000 to get tons. The steps panel shows each figure.
What do I need to use the HVAC Load Calculator?
The conditioned floor area, your climate, the insulation quality and the number of windows. The base loads and factors are built in.
What does the result from the HVAC Load Calculator show?
The total cooling load in BTU per hour and the equivalent tons of refrigeration, split between the area load and the window load.
How many BTU per square foot do I need?
Roughly 20 in a cold climate, 25 in a moderate one and 30 in a hot one, before adjusting for insulation. These are quick sizing rules, not a Manual-J result.
Is the HVAC Load Calculator really free?
Yes — 100 percent free, no sign-up, and everything runs in your browser.
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