CategoriesHvac

Hvac Calculators

HVAC calculators for heating and cooling load estimation. Calculate BTU requirements, unit sizing, and energy costs for any room or building.

5 Calculators

Heating + Cooling

Always Free

Heating and cooling are sized by heat load, not by floor area alone.

The rule of thumb people reach for is 20 BTU per square foot, and it is only a starting point. Real load depends on insulation, window area and orientation, ceiling height, air leakage and climate zone. Two identical-looking houses can differ by 30% in load, which is why oversized equipment is so common.

Oversizing is worse than it sounds. An oversized air conditioner cools the air quickly, shuts off before it has removed the humidity, and leaves a house that feels cold and clammy while short-cycling the compressor. Sizing down and running longer is more comfortable and lasts longer.

Settle this before you calculate

Conditioned floor area and ceiling height: Load follows volume, not floor area, so a 10 foot ceiling changes the answer against an 8 foot one.
Window area and which way it faces: Glazing is the weakest part of the envelope, and west-facing glass drives the afternoon peak that sizes the equipment.
Airflow, if you are sizing a fan: Ventilation is CFM, not BTU. A bathroom needs roughly 1 CFM per square foot with a 50 CFM floor.

Where the estimate goes wrong

Sizing up for a safety margin: Oversized cooling short-cycles and never dehumidifies. If the load calculation says 2.5 tons, fit 2.5 tons.
Replacing like for like: If the house has been re-insulated or had windows replaced since the old unit went in, its load has dropped. Recalculate rather than matching the old nameplate.
Ignoring duct capacity: A bigger unit on undersized ducts is noisy and starved of airflow. The duct system has to carry the CFM the equipment moves.