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HVAC load calculator

Estimate the BTU and AC tonnage a home needs from square footage, climate, and a few envelope details — a fast sizing ballpark for the truck or a customer call.

1,800 ft²

The living area being heated and cooled — exclude garages and unfinished space.

Climate

Insulation

Sun exposure

Ceiling height

Estimated cooling load

39,600 BTU/hr

3.3

tons (calc.)

3.5

rec. size

63k

heating BTU

≈ 3.5-ton AC (42,000 BTU) · furnace ≈ 63,000 BTU input

A quick sizing estimate — good for ballpark and customer conversations. Before installing, run a full Manual J: oversizing short-cycles the system and wastes energy.

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How this is estimated

BTU ≈ sq ft × climate factor × insulation × sun × ceiling height

It uses the standard BTU-per-square-foot rule of thumb, adjusted for climate and the home's envelope, then converts to tons (12,000 BTU = 1 ton) and rounds up to the nearest standard system size.

This is an estimate, not a Manual J.It's meant for quick ballparks and customer conversations. A certified ACCA Manual J — accounting for window orientation, infiltration, and duct losses — should always be run before buying and installing equipment. Oversizing hurts comfort and efficiency.

HVAC sizing: FAQ

How many BTUs do I need to cool a house?

A common rule of thumb is roughly 20–25 BTU per square foot in a moderate climate, adjusted for insulation, sun exposure, and ceiling height. For example, an average 1,800 sq ft home in a moderate climate needs around 36,000–45,000 BTU, or about 3 to 3.5 tons of cooling. This calculator estimates it from your specifics.

How many tons of AC do I need per square foot?

As a rough guide, plan on about one ton of cooling per 500–600 sq ft in a moderate climate — so a 1,800 sq ft home lands near 3 to 3.5 tons. Hotter climates, poor insulation, and lots of sun push that higher; well-insulated, shaded homes pull it lower. Use the calculator to adjust for your case.

Is this HVAC load calculator a Manual J?

No. This is a quick rule-of-thumb estimate for ballpark sizing and customer conversations — not a certified ACCA Manual J. A full Manual J accounts for window orientation, infiltration, duct losses, and more. Always run a proper Manual J before purchasing and installing equipment.

What happens if an AC unit is oversized?

An oversized system short-cycles — it cools the air fast but shuts off before removing humidity, leaving the home cold and clammy, wearing out the compressor, and wasting energy. That is why correct sizing (a proper Manual J at install) matters more than simply "bigger is better."

For HVAC contractors: see Hero365 for HVAC or the HVAC cost calculator.

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