AC Size Calculator for a Whole House

Explore a simplified house cooling estimate from geometry, illustrative insulation presets, windows and weather. See assumptions and separate sensible and moisture gains.

Conditioned floor area (sq ft)

Stories

Ceiling height (ft)

Outdoor design temperature (°F)Your area's 1% summer design temperature: exceeded only about 88 hours in a typical year — not the record high.

Outdoor design dew point (°F)Higher in humid climates; it sets the moisture (latent) load.

Indoor temperature (°F)Indoor humidity is taken as 50%.

Insulation and windows

Roof over the attic

Window area (% of floor area)Total glass area divided by floor area. Default 15% is an illustration, not a universal energy-code reference.

Window shading

BedroomsIllustrative occupancy equals bedrooms plus one; actual occupants and activity can differ.

Lights and appliances running on a hot afternoon (W)

Latitude (°)Sets the sun angles. Miami is about 26°N, Seattle about 48°N; use a negative value south of the equator.

This calculator explores a simplified whole-house cooling scenario using a square footprint and fixed envelope presets. It combines instantaneous conduction, window sunshine, leakage, occupants and appliances at one summer afternoon. Use the breakdown to see how assumptions change the estimate; it is not a measured peak load, a Manual J calculation or a final equipment selection.

How to use this tool

Enter conditioned floor area, stories and ceiling height.
Enter coincident outdoor temperature and dew point, plus the indoor setting.
Choose an illustrative envelope preset, roof assumption and window shading.
Enter window percentage, bedrooms, appliance watts and latitude.
Review the component table and nominal capacity comparison.

Formula

Estimate = wall + ceiling + glass conduction + window solar gain + leakage + people + appliances

Conduction uses area × temperature difference ÷ assembly R-value or window area × U-factor × difference. Leakage uses 1.08 × CFM × Δ°F sensible and nonnegative 0.68 × CFM × Δgr latent heat at sea level. Occupancy is bedrooms plus one, with assumed 230/200 BTU/h sensible/latent per person. Each watt adds 3.412 BTU/h. Presets are scenario assumptions, not certifications of a house’s age or code performance.

Example

For the default one-story 2,000 sq ft scenario at 95°F outdoors, 65°F dew point and 75°F indoors, sensible gain is about 26,300 BTU/h and moisture gain about 3,300 BTU/h. The total is about 29,600 BTU/h, or 2.47 tons. The next capacity in the illustrative list is 2.5 tons; actual equipment must satisfy sensible and moisture requirements at its operating conditions.

Frequently asked questions

Why does the solar result depend on latitude?

The model evaluates clear-sky sun at 4 pm solar time in July north of the equator and January south of it, splitting glass evenly among four sides. It is not a local weather file or a search for the annual peak.

Should I buy the next size shown?

No automatic purchase decision follows. ACCA Manual S uses equipment-specific performance data and design conditions. Nominal capacity, staging, airflow, moisture removal and local requirements must be checked separately.

What does a design temperature mean?

A 1% summer design value is exceeded during roughly 88 hours of an average year. Use local data and a compatible moisture condition; independently extreme temperature and dew point need not occur together.

What is omitted?

Ducts, mechanical ventilation, floor heat transfer, wall solar absorption, storage, external shading geometry and unusual layouts are omitted. The assumed attic is 20°F or 40°F above outdoor air. These omissions can change either the size or timing of the true peak.

Sources

Last updated: 2026-10-30