❄️ HVAC Cooling Load

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · A documented rule of thumb, pinned by automated tests · how PhDino checks its numbers

Related standards: ASHRAE 90.1

A rough-order cooling load estimate for sizing air conditioning equipment.

A building's cooling load comes from multiple sources: heat gain through the envelope (walls, roof, windows — driven by outdoor conditions and solar exposure) and internal gains from people, lights, and equipment. A proper load calculation (like ASHRAE's Radiant Time Series or Heat Balance methods) accounts for each source separately, with hour-by-hour solar and thermal-mass effects.

This calculator gives a fast order-of-magnitude estimate using a per-square-foot base load adjusted for sun exposure, plus a fixed per-person heat gain — useful for a rough sizing check, not for final equipment selection.

Key formula

Q_total = (Area × BaseLoad × SunFactor) + (Occupants × 250 BTU/h)
Tons = Q_total / 12,000
Airflow (CFM) = 0.75 × Q_total / (1.08 × ΔT), with ΔT = 20°F

Variables

BaseLoad

assumed BTU/h per square foot of floor area

SunFactor

multiplier for solar exposure (minimal/moderate/heavy)

250 BTU/h

roughly the sensible heat gain of one seated occupant (the moisture they add, the latent load, is not counted here)

0.75, 1.08, ΔT

the assumed sensible share of the load (the rest removes moisture), the sensible-heat constant for standard air, and an assumed 20°F difference between supply air and room air; together they give about 417 CFM per ton

How to use the HVAC Cooling calculator

Use this for a first estimate of how much cooling a room, an addition or a small building needs, in BTU per hour and in tons of air conditioning, plus the airflow that goes with it. It is the quick check to make before shopping for equipment or before a contractor arrives, so that you can tell whether a quote is in the right range.

It is a rough method by design: a base load per square foot, adjusted for sun, plus the heat that people give off. A proper load calculation, called a Manual J in North American practice, accounts for each window, wall, the roof, air leakage and the local design temperature, and this does not.

Floor Area (sqft)

The floor area being cooled by this equipment. Do not include spaces served by a separate system, and be careful with open stairwells and basements.

Base Load (BTU/h/sqft)

The base load per square foot. For houses, 20 to 30 BTU/h per ft² is a common range (roughly one ton per 400 to 600 ft²); hotter climates, poor insulation or lots of glass push it higher, and offices and shops run higher again. The default of 35 is a commercial value, so lower it for a well-built home.

Occupancy (people)

How many people are normally in the space. Each is counted as about 250 BTU/h of sensible heat. Large gatherings, kitchens and equipment rooms add loads that this simple input does not capture.

Sun Exposure

A multiplier for solar gain: minimal for shaded or north-facing rooms, moderate for typical exposure, heavy for large unshaded south- or west-facing glass.

Worked example: a sunny 1,200 square foot living space

An open living and dining area of 1,200 ft² in a well-insulated house has large west-facing windows with no shading, and 4 people use it. A base load of 25 BTU/h per ft² suits the construction, and the sun exposure is heavy. How large a system does it need?

Worked by hand:

1. Base load. 1,200 ft² × 25 BTU/h per ft² = 30,000 BTU/h.
2. Sun adjustment. Heavy exposure multiplies the base load by 1.3: 30,000 × 1.3 = 39,000 BTU/h.
3. People. 4 occupants × 250 BTU/h = 1,000 BTU/h, giving a total of 40,000 BTU/h.
4. Tons. One ton of cooling is 12,000 BTU/h, so 40,000 ÷ 12,000 = 3.33 tons.
5. Airflow. Assume about three quarters of the load is sensible heat carried by a 20°F air temperature difference: 40,000 × 0.75 ÷ (1.08 × 20) = 1,389 CFM, or about 417 CFM per ton.

The load is about 3.3 tons, so a 3.5-ton unit is the nearest standard size up. Going to 5 tons would be about 50% oversized. With moderate rather than heavy sun the total would drop to 34,000 BTU/h, which shows how much one assumption, the amount of unshaded glass, moves the answer. Treat the result as a range and let a proper load calculation settle the exact size.

Reading the result: why bigger is not safer

It is tempting to round the answer well up "to be safe", and that is the most common mistake in cooling. An oversized air conditioner cools the air so fast that it shuts off before it has removed much moisture. The room reaches the set temperature yet stays clammy, the system short-cycles, and equipment wears faster and costs more to run.

• Standard residential sizes step in half tons (1.5, 2, 2.5, 3, 3.5, 4, 5 tons). Pick the size nearest the calculated load, and treat anything more than about half a size above it with suspicion.
• Airflow of roughly 400 CFM per ton is the usual design target for cooling. It sizes the fan and the ducts together, and undersized ducts hurt performance far more than a slightly wrong tonnage does.
• The estimate is only as good as its exposure assumptions. Window area, orientation and shading are usually the largest single swing, followed by insulation and air leakage.
• Ducts running through a hot attic can lose a tenth or more of the delivered cooling, which a room-by-room estimate does not include.

Notes & limitations

• A real HVAC design load calculation (ACCA Manual J or ASHRAE-method software) accounts for window area/orientation, insulation levels, infiltration, and local design-day weather data — none of which this quick estimate uses.
• Undersizing cooling equipment leaves a space unable to keep up on the hottest days; oversizing wastes energy and hurts humidity control, so this estimate should be treated as a starting point, not a final answer.

Common mistakes

• Using the default base load of 35 BTU/h per ft² for a house. That figure suits commercial space; a well-built home is often 20 to 30, and using 35 can oversize the equipment by 40 percent or more.
• Sizing a whole house from one room's numbers, or one system for areas with very different exposures. Rooms with large west glass and shaded bedrooms need different amounts of air, and a single thermostat cannot balance them.
• Adding safety margin on top of a rule of thumb that is already conservative. Margin stacked on margin is how a 3-ton load becomes a 5-ton purchase.
• Forgetting equipment and lighting loads in kitchens, server closets and workshops. The people term covers only occupants.
• Treating tons as a weight. A ton of cooling is 12,000 BTU/h, the rate at which a ton of ice melting over 24 hours absorbs heat.
• Using this for heating. Heat loss is a different calculation, driven by the winter design temperature and the building envelope, not by sun or occupants.

Frequently asked questions

Papers worth reading

Qualitative comparison of North American and U.K. cooling load calculation methods Rees, S. J. et al. (2000), HVAC&R Research. Compares the main design methods for hourly cooling loads and how well the simplified ones predict the peak: a view of what a full load calculation does that a rule of thumb cannot.