Sizing basics
How to size an air conditioner (and why the rule of thumb is only a start)
The square-foot rule gets you to the right conversation. Here is what it leaves out, what Manual J adds, and how to tell whether the size on your quote is right.
What “size” means for an air conditioner
An air conditioner’s size is the rate at which it removes heat from the house, measured in BTU per hour or in tons, where one ton is 12,000 BTU per hour. A 3-ton unit removes 36,000 BTU of heat every hour it runs. The right size is the one that matches the rate at which heat enters the house on a hot design afternoon: through the roof and walls, through the windows, with the outdoor air that leaks in, and from the people, lights and appliances inside.
That rate is called the cooling load, and everything about sizing comes back to it. Get the load right and the equipment choice follows; get it wrong and no amount of efficiency or brand quality will make the system comfortable. The AC tonnage converter handles the units, and the rest of this guide is about the load.
The rule of thumb, and where it comes from
The quick method multiplies the floor area by a figure for BTU per square foot. The figure most often quoted is 20, which is reasonable for a mixed climate with average insulation and 8 ft ceilings. The central AC size calculator uses a figure that varies with the DOE climate zone, from 24 BTU per hour per square foot in the hot-humid south of Florida to 12 in northern Minnesota, before adding people and the kitchen.
For a 1,500 sq ft house with average insulation, three occupants and a kitchen, the calculator gives:
| Climate zone | Example cities | Cooling load | Size sold |
|---|---|---|---|
| 1 hot-humid | Miami, Honolulu | 40,600 BTU/hr | 3.5 tons |
| 2 hot | Houston, Phoenix | 37,600 BTU/hr | 3.5 tons |
| 3 warm | Atlanta, Dallas | 34,600 BTU/hr | 3 tons |
| 4 mixed | Washington DC, St. Louis | 31,600 BTU/hr | 3 tons |
| 5 cool | Chicago, Denver | 27,100 BTU/hr | 2.5 tons |
| 6 cold | Minneapolis | 24,100 BTU/hr | 2.5 tons |
| 7 very cold | Duluth, Fargo | 22,600 BTU/hr | 2 tons |
The spread between the top and bottom of the table is almost two to one, which is why a single national figure fails at both ends of the country. The guide to BTU per square foot by climate zone explains where each figure comes from, and the BTU per square foot table lists them with the adjustments.
The adjustments that matter most
Floor area and climate are the start. Five things move the load enough to change the size:
Insulation and windows. A poorly insulated older house with single-pane glass needs about a fifth more than the table; a well-insulated, air-sealed house about 15% less. Insulation is the adjustment you can change, and it is usually cheaper to add attic insulation than to buy a larger air conditioner.
Ceiling height. The table assumes 8 ft ceilings. Each foot above that adds about 10%, because there is more air to cool and the hot air collects at the top.
Sun and glass. A house with large west-facing windows and no shade gains far more in the afternoon than the same house facing north under trees. The ENERGY STAR room air conditioner guidance adds or subtracts 10% for sun and shade.
People. Each person gives off about 400 to 600 BTU per hour. The ENERGY STAR guidance adds 600 BTU per hour for each regular occupant beyond two.
The kitchen. Cooking, the refrigerator and the dishwasher add about 4,000 BTU per hour, which is why a kitchen gets more cooling than a bedroom of the same size. Rooms with unusual equipment, such as a home office with several computers, need the equipment heat load calculator on top.
What a Manual J adds
ACCA Manual J, published by the Air Conditioning Contractors of America, is the room-by-room method that building codes reference for sizing new equipment. Instead of a figure per square foot, it calculates the heat gain through every wall, window, door, ceiling and floor from their actual areas, R-values and orientations, adds the outdoor air that leaks in from a measured or estimated air tightness, adds people and appliances, and separates the sensible load (temperature) from the latent load (moisture).
Three things make it worth asking for. It is specific to your house, so the window you replaced last year counts. It separates the latent load, which matters in humid climates where the equipment has to be chosen for its dehumidification as well as its capacity. And it gives every room its own load, which the duct design (Manual D) needs to send the right air to each room. A Manual J for a typical house takes a contractor an hour or two with software, and many include it free in a replacement quote.
ACCA Manual S then turns the load into an equipment choice, using the manufacturer’s capacity at your design conditions rather than the nominal tonnage. It allows cooling equipment up to 115% of the calculated load, because sizes come in half-ton steps.
Why oversizing is the common mistake
Most sizing errors go in one direction. A contractor replacing a failed unit on a hot day often installs the same size as the old one, or a half size larger “to be safe”, and the old one was often oversized already. The result is a system that cools the house in short bursts, shuts off before it has removed much moisture, and leaves the house cool but clammy. The guide on why oversized systems cost comfort and money covers the symptoms and the reasons.
The “Is my AC the right size?” calculator compares an installed unit with the estimated load. In the example of a 4-ton unit on a 1,800 sq ft house in zone 4 with an estimated load of 37,000 BTU per hour, the installed capacity is 130% of the load, beyond the 115% that Manual S allows, and the calculator’s answer is 3.5 tons.
Sizing a typical house
A 1,500 sq ft house in climate zone 4, with average insulation, 8 ft ceilings, average sun and windows, three people and a kitchen.
- Base load: 1,500 × 18 = 27,000 BTU per hour
- People beyond two: 600 BTU per hour; kitchen: 4,000 BTU per hour
- Total load: 31,600 BTU per hour, or 2.63 tons
- Rounded up to the next half ton: 3 tons, 14% above the load
- Design airflow: 3 × 400 = 1,200 CFM
Fourteen percent is inside Manual S’s 115% limit. A 2.5-ton unit would be 5% short, which would show up only on the two or three hottest days of the year; for a house in a humid climate it would be the better choice, because it would run longer and remove more moisture.
Room units and mini-splits
The same logic applies room by room. The window AC size calculator uses the ENERGY STAR chart, which starts at 5,000 BTU per hour for rooms up to 150 sq ft, with the same sun, people and kitchen adjustments. The portable AC size calculator adds a correction for the heat that portable units leak back into the room. The mini-split size calculator sizes each head for its room and then the outdoor unit for the heads together, which is where the second sizing mistake, a 12k head in every bedroom, usually happens.
Heating sizes and airflow
If the system is a heat pump, the heating load also matters, and in cold climates it is usually larger than the cooling load. The heat pump size calculator sizes for cooling and then shows the backup heat needed on the coldest nights. For a furnace, the furnace size calculator does the heating side.
Whatever the size, the ducts have to carry the air: about 400 CFM per ton. A 3-ton unit on ducts that pass 900 CFM delivers well under 3 tons of cooling. The CFM per room calculator splits the airflow between rooms, and the duct size calculator checks the ducts can carry it.
Checking the size on a quote
Ask three questions. What is the calculated load, and may I see the Manual J? How does the proposed size compare with that load? And what airflow will the system deliver through the existing ducts? A good contractor answers all three. The guide to reading an HVAC quote covers the rest of the quote.