AC Tonnage Calculator

How the AC tonnage calculator works

Air conditioners are sold by capacity. One ton of cooling equals 12,000 BTU per hour. The name comes from the ice age of refrigeration: melting one ton of ice over 24 hours absorbs 288,000 BTU, which is 12,000 BTU every hour. So a 3-ton AC can remove about 36,000 BTU of heat from your home each hour.

To pick a size, you need your home's cooling load: how much heat leaks and shines in on a hot day. This AC tonnage calculator estimates it in five steps:

1. Start with a cooling factor in BTU per square foot for your climate, from 18 in cold areas to 26 in hot ones.
2. Adjust for ceiling height, insulation, windows and sun, number of stories, and ducts in a hot attic.
3. Add or subtract 400 BTU for each person above or below four, plus extra for a busy kitchen.
4. Divide by 12,000 to get tons.
5. Pick the smallest standard size (1.5, 2, 2.5, 3, 3.5, 4 or 5 tons) that covers at least 90% of that load.

Why 90% and not 100%? Rule-of-thumb loads tend to come out high, because they can't see your actual walls, windows and air leaks. A slightly small AC also runs longer cycles, and long cycles pull more moisture out of the air. Most homes are more comfortable with a snug fit than with extra capacity.

How many tons of AC do I need? Square feet to tons by climate

This chart uses the same math as the calculator, for an average home: 8-foot ceilings, average insulation and windows, four people, one story, and ducts inside the house. The first number is the standard size we'd suggest. The number in brackets is the estimated load in tons.

Residential central air conditioners top out at 5 tons. Bigger homes usually get two systems, often one per floor, which also fixes the classic "upstairs is always hot" problem. If your home is newer or tighter than average, read one row up. If it's older and drafty, read one row down.

The formula, with a worked example

Load (BTU/hr) = sq ft × climate factor × ceiling × insulation × windows × stories × ducts
    + 400 × (people − 4) + kitchen extra
Ceiling factor = 1 + 0.06 × (ceiling height − 8)
Tons = load ÷ 12,000
Size = smallest of 1.5, 2, 2.5, 3, 3.5, 4, 5 tons that is ≥ 0.9 × tons

Climate factors (BTU per sq ft): hot 26, warm 24, mixed 22, cool 20, cold 18. Multipliers: insulation 1.20 (poor) to 0.80 (very good); windows 0.92 (shaded) to 1.20 (big sunny glass); stories 1.00, 0.96 or 0.93; ducts 1.00 inside, 1.04 in a crawl space, 1.10 in a hot attic. A large kitchen adds 1,200 BTU.

Worked example: a 2,000 sq ft, one-story house in a mixed climate with average insulation, average windows, 8-foot ceilings and four people. The load is 2,000 × 22 = 44,000 BTU/hr. Divide by 12,000 and you get 3.67 tons. 90% of 3.67 is 3.30, and the smallest standard size at or above 3.30 is 3.5 tons. That 3.5-ton unit covers 95% of the estimated load.

Now move the ducts to a hot attic. The load becomes 44,000 × 1.10 = 48,400 BTU/hr, or 4.03 tons, and the suggestion moves up to 4 tons. Sealing and insulating attic ducts is often cheaper than buying the bigger unit and paying to run it for 15 years.

BTU calculator for one room: the ENERGY STAR chart

Window units, portable ACs and single mini-split heads are sold in BTU, not tons. The "Single room" mode uses the ENERGY STAR room air conditioner sizing chart, then applies its four adjustments: cut 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for each person beyond two, and add 4,000 BTU if the unit cools a kitchen.

Where a room sits exactly on a boundary, such as 300 sq ft, the calculator uses the lower row. A 300 sq ft bedroom with average sun and two people needs about 7,000 BTU, so you'd shop for an 8,000 BTU window unit, the next common size up. The chart assumes 8-foot ceilings. For a room with a vaulted ceiling, a top-floor room under a hot roof, or lots of west-facing glass, step up one size.

Why a bigger AC is not better

Many replacement quotes simply match the old unit's size, or bump it up "to be safe." Oversizing causes real problems:

• Short cycling. A big unit hits the thermostat setting in a few minutes and shuts off, then starts again soon after. Frequent starts wear the compressor and contactor. Our guide to AC short cycling covers the other causes and fixes.
• Clammy air. An AC removes moisture only after the coil has been running cold for several minutes. Short runs leave the house cool but sticky, so people turn the thermostat down and pay more.
• Hot and cold rooms. Short blasts of air don't mix well. Rooms far from the air handler never catch up.
• Higher cost. Every half ton adds a few hundred dollars to the price of the unit, and bigger units may need larger ducts and a bigger breaker.

An undersized AC has the opposite issue. On the hottest afternoons it runs nonstop and the house may drift a few degrees above the setting. That's why the calculator flags any choice outside 90% to 115% of the estimated load.

Rule of thumb vs. a Manual J load calculation

This page gives a quick, honest estimate. It is not a substitute for Manual J, the load calculation method published by ACCA (Air Conditioning Contractors of America). A Manual J looks at each room: wall and attic insulation values, window sizes, glass type and direction, air leakage, duct losses and your local design temperatures. It often comes out smaller than any square-foot rule, especially for newer homes.

When you collect replacement quotes:

• Ask each contractor whether they ran a Manual J, and for a copy of the summary.
• Be wary of a quote that sizes the new unit only from the old unit's label or a quick look around.
• If your load falls between two sizes, ask about a two-stage or variable-speed unit. It runs at low speed most of the time and only uses full capacity on the hottest days.
• Ask whether the ducts can handle the airflow. Each ton typically needs about 400 cubic feet per minute of air, and undersized ducts make a new unit noisy and less efficient.

Once you know the size, our HVAC replacement cost calculator prices the whole job, and the AC replacement cost guide lists installed prices for each size.

Sizing tips for heat pumps, mini-splits and cold climates

• Heat pumps are sized by their cooling capacity too, so the tonnage is the same. In cold climates the heating load can be larger than the cooling load. A cold-climate model or backup heat covers the gap. See heat pump vs. furnace.
• Mini-splits are sized room by room. Run the single-room mode for each space and add the heads together for the outdoor unit. Common head sizes are 6,000, 9,000, 12,000, 18,000 and 24,000 BTU.
• Fix the house first. Air sealing, attic insulation and sealing duct leaks can cut the load enough to drop half a ton, which saves money on the unit and every summer after.
• Two-story homes often need more cooling upstairs. Zoning or a second small system can work better than one large unit.

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