Calculators
Air conditioner sizing calculators
Central AC tons, window and portable units, mini-splits, equipment heat loads and what a system costs to run.
Every air conditioner question starts the same way: how much heat enters the space on a hot afternoon, and how fast must the equipment remove it. The answer is a load in BTU per hour, and the first estimate of it is the floor area multiplied by a figure for your climate zone, then adjusted for ceiling height, insulation, sun, windows, people and a kitchen. These calculators do that arithmetic and round the result to the sizes that are actually sold, in half-ton steps for central systems and in the standard BTU classes for room units.
The common mistake is to size up "to be safe". An oversized air conditioner reaches the thermostat setting quickly and shuts off before it has removed much moisture, so the house feels clammy and the compressor wears out on short cycles. Treat every result here as a starting estimate; the standard a contractor follows for the final figure is ACCA Manual J for the load and Manual S for the equipment.
8 calculators
Central AC Size Calculator
Tons and BTU/hr for a house from its floor area, climate zone, insulation, sun, windows and occupants.
AC Tonnage Converter
BTU/hr, tons, kW, watts, kcal/h and the Asian "horsepower" class, converted in one table.
Window AC Size Calculator
The BTU/hr a room unit needs from the ENERGY STAR chart, the nearest size sold and its electrical draw.
Portable AC Size Calculator
Real cooling needed (SACC) and the ASHRAE label to look for, single or dual hose.
Mini-Split Size Calculator
BTU/hr and head size for up to five zones, and the outdoor unit that serves them.
Equipment Heat Load Calculator
The cooling load that computers, appliances, lighting and people add to a room, in BTU/hr and tons.
AC Running Cost Calculator
What an air conditioner costs per hour, day and month from its size, SEER2, run time and your electricity price.
SEER Upgrade Savings Calculator
Electricity and dollars saved by a higher SEER2 rating, and how long the extra cost takes to pay back.
Guides for this topic
BTU per square foot by climate zone
Twenty BTU per square foot is right in St. Louis and wrong in Miami and Minneapolis. Here are the figures by climate zone, what changes them, and when to stop using them.
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.
Mini-splits: sizing each zone and the outdoor unit
A mini-split is sized twice, once for each room and once for the outdoor unit that serves them all. Most mistakes happen in the first step, with a 12k head in every bedroom.
Why an oversized AC or furnace costs you comfort and money
Bigger feels safer and works worse. Here is how an oversized system leaves a house clammy, cycles itself to an early end, and what to check before the next one is the same size.
How to read an HVAC quote (tonnage, SEER2, ductwork, warranty)
Three quotes for the same house can differ by thousands of dollars and still not be comparable. Here is what each line should say and what to ask when it does not.
SEER2, HSPF2, AFUE and COP: the ratings explained
Every efficiency rating is output divided by input. Once you know what each one counts, comparing equipment and estimating what it costs to run is simple arithmetic.
Window and portable AC sizing: BTU, SACC and the room
A window unit is sized from a chart; a portable unit is sized from a chart and then corrected for the cooling it loses to its own hose. Here is how both work, with the numbers.
Questions people ask
- How many tons of AC does a 1,500 square foot house need?
- At the rule-of-thumb loads this site uses, a 1,500 sq ft house with average insulation, 8 ft ceilings, three occupants and a kitchen needs about 3 tons in a mixed climate (zone 4), 3.5 tons in a hot climate (zones 1 to 2) and 2 to 2.5 tons in a cool one (zones 5 to 7). Sun exposure, many windows and a poorly insulated attic push the figure up by 10 to 20%; good insulation brings it down a half size. A Manual J load calculation replaces the rule of thumb for the final size.
- How many BTU do I need for a 12 by 12 room?
- A 144 sq ft room falls in the 150 sq ft class of the ENERGY STAR room air conditioner chart, which calls for about 5,000 BTU per hour. Add 10% if the room is very sunny, subtract 10% if it is heavily shaded, add 600 BTU for each person beyond two who regularly uses it, and add 4,000 BTU if it is a kitchen. A 5,000 or 6,000 BTU window unit is the usual answer.
- What size mini-split do I need for a garage?
- Garages are sized like any other room, then adjusted for their weak points: uninsulated walls and a big door act like "poor" insulation, and a west-facing door adds sun load. A typical two-car garage of 400 to 500 sq ft lands between 12,000 and 18,000 BTU per hour, which means a 12k or 18k head. If the garage is uninsulated in a hot climate, insulate it first; cooling an uninsulated garage costs more than the insulation.
- How much does a higher SEER2 rating save?
- Electricity use falls in proportion to the ratio of the ratings. Replacing a 10 SEER system with a 15.2 SEER2 unit cuts cooling electricity by about a third, because 10 ÷ 15.2 is 0.66. For a 3-ton system running 1,200 hours a season at 17 cents per kWh that is roughly $200 a year. The SEER upgrade savings calculator does the sum with your hours and your price, and shows how long the extra cost takes to pay back.
- How many CFM of airflow does an air conditioner need per ton?
- About 400 cubic feet per minute per ton is the design rule for most of the United States. Humid climates use 350 CFM per ton so the coil runs colder and removes more moisture; dry climates use up to 450. A 3-ton system therefore needs 1,200 CFM through its ducts (1,050 in a humid climate), which is why duct size and return grille size matter as much as the equipment.
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