Sized from your house and your climate

How many tons do you actually need?

61 free heating and cooling calculators for homeowners, DIYers and contractors: air conditioners, furnaces, heat pumps, ducts and vents, ventilation, running costs, installation prices and water heaters. Each one shows its formula, its sourced assumptions and the arithmetic next to the answer.

A 1,500 sq ft house in climate zone 4

Air conditioner size
3 tons
Furnace size at 95% AFUE
60,000 BTU/hr input
Window unit for a 12 × 15 ft bedroom
6,000 BTU/hr
A month of cooling at 3 tons, SEER2 15, 8 hours a day
$97.92

Average insulation, 8 ft ceilings, three people and a kitchen; electricity at 17 cents per kWh. Change any of it in the calculators.

Calculators

Air conditioning

Central AC tons, window and portable units, mini-splits, equipment heat loads and what a system costs to run.

Heating

Furnace output and input, heat pump capacity and balance point, boilers, radiators, baseboards, space heaters, radiant floors and the cost per BTU of every fuel.

Ducts & airflow

CFM per room and per ton, duct sizes, velocity, return grilles, registers, equivalent lengths, air changes and exhaust fans.

Ventilation & humidity

Whole-house ventilation by ASHRAE 62.2, dehumidifiers and humidifiers, ERV and HRV sizing, air purifier CADR, ceiling and whole-house fans.

Running costs

What a system costs to run, fuels compared per million BTU, thermostat setbacks, degree days, filters, upgrade paybacks and duct leakage.

Installation costs

Central AC, furnaces, heat pumps, mini-splits, ductwork, repairs and maintenance, with sourced and dated ranges.

Water heating

Tank size by first-hour rating, tankless GPM and temperature rise, recovery time, daily hot water use, running cost and replacement cost.

Planning & comfort

Zoning, lifespan and replacement timing, checking the size of an existing unit, humidity and dew point comfort, heat index, heat loss through walls and generator sizing for HVAC.

How the estimates work

Read the full methodology

  1. Your room or houseFloor area, ceiling height, insulation, sun, windows, people and the kitchen: the things that move a heating or cooling load the most.
  2. A climate-zone loadEach DOE climate zone has its own BTU per square foot for cooling and for heating, adjusted by those factors, so a Miami house and a Minneapolis house of the same size get different answers.
  3. A size that is actually soldThe load is rounded to the next half ton, the next furnace input or the next mini-split head, and checked against the ACCA Manual S limits that stop a system being oversized.
  4. The standard that confirms itEvery result says which standard a contractor would use to confirm it: Manual J for loads, Manual D for ducts, ASHRAE 62.2 for ventilation, the IRC for exhaust fans.

Most sizing tools multiply a floor area by one national figure and stop. These start from the climate, show every adjustment, round to the equipment you can buy, and price with dated, named survey ranges rather than a single number. They estimate; they never replace a load calculation, a licensed installer, or the gas, refrigerant and electrical work that only a professional should do.

Questions people ask first

How many BTU per square foot do I need for air conditioning?
It depends on the climate zone. Before people and the kitchen, an average-insulated house with 8 ft ceilings needs about 24 BTU per hour per square foot in southern Florida, 20 in Atlanta or Dallas, 18 in Washington DC or St. Louis, 15 in Chicago and 12 in northern Minnesota. Insulation, ceiling height and sun move it by 10 to 20 percent either way.
What size air conditioner does a 2,000 square foot house need?
In climate zone 4, with average insulation, three people and a kitchen, the load is about 40,600 BTU per hour, so a 3.5-ton unit. The same house needs about 4 tons in Atlanta and 3 tons in Chicago. A Manual J load calculation from the contractor should confirm the size before you buy.
How many CFM per ton does an air conditioner need?
About 400 CFM of airflow per ton of cooling is the standard design figure, so a 3-ton system moves about 1,200 CFM. Humid climates often use 350 CFM per ton to remove more moisture, and dry climates up to 450 for more sensible cooling.
Is a heat pump cheaper to run than a gas furnace?
It depends on the prices. A heat pump at a seasonal COP of 2.8 beats a 95 percent gas furnace when electricity costs less than about 3.6 times the gas price per therm times the COP, in cents per kWh, which is 15.1 cents at $1.50 gas. Against propane, oil or electric resistance heat, a heat pump almost always wins.
What size bathroom exhaust fan do I need?
The building code minimum is 50 CFM for a fan switched on and off. The Home Ventilating Institute rule is 1 CFM per square foot for bathrooms up to 100 square feet, and 50 CFM per toilet, shower and tub above that. An 8 by 10 ft bathroom needs about 85 CFM, so a 110 CFM fan.
What size dehumidifier do I need for a basement?
A damp 1,000 square foot basement needs about 25 pints per day under the 2019 DOE test, so a 30-pint unit, which would have been sold as about a 42-pint unit before the test changed. Add more for a very wet basement or a humid climate.

From the guides

All 25 guides

Built to be checked. Every calculator is tested against its own worked examples before it goes up. Loads come from published climate-zone sizing charts and ENERGY STAR guidance, duct figures from the ASHRAE friction relation, ventilation from ASHRAE 62.2 and the International Residential Code, energy content and prices from the EIA, and costs from dated 2025 and 2026 surveys, all listed on the methodology page and last reviewed on October 7, 2026. The site is written by its publisher with AI assistance and human review. Found a figure that looks wrong? Tell us.

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