Everyday
Furnace Size Calculator
Pick your city for its design temperature and heating degree days, and the page estimates your heating load, the furnace size it calls for, a heat pump size with auxiliary heat, and what each system costs to run for a year.
How it works
Behind the numbers
The load estimate starts from your city's 99% design temperature, the outdoor temperature your area stays above 99% of the year. The load factor rises as that temperature falls: about 18 BTU per square foot in Miami, about 30 in Atlanta, about 40 in Chicago and about 45 in Minneapolis, before adjustments for ceiling height and insulation quality. This is a planning estimate in the same family as the rules of thumb contractors quote. A Manual J calculation, which models your actual walls, windows, ducts and air leakage, is the number to buy equipment on.
Furnace sizing divides the load by efficiency. A home that needs 80,800 BTU/h with a 96% AFUE furnace calls for about 84,200 BTU/h of input, which lands on the 100,000 BTU standard size and delivers 96,000 BTU/h. ACCA Manual S allows gas furnaces to oversize the load by up to 40%; beyond that, short cycling, temperature swings and extra wear follow. In mild climates even the smallest common furnace (40,000 BTU input) oversizes most homes, which is one reason two-stage and modulating models exist.
The heat pump is sized to carry the house on its own down to a 30°F balance point, or down to your design temperature if your winters never get that cold. Below the balance point its capacity keeps falling, roughly 56% of nameplate at 0°F for a standard unit, and auxiliary heat strips cover the gap shown in the results. Annual costs use your city's heating degree days to estimate the season's total heat, then divide by each system's efficiency at the fuel prices you enter. Change the gas and electricity prices and the ranking can flip, especially in milder climates.
FAQ
Questions, answered
What size furnace do I need for a 2,000 square foot house?
It depends mostly on climate. A 2,000 sq ft home with average insulation needs roughly 37,000 BTU/h of heat in Miami, about 59,000 in Atlanta and about 81,000 in Chicago. At 96% AFUE those loads point to a 40,000, an 80,000 and a 100,000 BTU input furnace respectively. Insulation quality and ceiling height move the answer by 20% or more.
What happens if a furnace is oversized?
An oversized furnace heats in short bursts, then shuts off. Rooms swing between too warm and too cool, the blower noise comes and goes all day, and parts wear faster from constant starting. Manual S caps gas furnace sizing at 140% of the calculated load. A little oversize is normal because furnaces come in steps; a lot is a comfort problem.
Is a heat pump cheaper to run than a gas furnace?
At typical 2026 prices, gas usually wins in cold climates and the heat pump wins in mild ones, but the gap is set by your local gas and electricity prices, which is why this page asks for both. A heat pump moves two to three units of heat per unit of electricity across most of the season, yet it still loses ground on the coldest nights when auxiliary strips take over.
What is a design temperature?
The 99% design temperature is the outdoor temperature a location stays at or above for 99% of the hours in a year, published by ASHRAE for thousands of weather stations. Sizing equipment to it covers nearly every night of the winter without paying for capacity you would use a few hours per decade.
Should I still get a Manual J calculation?
Yes, before you sign a quote. This page gives a planning estimate from floor area and climate. A Manual J models your actual construction, window area, duct losses and air leakage, and it is what building codes and equipment warranties expect. If a contractor sizes from square footage alone, ask why.