What size furnace do I need?
Furnace sizing runs on climate: from about 30 BTU per square foot in the mild South to 60 in the cold North. Enter your home size and zone to get the output load, plus the input rating to look for on the nameplate.
Furnace size calculator
BTU output · inputNothing you enter here leaves your browser. The math runs on your device. Figures round to sensible increments; your home's real load depends on details a rule of thumb cannot see.
Output, input, and efficiency
A furnace estimate starts from a heating load in BTU per square foot, set by your climate zone. Mild zones need around 30 BTU per square foot; cold zones need up to 60. Tall ceilings raise it, since there is more air to heat. This gives the output heat the furnace has to deliver.
Furnaces are sold by their input rating, not their output, and the gap between them is the AFUE efficiency. A 96% furnace turns almost all of its gas into heat, so its input is close to its output. An 80% furnace needs a noticeably higher input to deliver the same warmth. The calculator shows the output load you need and the input rating that delivers it at the efficiency you pick.
This is an estimate, not a Manual J
Heating is where undersizing and oversizing both bite. An undersized furnace runs nonstop and still loses ground on the coldest night. An oversized one short-cycles: it heats fast, shuts off, and cools down again, over and over, which wastes fuel and wears the equipment.
A per-square-foot rule cannot see your insulation, your windows, or how tight the house is. An ACCA Manual J load calculation can, and a licensed HVAC installer runs one before sizing and fitting the system. Use this number to sanity-check a contractor's quote, not to replace the calculation.
Common questions
What size furnace do I need for a 2,000 sq ft house?
It depends heavily on climate. Heating load runs from about 30 BTU per square foot in the mild South to about 60 in the cold North, so a 2,000 square foot home needs roughly 60,000 BTU of output in a warm zone and up to 120,000 in a cold one. The calculator asks for your climate zone because a single national number would be wrong for most people.
How many BTU of furnace do I need per square foot?
Roughly 30 to 60 BTU per square foot of output heat, rising as you move north into colder climate zones. That is an HVAC-industry rule of thumb and a planning band, not an official standard or a Manual J (there is no government BTU-per-square-foot-by-zone table). Insulation, window area, air leakage, and ceiling height all move it, which is why a load calculation from an HVAC contractor is the number to buy on.
What is AFUE, and why is furnace input higher than output?
AFUE is the furnace's efficiency: the share of the fuel it turns into delivered heat. A 95% AFUE furnace delivers 95 BTU of heat for every 100 BTU of gas it burns. Furnace nameplates list the input rating, so to deliver the output you need, you divide by the efficiency. The calculator shows both the output load and the input rating to shop for.
Is a bigger furnace better?
No. An oversized furnace heats fast, hits the thermostat, and shuts off, then repeats, which is called short-cycling. It wears parts, wastes fuel, and leaves uneven temperatures. An undersized furnace never catches up on the coldest night. The goal is to match the load, which is exactly what a Manual J calculation does before installation.