Furnace Size Calculator
Furnaces are sold by their input rating — the gas going in — and heated by their output, which is input multiplied by efficiency. An eighty thousand BTU furnace at eighty percent AFUE delivers sixty-four thousand, and comparing that eighty thousand against a heating load overstates the furnace by a quarter before anything else has gone wrong. Above two thousand feet the input drops again. This calculator works in output from the start, applies the altitude derate, and checks the airflow the furnace needs to stay inside its nameplate temperature rise, which is the constraint that turns an oversized furnace into one that shuts itself off.
- Heat this furnace actually delivers
- 80,000 BTU/hr 100,000 BTU/hr input, × 80% AFUE. The number on the model badge is the input — the gas going in, not the heat coming out.
- Heating load at design temperature
- 70,650 BTU/hr 1,800 ft² at 8 ft, average insulation, 10 °F design temperature. Same model the mini split page uses.
- Oversizing
- +13% Within a sensible margin. Some headroom covers recovery from setback and a colder-than-design night without pushing the equipment into short cycling.
- Airflow needed for a 50 °F rise
- 1,481 CFM Output ÷ (1.08 × rise). The nameplate specifies a permitted rise range, and the blower has to move enough air to stay inside it. This is where an oversized furnace on existing ductwork fails.
- Rise at 1,600 CFM
- 46 °F Inside a normal range for the target rise. Airflow and output are matched.
- Input rating that would match the load
- 90,000 BTU/hr Working backwards: 70,650 BTU/hr of load ÷ 80% efficiency, rounded up to the next common size. Furnaces are sold in 20,000 BTU steps in most lines, so an exact match is rarely available — round up once, not twice.
- What this does not model
- Manual J A screening estimate from floor area. It ignores window area and orientation, air leakage, duct losses into unconditioned space, and internal gains. Duct losses alone commonly account for twenty percent of delivered heat in an attic or crawl space system.
Input is what you buy, output is what you get
Every gas furnace carries two ratings. Input is the rate at which it burns fuel. Output is the rate at which it delivers heat to the air. The difference goes up the flue, and the ratio between them is the AFUE.
Model numbers and price labels use input, because it is the larger number and because it is what the gas supply has to be sized for. An eighty percent furnace rated at a hundred thousand BTU delivers eighty. A ninety-six percent condensing furnace at the same input delivers ninety-six, which is a genuinely different machine even though the badge reads the same.
This matters twice. Comparing input against a heating load oversizes the furnace by the inverse of the efficiency. And comparing an eighty percent unit against a ninety-six percent unit by input compares two things that are not equivalent — the condensing unit at the same input is a bigger heater as well as a cheaper one to run.
When a contractor quotes a furnace size, ask which number it is. If the answer is not immediate, that is informative in itself.
Altitude takes another slice
Combustion needs oxygen, and there is less of it per cubic foot as you go up. The long-standing rule in the fuel gas codes is to reduce input by four percent for every thousand feet above two thousand.
In Denver at about five thousand two hundred feet that is a thirteen percent reduction. A furnace rated at a hundred thousand BTU delivers eighty-seven thousand of input, and after eighty percent efficiency, under seventy thousand of heat. Sized on the badge, it is now nearly a third smaller than it appeared.
Manufacturers handle this in different ways. Some list altitude-rated capacities directly, some require a high-altitude orifice or conversion kit above a threshold, and some models are simply not listed for use above a certain elevation. The kit is not optional where it is specified — running at sea-level orifices in thin air produces incomplete combustion.
The same derate applies to water heaters, boilers and gas ranges, which is worth remembering when a whole house is being planned at elevation.
Temperature rise is where oversizing shows up
A furnace nameplate specifies a permitted temperature rise, typically a thirty degree window such as forty to seventy degrees Fahrenheit. That is the difference between the air going into the furnace and the air coming out, and it is not advice — it is a listing condition.
The physics is one equation: rise equals output divided by 1.08 times airflow. Fix the output and the rise is set by how much air the blower moves. A furnace with too much output for its duct system runs a rise above the permitted range, the heat exchanger runs hotter than designed, and the high limit switch cuts the burner.
What the homeowner sees is a furnace that runs for a few minutes, stops, and starts again — and what they are usually told is that the ducts are undersized. Sometimes that is true. Very often the ducts were adequate for the house and the replacement furnace was simply larger than the one it replaced, because it was chosen by matching the old badge number rather than calculating the load.
Running below the rise range is a problem too, in the other direction. Too much air cools the flue gases, and in a non-condensing furnace that lets moisture condense inside the heat exchanger and vent, where it corrodes both.
What this is based on
- AFUE — annual fuel utilization efficiency, the ratio of output to input
- Fuel gas code altitude derating practice — 4% per 1,000 ft above 2,000 ft
- Sensible heat equation — BTU/hr = 1.08 × CFM × temperature rise
A screening estimate, not a Manual J load calculation or a combustion analysis. It does not size gas piping, venting or combustion air, and it does not account for duct losses, which are substantial where ductwork runs through unconditioned space. Altitude derating requirements and any required conversion kit are set by the manufacturer and the fuel gas code adopted locally.
Frequently asked questions
Is a furnace rated by input or output?
The model badge and the price label almost always show input — the rate at which it burns gas. Output, which is what heats the house, is input multiplied by the AFUE. An 80,000 BTU furnace at 80% efficiency delivers 64,000.
How much should a furnace be oversized?
Fifteen to thirty percent above the calculated load is a normal margin, covering recovery from setback and colder than design nights. Beyond about forty percent the losses start: temperature swings, duct noise on every start, and more thermal cycling of the heat exchanger.
Do I need to derate for altitude?
Above 2,000 feet, yes — roughly four percent of input per thousand feet. Many manufacturers also require a high-altitude conversion kit above a threshold, and some models are not listed above a certain elevation at all.
What is temperature rise and why does it matter?
The difference between return air and supply air temperature, and the nameplate specifies a permitted range. It equals output divided by 1.08 times airflow, so a furnace with too much output for its ductwork runs above the range and trips the high limit switch — which looks like a duct problem and is often a sizing problem.
Can I just match the size of my old furnace?
It is the most common approach and the least reliable, because the old one was very likely oversized too, and because the house may have been insulated, re-windowed or extended since. Matching the old badge number also carries any input-versus-output confusion forward into the new equipment.