What Size Furnace Do I Need? Why the Charts Disagree

By Ray Bennett · Updated 26 September 2026 · 6 min read · Checked against the sources at the end

What Size Furnace Do I Need? Why the Charts Disagree

Short answer

Ask for a Manual J load calculation, and compare the furnace's output with it, not the input. Bryant's own chart puts a 2,000 square foot house anywhere from 60,000 to 120,000 BTU. With 60,000 BTU input, a Goodman GM9S80 at 80 percent gives out 48,000 and a Carrier 59SC5A at 95 to 96 percent gives 58,000. Too big short cycles, too small never catches up.

What size furnace do I need? The makers’ own answer is a load calculation, because square footage alone cannot settle it. Bryant and Carrier’s charts, read for the same 2,000 square foot house, run from 50,000 BTU to 120,000.

There is a second trap in the numbers. The size printed in the model number is the gas going in, not the heat coming out, and two furnaces with the same number can heat a house very differently. This page covers both.

Before you start

What to carry: a tape measure and a torch for the label inside the furnace door. Twenty minutes.

Gas or electric? This page is about gas furnaces, which are sized in BTU per hour. An electric furnace or a heat pump is sized differently: heat pump or furnace.

Replacing an old one? Note its model number from the label. You will want it below, but not to copy.

Two shortcuts the makers warn against. Copying the old furnace. Carrier says replacing a furnace with the same BTU rating as the old one can result in the wrong size. The house may have new windows, insulation or an extension since the old one went in.

Going bigger to be safe. A contractor quoted on Carrier’s page says the old belief that bigger was always better no longer holds. An oversized furnace heats too quickly and shuts off before the cycle is complete, which Carrier ties to uneven heat, higher bills and extra wear.

Why the charts disagree

The same 2,000 square foot house, read off the makers' own charts. The spread is the reason they all end by asking for a load calculation.

Bryant’s chart multiplies the floor area by 30 to 60 BTU per square foot, depending on the climate. It uses 30 to 35 in the warm South and Southwest, 40 to 50 in the Midwest and Mid-Atlantic, and 50 to 60 in the Northeast and northern Midwest. For 2,000 square feet that is 60,000 to 120,000 BTU. Bryant’s own questions section then gives the same house 60,000 to 100,000.

Carrier’s table gives ranges that overlap. Its 50,000 to 60,000 BTU furnaces are listed for 1,000 to 2,000 square feet, and its 110,000 to 120,000 BTU ones for 2,200 to 3,500 or more. Carrier calls them general reference ranges, not sizing advice.

Where they agree. Both put a 60,000 BTU furnace at about 1,200 to 2,000 square feet, and both say the chart is a starting point. Carrier says square footage alone should not be used to size a furnace.

Heat out, not gas in. Bryant’s chart numbers are the heat the house needs, so match them to the furnace’s output, not to the 060 in the model number.

Get the number that settles it

Check 1Often free from a utility energy audit, Trane says

Ask for a Manual J load calculation

What it is. Manual J is the load calculation published by the Air Conditioning Contractors of America. Carrier says ACCA identifies it as the ANSI recognised standard for sizing home heating and cooling.

What it looks at. Trane lists the square footage, the climate zone, the ductwork, the number and style of windows, shade and sunlight, the insulation, the number of people and heat producing appliances. Carrier adds which way the house faces, and Bryant adds ceiling heights and air leakage.

Where to get one. Trane says many utility companies do a free energy audit that gives you one, and an energy auditor or a dealer can do it too. Bryant says its dealers use Manual J as part of fitting a furnace.

What the answer is for. Carrier’s product data says a furnace should cover 100 percent of the design heating load, plus whatever margin comes from the jump between model sizes.

Input is not output

Both of these are sold as 60,000 BTU furnaces. The efficiency decides how much of it reaches the house.

Check 2Free, five minutes with a torch

Read the size of the furnace you have now

The model number. Carrier’s nomenclature puts the heating size in three digits, so 060 means 60,000 BTU. Goodman reads the same way: 060 is 60,000 BTU per hour. That number is the input, the gas it burns.

The output. The maker’s data lists the output separately. A Goodman GM9S80 with 60,000 BTU input gives 48,000 BTU of heat at 80 percent AFUE. A Carrier 59SC5A with the same 60,000 input gives 58,000.

Why it matters. Bryant says to compare the actual heating output, not just the input, when choosing between furnaces. Swapping an old 80 percent furnace for a 95 percent one of the same input puts about a fifth more heat into the same house, so keeping the same 060 can end up too big. Check the new output against your load.

More on reading the label, including the date it was made: how old is my furnace or AC.

Check 3Free, on the dealer's quote

Pick between the sizes on offer

Between two sizes. Trane says to go with the bigger unit when your load falls between sizes, because a little extra covers the coldest days.

But not far over. Trane’s rule of thumb is no more than 40 percent over the heating load for a furnace, and 25 percent for a heat pump. Hold that against the output, not the size in the model number.

Stages help. Carrier says a two stage furnace runs at a lower output when less heat is needed, and a modulating one adjusts in small steps to match the demand. Bryant says a two stage furnace runs on the lower setting most of the time and steps up only when it turns very cold, which cuts temperature swings.

How a wrong size shows up

Too big. Bryant lists short cycling, uneven temperatures and poor humidity control, more wear and higher bills. The furnace roars for a few minutes and stops, over and over: a furnace short cycling.

Too small. Bryant lists struggling in cold weather, running constantly without reaching the set temperature, and cold spots. Before blaming the size, rule out a filter or a fault: heat on but the house is cold.

Normal is not always short. Carrier sizes a furnace to cover 100 percent of the load on the coldest design day, so on the coldest nights a correctly sized one runs long, and that is not a fault: how long a furnace should run.

What each one costs

The step	Cost
Reading your furnace’s label	Nothing
Manual J through a utility energy audit	Often free, Trane says
Manual J from the dealer quoting the job	Part of the fitting at Bryant dealers, Bryant says. Ask if your quote includes it
New 60,000 BTU furnace, fitted	3,800 to 10,200 dollars, Carrier says; 2,000 to 14,200, Bryant says

Questions people ask

How many square feet does a 60,000 BTU furnace heat?

About 1,200 to 2,000, say both Bryant and Carrier, with colder climates and poor insulation at the low end, though Carrier also says a 60,000 BTU furnace is not tied to a set square footage. Check the output against a load calculation.

What size furnace for a 2,000 square foot house?

Bryant’s chart gives 60,000 to 120,000 BTU depending on the climate, and its own questions section 60,000 to 100,000. A Manual J narrows that to one number for your house.

How big a house will an 80,000 BTU furnace heat?

Bryant puts it at about 1,600 to 2,400 square feet, depending on insulation and climate.

Is it better to oversize or undersize a furnace?

Neither, Bryant and Carrier say. If the load falls between two sizes, Trane says take the bigger one, but no more than 40 percent over the load.

Can I just replace my furnace with the same size?

Carrier says that can result in the wrong size. The house may have changed since the old one went in, and the old one may never have been right.

Is a 95 percent furnace bigger than an 80 percent one?

At the same input it puts out more heat. With 60,000 BTU input, Goodman’s 80 percent GM9S80 gives 48,000 and Carrier’s 59SC5A, rated 95 to 96 percent, gives 58,000. Compare the output with your load.

Sources: Bryant "What Size Furnace Do I Need?" and "60,000 BTU Furnace Guide", Carrier "Calculating Furnace Size" and "What Is a 60,000 BTU Furnace?", Trane "What Size HVAC Unit Do I Need?", Carrier 59SC5A product data (input, output and model number nomenclature) and Goodman GM9S80 product specifications (input and output at 80 percent AFUE)