How to size a furnace by state and square footage (the climate-zone rule, step by step)
· 6 min read
The published furnace rule takes four numbers: your climate zone, which you read from your state; the zone's BTU per square foot; your square footage; and an insulation factor. Multiply, take the midpoint, round up to the next 10,000 BTU/h, and you have the furnace class an installer's quote should be near. Done here for a 1,800 sq ft house in Michigan: 99,000 BTU/h, a 100,000 BTU/h furnace class. The same house in Texas is 67,500 BTU/h, a 70,000 BTU/h class.
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Step 1: find your climate zone from your state
The rule has five zones, from zone 1 (hot) to zone 5 (cold). The calculator's state list (BTUtally's reading of the published five-zone heating maps, by where most of each state's people live) puts Michigan in zone 5 and Texas in zone 2. If you know your zone from a heating map, use it; the sources place some border states differently.
| Zone | States | BTU per sq ft | 1,500 sq ft | 2,000 sq ft | 2,500 sq ft |
|---|---|---|---|---|---|
| Zone 1 | FL, HI | 30-35 | 50,000 BTU/h | 70,000 BTU/h | 90,000 BTU/h |
| Zone 2 | AL, AZ, CA, GA, LA, MS, SC, TX | 35-40 | 60,000 BTU/h | 80,000 BTU/h | 100,000 BTU/h |
| Zone 3 | AR, DE, DC, KS, KY, MD, NV, NM, NC, OK, TN, VA | 40-45 | 70,000 BTU/h | 90,000 BTU/h | 110,000 BTU/h |
| Zone 4 | CO, CT, ID, IL, IN, IA, MA, MO, NE, NJ, NY, OH, OR, PA, RI, UT, WA, WV | 45-50 | 80,000 BTU/h | 100,000 BTU/h | 120,000 BTU/h |
| Zone 5 | AK, ME, MI, MN, MT, NH, ND, SD, VT, WI, WY | 50-60 | 90,000 BTU/h | 110,000 BTU/h | 140,000 BTU/h |
Read across: a 2,000 sq ft house is 70,000 BTU/h in zone 1 and 110,000 BTU/h in zone 5, the same house. Every cell is the midpoint of the zone's range rounded up to the next 10,000 BTU/h.
Step 2: multiply your square footage by the zone's BTU per sq ft
The published range (HVACDirect.com, How to Size Your Air Conditioner or Heater, heating factor) is 30-35 BTU per hour per sq ft in zone 1, rising to 50-60 in zone 5. For 1,800 sq ft in Michigan: 1,800 x 50 = 90,000 and 1,800 x 60 = 108,000 BTU/h. Use the conditioned area, the floors the furnace heats, not the lot or the garage.
Step 3: adjust for insulation and take the midpoint
The range is multiplied by 1.1 for a poorly insulated house and by 0.9 for a well insulated one (Inch Calculator, Furnace BTU Calculator); average insulation leaves it alone. The single figure carried forward is the midpoint of the adjusted range. In Michigan:
| Step | Figure |
|---|---|
| Zone from the state | Michigan: Zone 5 |
| Zone's BTU per sq ft | 50-60 BTU/h per sq ft |
| Range for 1,800 sq ft, average (x1) | 90,000-108,000 BTU/h |
| Midpoint (the load) | 99,000 BTU/h (29.01 kW) |
| Furnace class: next 10,000 BTU/h | 100,000 BTU/h |
| Heat pump class: next half ton | 8.25 tons, class 8.5 tons |
| Same house, poor (x1.1) | 108,900 BTU/h, class 110,000 BTU/h |
| Same house, good (x0.9) | 89,100 BTU/h, class 90,000 BTU/h |
| Same house in Texas (zone 2), average | 67,500 BTU/h, class 70,000 BTU/h |
Insulation moves the class by 1 step down with good insulation (90,000 BTU/h) and 1 step up with poor (110,000 BTU/h); the zone moves it by 3 steps between Texas and Michigan. If the house has new windows and blown-in attic insulation, use the good factor; single glazing and an uninsulated attic, the poor one.
Step 4: round up to the furnace class
The calculator rounds the midpoint up to the next 10,000 BTU/h and calls that the furnace class: 99,000 BTU/h becomes a 100,000 BTU/h class. It is a rounding rule, not a catalogue, and whether to buy the size above is the installer's call, made on the load calculation. The heat pump and furnace calculator does the four steps from your state, area and insulation and shows the heat pump class beside the furnace class.
Step 5: use the figure the right way
The rule is a published shortcut for a first estimate and a check on a quote. A quote for 1,800 sq ft in Michigan that lands near 100,000 BTU/h is in the range the rule expects; one far above it deserves a question about the load calculation behind it. What replaces the rule is a Manual J, which counts windows, orientation, air leakage, the envelope and the local design temperature; that is the figure equipment is bought on. Two things the rule does not do: it does not size ductwork, and it does not size cooling, which is a separate chart read room by room in the 1,500 sq ft house guide.
For other sizes: the BTU per square foot guide has 1,000 to 3,000 sq ft in every zone, the 1,000 sq ft guide has a small home at three insulation levels, and the 2,000 sq ft heat pump guide has the tons and half-ton classes by zone. All of them are produced by the same code as the calculator, so the figures agree.
Frequently asked questions
- What size furnace do I need for 1,800 sq ft?
- In Michigan (Zone 5) with average insulation, 90,000-108,000 BTU/h, 99,000 BTU/h at the midpoint, a 100,000 BTU/h output class (HVACDirect.com, How to Size Your Air Conditioner or Heater, heating factor). In Texas (Zone 2), 67,500 BTU/h and a 70,000 BTU/h class. The state decides the zone, and the zone moves the answer by more than anything else.
- Is there a furnace size calculator by zip code?
- The published rule works by climate zone, and the calculator sets the zone from your state (50 states and DC, BTUtally's reading of the published five-zone heating maps, by where most of each state's people live); a zip code adds nothing the rule can use. The sources place some border states differently, so if you know your zone from a heating map, pick it directly.
- Is the furnace figure input or output BTU?
- Output: the heat delivered to the house, which is what the load in BTU/h describes. A furnace label carries an input figure and a rated output, and the output is the one to set against 100,000 BTU/h; the class here rounds the midpoint up to the next 10,000 BTU/h as a rounding rule, not a product list.
- Does the same figure size a heat pump?
- The load does, the class differs: 99,000 BTU/h is 8.25 tons, rounded up to a heat pump class of 8.5 tons. A heat pump also cools, so its size is checked against the cooling side too, which the AC calculator gives room by room. The 2,000 sq ft heat pump guide has every zone and insulation level.
Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.