How to use this furnace sizing chart (step-by-step)
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Find your zone. Use the map summary above: Zone 1 = southern FL/HI/south TX; Zone 5 = MN/near Canadian border.
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Calculate base BTU. Square footage × zone BTU/ft² (e.g., 2,200 sq. ft. in Zone 3 at 40–45 → 88k–99k BTU).
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Adjust for your home.
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Great insulation? Use the low end of the range.
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Older leaky home? Lean high.
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10′+ ceilings? Add 10–15%.
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Leaky ducts? Add 15–25%.
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Pick the nearest size your preferred model offers (most furnaces come in 15k–20k BTU steps).
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If your home is large, complex, or has lots of glass, schedule a Manual J for precision.
Want a quick second look from a tech? Share pics of your equipment and rooms via Quote by Photo.
Climate zones & BTU per square foot (why it matters)
Your climate sets the heating load baseline. Warmer zones need fewer BTUs; colder zones need more to hold temperature on design-day (the coldest typical day of the year).
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Zone 1 (Hot): 30–35 BTU/ft² — Southern FL, HI, south TX. Mild winters; short runtimes.
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Zone 2 (Warm): 35–40 BTU/ft² — Much of the South, AZ/NV low elevations.
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Zone 3 (Moderate): 40–45 BTU/ft² — Mid-latitudes (e.g., Carolinas).
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Zone 4 (Cool): 45–50 BTU/ft² — Upper Midwest/Northeast suburbs.
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Zone 5 (Cold): 50–60 BTU/ft² — Northern tier/MN/ME. Longer, harsher seasons.
These are output targets, and they assume standard 8–9′ ceilings and “average” construction. After you get a base number, adjust for insulation, windows, ceiling height, solar gain, and ducts (we’ll cover those next). If you’re on a zone border, start from the lower zone and increase only if your home’s build demands it.
Quick estimates by home size (1,000–5,000 sq. ft.)
Use these to sense-check your math. Numbers reflect typical homes; insulation and duct losses can swing results.
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1,000 sq. ft.:
Zone 1: 30k–35k | Zone 3: 40k–45k | Zone 5: 50k–60k -
2,000 sq. ft.:
Zone 1: 60k–70k | Zone 3: 80k–90k | Zone 5: 100k–120k -
3,000 sq. ft.:
Zone 1: 90k–105k | Zone 3: 120k–135k | Zone 5: 150k–180k -
4,000 sq. ft.:
Zone 1: 120k–140k | Zone 3: 160k–180k | Zone 5: 200k–240k -
5,000 sq. ft.:
Zone 1: 150k–175k | Zone 3: 200k–225k | Zone 5: 250k–300k
If your math lands between sizes, choose the nearest model—not automatically the bigger one. When in doubt, ask our team through the Design Center.
Insulation & air sealing: the 20–30% swing most folks miss
Insulation quality decides whether you can stay at the low end of the BTU range or need to bump up. Tight, modern homes (sealed attic, insulated rim joists, proper weather-stripping) often need 20–30% less heat than the same size older home. On the flip side, a 1960s house with original attic insulation and gaps around can lights or top plates can demand 20%+ more.
Pro tip: if your furnace room is warm while the far bedrooms are chilly, you probably have envelope issues and duct imbalances—not a too-small furnace. Fixing air sealing and insulation often shrinks the correct furnace size and evens out temperatures. For DIY/equipment questions, check the Help Center.
Windows, doors & infiltration (why glass costs BTUs)
Glass loses heat faster than insulated walls. Lots of single-pane or older double-pane windows push you toward the high end of the range. As a rule of thumb, every inefficient window can add ~1,000 BTU to your load. Big sliders and leaky doors count, too.
If you’re replacing windows soon, size the furnace for the post-upgrade condition—otherwise you’ll be oversized later. Weather-stripping, door sweeps, and proper threshold adjustments cut infiltration and may let you choose the smaller of two sizes. For homes with massive window walls, two-stage or modulating furnaces handle morning solar gain and nighttime losses more gracefully than single-stage units.
Ceiling height & layout (volume, open plans, and sun)
Sizing rules assume 8–9′ ceilings. If you’ve got 10′+, add 10–15% for the extra air volume. Open floor plans act like one big zone—great for comfort if ducts are balanced, but they make short cycling more likely if you oversize.
Orientation matters, too. South/west-facing homes see more winter sun; you can reduce load ~10%. North-facing or shaded homes often need ~10% more. Basements? Conditioned basements usually lower whole-home heat loss; uninsulated crawl spaces do the opposite. When you’re close between two sizes, these layout cues help you choose correctly.
Ductwork: leaks, insulation, and location (the hidden tax)
Leaky or uninsulated ducts can add 15–25% to real-world heating needs. A tight, well-balanced duct system lets a furnace meet its rating; a leaky attic trunk line won’t. If ducts run through attics or garages, insulate and seal before upsizing equipment.
Red flags I see in the field:
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Rooms far from the furnace run hot/cold compared to nearby rooms.
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Dusty supply boots or dirty lines in insulation (classic leak trails).
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Whistling returns, weak supplies, or banging ducts on startup.
If these show up, fix the distribution first. It’s cheaper than buying extra BTUs you’ll never feel. Need a second opinion? Reach out via Contact Us.
Manual J vs. rule-of-thumb (when to call a pro)
BTU per sq. ft. is a reliable starting point. Manual J is the exact way to size: it accounts for window specs, R-values, orientation, infiltration, and local weather data. For homes 3,000+ sq. ft., complex layouts, heavy glass, or comfort complaints, a Manual J usually pays for itself by preventing a mis-size.
What we recommend: run the chart, then get a Manual J if you’re within one size either way, or if you plan big envelope changes (windows/insulation). You can share details through Quote by Photo and our team will advise whether a quick check or full calc makes sense. Prefer a guided path? Start in our Design Center.
Oversized vs. undersized: what really happens
Oversized furnaces heat fast and shut off fast—called short cycling. That leads to temperature swings, higher bills (startup is inefficient), premature wear, and poor air mixing. You’ll also get worse humidity control and more noise. Heat exchangers that never see steady-state are more likely to crack early.
Undersized furnaces run non-stop on cold snaps and still may not hold setpoint. That can shorten blower life and raise bills, too. Aim to be within +20% / −10% of the Manual J or your adjusted chart number. Two-stage or modulating units can mask mild oversizing, but they won’t fix big mistakes.
Choosing the nearest furnace size (AFUE & staging)
Manufacturers build furnaces in ~15k–20k BTU increments. Pick the size closest to your target, not automatically the bigger one. Also consider AFUE: a 95% AFUE furnace delivers more usable heat output than an older 80% unit at the same input. When replacing, re-check sizing—many homes can drop a size thanks to higher efficiency and envelope upgrades.
Staging matters:
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Single-stage: on/off—fine if sizing is spot-on.
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Two-stage: longer low-fire cycles, quieter, better comfort if you’re near the high end of a size.
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Modulating: best comfort across wide swings (big glass, mixed exposures).
Ready to compare options? Browse Furnaces and, if useful, explore HVAC financing to plan the project.
FAQ: Furnace sizing quick answers
What size furnace do I need for 2,000 sq. ft.?
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Zone 1: ~60k–70k BTU
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Zone 3: ~80k–90k BTU
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Zone 5: ~100k–120k BTU
Adjust for insulation (±20–30%), ceilings (+10–15% for 10′+), and ducts (+15–25% if leaky).
Is 100,000 BTU enough for 3,000 sq. ft.?
It depends on the climate and house. In Zone 1, yes (90k–105k). In Zone 3, likely undersized (needs 120k–135k). In Zone 5, definitely undersized (150k–180k).
Do high ceilings change the size?
Yes. Above 10′, add 10–15%. Big open plans also favor two-stage or modulating furnaces for steadier temperatures.
My windows are old how much does that matter?
A lot. Expect ~1,000 BTU per inefficient window added to the load. Consider replacing or improving air sealing; otherwise choose the higher end of your range.
How close should I be to the “perfect” size?
Within +20% / −10% of the target is the sweet spot. Pick the nearest available model size—don’t automatically bump up.
Do I need a Manual J?
Highly recommended for 3,000+ sq. ft., unusual layouts, lots of glass, or if past equipment struggled. Start a request in the Design Center or send a Quote by Photo.
Does AFUE affect sizing?
Yes. Higher AFUE means more output per input. A 95% furnace with the same input provides more usable heat than an 80% unit, so don’t size only by input BTU.
Where can I get help or shop models?
Check our Sizing Guide, browse or contact support via the Help Center.
