1,600 sq ft, zone 4, average envelope
8 ft ceilings, average insulation and tightness.
Often near 70,000–85,000 BTU/h load and 80k–100k input on 80% AFUE — confirm with Manual J.
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Furnace Sizing Calculator
Square footage, climate zone, and envelope quality → heating BTU range and nominal furnace input with a Manual J caution.
Heating load
54,000 BTU/h
Range 32,500–84,500 BTU/h · Zone 4 — Washington DC, Nashville, St. Louis
- 80% AFUE nominal input
- 70,000 BTU/h
- 95% AFUE nominal input
- 60,000 BTU/h
Get a Manual J before you buy
ACCA Manual J (or equivalent) is the permit-grade load calculation. This screening estimate cannot replace it — use the numbers to sanity-check contractor proposals.
Shareable summary
Quick answer
Furnace size is driven by heating load in BTU/h, not square footage alone — a typical 1,800 sq ft home in IECC zone 4 often lands near 60,000–90,000 BTU/h furnace input after envelope adjustments, but ACCA Manual J is required before buying equipment. Use the form below for a screening range and insist on Manual J on the quote.
What this assumes
- Uses climate-zone heat factors and envelope multipliers — same screening family as the HVAC sizing tool.
- Nominal furnace sizes round up to common catalog inputs (40k, 60k, 80k, 100k, 120k BTU/h).
- Does not model duct losses, altitude, or ventilation load.
Important
Furnace input must be verified with ACCA Manual J or equivalent before permit and equipment order. Oversized furnaces short-cycle, waste fuel, and deliver uneven comfort.
Understanding the result
Output versus input
Furnace labels show input BTU/h. Output equals input × AFUE. An 80% furnace with 100,000 BTU/h input delivers about 80,000 BTU/h to the house.
Why square footage alone fails
Windows, insulation, infiltration, and ceiling height swing load by 30% or more on the same footprint. This tool adjusts for those factors but still replaces — not replaces — Manual J.
How the math works
Heating BTU/h = sq ft × zone heat factor × ceiling height factor × insulation × tightness. Furnace input = load ÷ AFUE (80% or 95%), rounded up to catalog nominal input.
Where to find each input
- Conditioned floor area — assessor or measure
- Climate zone — IECC map
- Ceiling height — weighted average
- Insulation quality — attic/wall judgment vs neighbors
- Air tightness — blower door if available, else estimate
Worked examples
2,400 sq ft, zone 6, poor insulation
9 ft ceilings, poor windows.
Load band widens upward — risk of oversizing if contractor uses rule-of-thumb only.
1,200 sq ft tight cottage, zone 3
Good insulation, tight construction.
Lower nominal furnace — may fit 40k–60k input class on 95% equipment.
What this does not include
- Manual J report
- Duct design
- Combustion air
- Altitude correction
Regional adjustment
Design temperatures vary by city within a zone — local Manual J uses county design days.
Accuracy and references
ACCA Manual J intent; IECC zone heat factors used in screening tools.
Counterpart bridge
Pricing this on the contractor side?
This page interprets quotes and budgets from the buyer’s chair. The matching pro tool uses the same scope logic to build a defensible price.
Open HVAC Sizing CalculatorFor trade owners
Pricing this job as a contractor?
Homeowners see national ranges on this page. If you are building the estimate, use the same scope assumptions — then price to burdened labor, overhead recovery and a real margin.
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When you are ready to hire for furnace sizing calculator, start with licence verification — not with whoever answered first.
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Rankings are never sold. Order reflects verification depth and relevance, not payment.
Verify any licence yourself before signing: state licensing hub.
Common questions
What size furnace do I need for 2000 sq ft?
Depends on climate zone and envelope — often 60,000–100,000 BTU/h input in zone 4 with average insulation. Use the calculator with your zone and home details.
What is Manual J?
ACCA Manual J is the standard residential heating and cooling load calculation. Permits and reputable HVAC contractors rely on it.
80% vs 95% AFUE — which size?
Higher efficiency needs less input BTU for the same output. Compare both nominal lines in the result.
Can my furnace be too big?
Yes. Oversizing causes short cycling, humidity issues, and higher wear. Stay within the calculated band unless Manual J says otherwise.
Do I count the basement?
Include only conditioned floor area connected to the duct system.
How do ceiling height and insulation affect size?
Taller ceilings and poor insulation raise BTU/h requirements. Tight, well-insulated homes need less.
What about a heat pump?
Heating load in BTU/h is similar; equipment type changes efficiency and backup heat — see HVAC sizing for dual-fuel context.
Who should run the real calculation?
A licensed HVAC contractor or engineer using Manual J software tied to your zip code design temperatures.
Is this a quote from a contractor?
No — it is a planning band from published medians and engineering rules. Always collect written, itemized bids.
Can I save my results?
Use Print / save PDF on the shareable card or save to your homeowner account when signed in.
Why does my metro change the number?
Labor and permit costs vary by market. We apply a regional cost index from our metro table.
What vintage are the cost ranges?
2024–2026 residential medians cross-checked with True Trade Path cost guides and RSMeans-style repair data.
Does this include tax?
Unless your quote says otherwise, assume sales tax may apply to materials in your state.
Should I get three quotes?
Yes — three comparable scopes are the standard check before you sign.
What if my house is unusual?
Historic homes, steep sites, and commercial-grade equipment need site-specific quotes — treat this as a screen.
Where is the pro counterpart?
See the counterpart bridge at the bottom — contractors use the paired tool to build defensible pricing.
You may also need
The next decision usually follows directly from this one.
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- CalculatorDuct Size CalculatorSize round and rectangular duct from airflow and friction rate, with velocity checks.
- ComparisonRepair or Replace CalculatorCompare the economics of repairing a failing system against replacing it outright.
- Cost guideFurnace Replacement CostReplacing a gas or propane furnace while keeping the existing air conditioning system.