A properly sized furnace delivers 30–60 BTU per square foot of heating output, depending on your climate zone and home construction — meaning a 2,000 sq ft home in a cold climate (Zone 5–6) needs roughly 80,000–120,000 BTU of input capacity. Oversized furnaces are the #1 installation mistake in the industry, causing short cycling, uneven heating, excessive noise, and wasted energy.
This guide covers the complete sizing process: climate zone analysis, square footage calculations, the critical difference between input and output BTU, and the 10 adjustment factors that separate a rough guess from an accurate load calculation.
Interactive Furnace Sizing Calculator
Use our calculator to get a personalized furnace size recommendation based on your specific home:
Input BTU vs. Output BTU: The Number That Matters
Every furnace has two BTU ratings, and confusing them is the most common sizing error.
- Input BTU: Total fuel energy the furnace burns per hour
- Output BTU: Actual heat delivered to your home after efficiency losses
The relationship: Output BTU = Input BTU × AFUE
| Furnace Type | AFUE Rating | Input BTU | Output BTU |
|---|---|---|---|
| Standard efficiency (old) | 80% | 100,000 | 80,000 |
| Standard efficiency (2026 min) | 80% | 80,000 | 64,000 |
| Mid-efficiency | 90% | 80,000 | 72,000 |
| High-efficiency condensing | 95% | 80,000 | 76,000 |
| Premium condensing | 97–98% | 80,000 | 77,600–78,400 |
Always size by output BTU. When a Manual J calculation says your home needs 60,000 BTU of heating, that's output BTU. If you're installing a 95% AFUE furnace, you need an input rating of 60,000 ÷ 0.95 = 63,158 BTU — so a 60,000 or 80,000 BTU input furnace (standard sizes) would work. If you size by input BTU and ignore efficiency, you'll overshoot by 20% with a high-efficiency unit.
Furnace Sizing Chart by Square Footage and Climate Zone
This chart shows recommended input BTU for a furnace with 95% AFUE. Adjust for your actual efficiency rating.
| Home Size (sq ft) | Zone 3 (Warm) | Zone 4 (Mixed) | Zone 5 (Cool) | Zone 6 (Cold) | Zone 7 (Very Cold) |
|---|---|---|---|---|---|
| 800 | 30,000 | 40,000 | 45,000 | 55,000 | 65,000 |
| 1,000 | 40,000 | 45,000 | 55,000 | 65,000 | 80,000 |
| 1,200 | 45,000 | 55,000 | 65,000 | 80,000 | 90,000 |
| 1,500 | 55,000 | 65,000 | 80,000 | 90,000 | 110,000 |
| 1,800 | 60,000 | 80,000 | 90,000 | 110,000 | 120,000 |
| 2,000 | 65,000 | 80,000 | 100,000 | 120,000 | 135,000 |
| 2,500 | 80,000 | 100,000 | 120,000 | 135,000 | 160,000 |
| 3,000 | 100,000 | 120,000 | 135,000 | 160,000 | 180,000+ |
| 3,500 | 110,000 | 135,000 | 160,000 | 180,000+ | 200,000+ |
| 4,000 | 120,000 | 160,000 | 180,000+ | 200,000+ | 2 units |
Assumes average insulation, 8-foot ceilings, and standard window area. Apply adjustment factors below for accuracy.
Standard residential furnace sizes (input BTU): 40,000 / 60,000 / 80,000 / 100,000 / 120,000 / 140,000. Furnaces come in fixed sizes, so you'll round to the nearest available model. When between sizes, round up by no more than 15% — larger jumps indicate oversizing.
Heating Load Calculation: Step-by-Step
Step 1: Determine Your Design Temperature Difference
Your furnace must maintain indoor temperature (68–72°F) when outdoor temperature hits the local "design temperature" — the coldest 1% of winter hours.
| City | Winter Design Temp (°F) | Design ΔT from 70°F |
|---|---|---|
| Miami, FL | 47°F | 23°F |
| Houston, TX | 29°F | 41°F |
| Atlanta, GA | 22°F | 48°F |
| Nashville, TN | 14°F | 56°F |
| St. Louis, MO | 6°F | 64°F |
| Chicago, IL | −3°F | 73°F |
| Denver, CO | 1°F | 69°F |
| Boston, MA | 7°F | 63°F |
| Minneapolis, MN | −12°F | 82°F |
| Fargo, ND | −18°F | 88°F |
| Anchorage, AK | −15°F | 85°F |
Step 2: Calculate Base Heating Load
Base heating load (BTU/hr) = Square footage × Design ΔT × Heat Loss Factor
The heat loss factor depends on your home's construction:
| Construction Quality | Heat Loss Factor (BTU/hr per sq ft per °F ΔT) |
|---|---|
| Poor (pre-1970, no insulation) | 0.9–1.1 |
| Below average (pre-1990, some insulation) | 0.7–0.9 |
| Average (1990–2010, code insulation) | 0.5–0.7 |
| Good (2010–2020, above-code insulation) | 0.4–0.5 |
| Excellent (2020+, high-performance envelope) | 0.25–0.4 |
| Passive house standard | 0.10–0.20 |
Step 3: Apply Adjustment Factors
The same factors that affect cooling loads also affect heating loads, with some differences:
| Factor | Heating Adjustment | Notes |
|---|---|---|
| Ceiling height (9 ft) | +12% | More volume to heat |
| Ceiling height (10 ft) | +25% | — |
| Ceiling height (12 ft) | +50% | — |
| Many/large windows | +10–20% | Heat loss through glass |
| Poor air sealing | +15–25% | Cold air infiltration |
| Ductwork in attic/crawl | +15–25% | Heat lost before reaching rooms |
| Ducts in conditioned space | −10% | No duct losses |
| Above-average insulation | −10–20% | Less heat transfer |
| Basement (heated) | +10–15% of basement sq ft | Below-grade heat loss |
| Slab on grade | +5% | Edge losses |
| Fireplace (no damper) | +10% | Acts as a chimney for warm air |
| Attached garage (unheated) | +5% | Shared wall heat loss |
| South-facing glass | −5% | Solar gain helps in winter |
Worked Sizing Examples
Example 1: 1,600 sq ft ranch in Chicago, IL
- Design ΔT: 73°F (outdoor −3°F to indoor 70°F)
- Construction: Average (1995 build, code insulation) → heat loss factor 0.6
- Base load: 1,600 × 73 × 0.6 = 70,080 BTU/hr
- Adjustments: 9-ft ceilings (+12%) = 78,490 BTU/hr; average ductwork (0%) = 78,490; average windows (0%) = 78,490
- Needed output: ~78,500 BTU/hr
- With 95% AFUE: 78,500 ÷ 0.95 = 82,632 BTU input
- Recommended: 80,000 BTU input furnace (95% AFUE)
The 80,000 BTU unit delivers 76,000 output — 3% below the calculated load. On the coldest 1% of hours, the furnace runs near-continuously, which is acceptable. A 100,000 BTU unit would be oversized by 20%.
Example 2: 2,800 sq ft colonial in Minneapolis, MN
- Design ΔT: 82°F (outdoor −12°F to indoor 70°F)
- Construction: Below average (1985, some insulation added) → heat loss factor 0.75
- Base load: 2,800 × 82 × 0.75 = 172,200 BTU/hr
- Adjustments: 8-ft ceilings (0%); large windows on north side (+15%) = 198,030; leaky ductwork in attic (+20%) = 237,636; poor air sealing (+15%) = 273,281
- Needed output: ~273,000 BTU/hr
- This exceeds single-furnace capacity. Options: two furnaces (zoned system), massive insulation upgrades first, or a combination approach.
After recommended upgrades (seal ducts, add attic insulation, weather-strip windows): recalculated load drops to ~160,000 BTU/hr → two 80,000–100,000 BTU furnaces (one per floor) or a single 160,000+ BTU commercial-grade unit.
Example 3: 1,200 sq ft new-build condo in Denver, CO
- Design ΔT: 69°F (outdoor 1°F to indoor 70°F)
- Construction: Excellent (2026 code, R-21 walls, R-49 attic, triple-pane) → heat loss factor 0.35
- Base load: 1,200 × 69 × 0.35 = 28,980 BTU/hr
- Adjustments: 9-ft ceilings (+12%) = 32,458; ducts in conditioned space (−10%) = 29,212; minimal windows (+0%) = 29,212
- Needed output: ~29,000 BTU/hr
- With 95% AFUE: 29,000 ÷ 0.95 = 30,526 BTU input
- Recommended: 40,000 BTU input furnace (smallest available standard size)
Modern construction dramatically reduces heating loads. This new condo in Denver needs less heating capacity than a 1985 ranch half its size. The 40,000 BTU furnace will still be slightly oversized, but it's the smallest commonly available residential unit.
Two-Stage and Variable-Speed Furnaces: Sizing Advantages
Modern furnaces offer multi-stage and modulating operation that changes the sizing equation:
| Furnace Type | Operation | Oversizing Tolerance | Comfort Level |
|---|---|---|---|
| Single-stage | 100% on or off | ±10% of load | Good |
| Two-stage | Low fire (60–70%) / High fire (100%) | ±20% of load | Better |
| Modulating/variable | 40–100% capacity in 1% increments | ±30% of load | Best |
A modulating furnace rated at 100,000 BTU can fire as low as 40,000 BTU, effectively acting like a perfectly sized furnace for mild and extreme conditions alike. If you're between sizes, a modulating furnace lets you safely choose the larger option without the short-cycling penalties of a single-stage unit.
Best practice for 2026: Unless budget is the only constraint, choose at minimum a two-stage furnace. The low-fire stage runs longer, distributes heat more evenly, operates more quietly, and filters air better. Modulating furnaces cost $500–$1,200 more but deliver the best comfort and efficiency.
2026 Furnace Efficiency Standards
The DOE's minimum AFUE requirements for 2026:
| Region | Minimum AFUE (Non-Weatherized Gas Furnace) |
|---|---|
| Northern states (DOE North region) | 95% (condensing) |
| Southern states (DOE South region) | 80% (non-condensing allowed) |
As of 2026, the northern US requires condensing furnaces (95% AFUE minimum) for all new installations. This means PVC venting, condensate drains, and slightly higher upfront costs — but 15–20% lower fuel bills compared to the old 80% minimum.
| AFUE | Annual Gas Cost (100,000 BTU furnace, $1.20/therm) | Annual Savings vs 80% |
|---|---|---|
| 80% | $1,800 | — |
| 90% | $1,600 | $200 |
| 95% | $1,516 | $284 |
| 97% | $1,485 | $315 |
| 98% | $1,469 | $331 |
Based on 1,500 equivalent full-load hours (typical Zone 5 heating season).
Furnace Sizing Mistakes to Avoid
Mistake 1: Using the "Replace with the Same Size" Rule
Just because your old furnace was 120,000 BTU doesn't mean you need another 120,000 BTU unit. Old furnaces were routinely oversized by 50–100%. If your old unit short-cycled frequently (running 5–8 minutes then shutting off), it was almost certainly oversized. A proper load calculation for the replacement may reveal you only need 80,000 BTU.
Mistake 2: Sizing for the Absolute Worst-Case Day
Your furnace should handle the coldest 1% of winter hours — not the polar vortex record low. Designing for a once-in-50-years extreme event leads to massive oversizing for the other 99.9% of the heating season. If you're concerned about extreme cold events, choose a two-stage or modulating furnace that can handle the typical load on low fire and ramp up to high fire during extreme events.
Mistake 3: Ignoring Duct Design
A perfectly sized furnace paired with undersized or leaky ductwork performs like an undersized furnace. Your furnace can produce 80,000 BTU, but if only 60,000 BTU reaches your rooms, you have an effective 60,000 BTU system. ACCA Manual D duct design should accompany every furnace installation.
Mistake 4: Not Accounting for Home Improvements
If you plan to add insulation, replace windows, or seal your envelope within the next few years, size the furnace for the improved home, not the current one. A modulating furnace accommodates this well since it adjusts output to match the actual load.
Key Takeaways
- Size furnaces by output BTU (input × AFUE), not input BTU — a 95% efficient furnace delivers 5–20% more heat per input BTU than an 80% unit
- Use 30–60 BTU per square foot as a starting range, with climate zone being the biggest variable
- Oversizing causes short cycling, uneven heat, noise, and higher bills — a furnace that runs 15–20 minutes per cycle is correctly sized
- Two-stage and modulating furnaces tolerate sizing imprecision better than single-stage units
- Northern US now requires 95% AFUE minimum (condensing furnaces) for new installations in 2026
- Ductwork matters as much as furnace size — seal and insulate ducts before upgrading the furnace
Frequently Asked Questions
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