Furnace Size Calculator
The single most common furnace-buying mistake is confusing input with output. Furnaces are advertised by their input rating — the "80,000 BTU furnace" on the box is 80,000 BTU/hour of gas going in. What actually heats your home is the output, which is input times the AFUE efficiency: that same 80,000 BTU furnace at 95% AFUE only delivers about 76,000 BTU/hour. This calculator sizes by output — it figures the heat your home loses, then tells you the input rating to shop for so the output covers it, applying the ACCA Manual S rule that a furnace should be no more than about 140% of the load.
It runs on the same published physics as a professional load calc — conduction (U-value times area times the design temperature difference) plus air infiltration (1.08 times the leakage airflow times the temperature difference), with a duct-loss adder for ductwork in unconditioned space — using the ASHRAE 99% winter design temperature for your climate zone. Because heating ignores solar and internal gains, the heating load is simpler than a full cooling calc, which is why a focused furnace tool can give a solid screening number from a handful of inputs. It also handles the details that trip people up: electric-furnace kilowatts, the higher airflow a big furnace needs, and the capacity gas furnaces lose at high altitude.
What it is not is a substitute for a real Manual J. It is a screening estimate to help you sanity-check a contractor’s proposal, right-size before you shop, and understand why a bigger furnace is usually a downgrade — an oversized unit short-cycles, blows cooler air, and wears out faster. For the cooling side and equipment tonnage, pair it with the Manual J load calculator. A full ACCA Manual J and Manual S by a professional are still required for a permit and the final install. Free, no signup — part of the HVAC and ventilation section.
Furnace Size Calculator
What size furnace do you need? This free calculator estimates your home’s heating load from its size, climate, insulation, and air leakage — the way a Manual J does — then converts it to a furnace size in BTU/hour input and output, or kilowatts for electric. It shows the load-based size next to the “50 BTU per square foot” rule of thumb so you can see how much rules of thumb oversize, and it untangles the input-vs-output rating that trips up most buyers. Screening estimate — a full Manual J is still required for a permit. Free, no signup.
Your home
Conditioned floor area, ceiling height, and stories. The heating load is estimated from the envelope, not from a rule of thumb.
Insulation & construction
These set the envelope U-values. Air tightness and duct location are the biggest swing factors in a heating load — ducts in an unconditioned attic add 15–30%.
Furnace
Pick the fuel and, for gas, the efficiency. Furnaces are rated by input; the output that actually heats your home is input × AFUE.
Calculation Formulas
Heat lost through each surface (walls, ceiling, floor, windows, doors) equals its U-value times area times the indoor-to-outdoor design temperature difference. Manual J excludes solar and internal gains from the heating load, which keeps it conservative.
Example:
1,300 ft² of R-11 wall (U ≈ 0.09) at a 68°F design ΔT = 0.09 × 1,300 × 68 ≈ 7,960 BTU/h.
Indoor design is 70°F. The outdoor value is the ASHRAE 99% winter dry-bulb — the temperature your area stays above 99% of the time — not the record low, so the furnace is sized for a normal cold day, not a once-a-decade extreme.
Example:
Chicago (Zone 5) 99% design ≈ 2°F, so ΔT = 70 − 2 = 68°F.
Cold air leaking in must be heated. The 1.08 combines air density, specific heat, and 60 min/h. Winter air changes run higher than summer because of stack effect, so a winter multiplier is applied.
Example:
0.56 ACH in a 16,000 ft³ house ≈ 149 CFM; at ΔT 68°F, 1.08 × 149 × 68 ≈ 10,950 BTU/h.
Furnaces are sold by input rating, but only the output heats your home. A 95% AFUE furnace turns 100,000 BTU/h of gas input into 95,000 BTU/h of heat. Always size so the OUTPUT covers your heating load.
Example:
An 80,000 BTU/h input furnace at 95% AFUE delivers 80,000 × 0.95 = 76,000 BTU/h of heat.
Manual S allows heating equipment from 100% up to about 140% of the calculated load. Bigger than that short-cycles and hurts comfort and efficiency, so you pick the smallest standard size that covers the load within that window.
Example:
A 41,000 BTU/h load allows up to ~57,000 BTU/h of output; a 45,000-input × 95% (≈42,750 output) furnace fits.
Electric resistance is 100% efficient at the unit, so input equals output. One kilowatt delivers 3,412 BTU/h. Running cost is high, so a heat pump with electric backup is usually cheaper to operate.
Example:
A 22,000 BTU/h load ÷ 3,412 = 6.5 kW, so a 10 kW electric furnace is the smallest standard size.
The blower must move enough air to carry the furnace output at the furnace’s design temperature rise (typically 35–65°F). This is a quick check that the ductwork can handle the airflow.
Example:
42,750 BTU/h output ÷ (1.08 × 50°F) ≈ 790 CFM.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Sensible air multiplier | 1.08 | 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/h (standard air). |
| Indoor heating design temp | 70 °F | ACCA Manual J default heating setpoint. |
| BTU per kilowatt | 3,412 | Electric resistance heat: 1 kW = 3,412 BTU/h at 100% efficiency. |
| Manual S heating limit | ≤ ~140% | Furnace output up to about 140% of the heating load. |
| AFUE range | 80–98% | Standard (80%) to condensing high-efficiency (95–98%) gas furnaces. |
| Design temperature rise | ~50 °F | Typical furnace air-temperature rise (label range 35–65°F) used for blower CFM. |
| Altitude derate (gas) | ~4% / 1,000 ft | Above 2,000 ft, gas furnaces lose input capacity (IFGC) and need a larger input. |
| Winter infiltration bump | × 1.4 | Winter air changes run higher than summer from stack effect. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
ACCA Manual J — Residential Load Calculation(ANSI/ACCA 2-2016 (8th ed.))
View StandardThe standard for the heating load a furnace must meet. Required for most permits. This tool uses a simplified whole-house version of the Manual J heating method.
Key Requirements:
- •Use the ASHRAE 99% heating design temperature for the location
- •Account for every envelope surface plus infiltration and duct losses
ACCA Manual S — Equipment Selection(ANSI/ACCA 3)
View StandardMatches the furnace to the load and limits oversizing. Heating equipment is allowed up to roughly 140% of the calculated load; beyond that, short-cycling degrades comfort and efficiency.
Key Requirements:
- •Furnace output ≥ 100% and ≤ ~140% of the heating load
- •Confirm blower airflow suits the ductwork
AHRI Furnace Ratings (AFUE)(AHRI / DOE test procedure)
View StandardAnnual Fuel Utilization Efficiency (AFUE) is the ratio of heat output to fuel input over a season. Furnaces are marketed by input BTU/h; output = input × AFUE. Federal minimums are 80% (some regions 90%+).
Key Requirements:
- •Size by OUTPUT (input × AFUE), not the nameplate input
- •Condensing furnaces (90%+ AFUE) require a condensate drain and different venting
International Fuel Gas Code — altitude(IFGC / NFPA 54)
View StandardGas appliances lose input capacity at altitude (roughly 4% per 1,000 ft above 2,000 ft), so a larger input rating — or a high-altitude conversion kit — is needed in the mountains.
Key Requirements:
- •Derate gas input above 2,000 ft elevation
- •Verify the specific derate and any conversion kit with the manufacturer
International Energy Conservation Code(IECC 2021 R402 / R403.7)
View StandardPrescriptive envelope U-factors by climate zone (used by the "current code" insulation tier) and the requirement that equipment be sized from a Manual J/S calculation.
Key Requirements:
- •Envelope U-factors vary by climate zone
- •Equipment sized per Manual S from a Manual J load (R403.7)
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Cold climates need real capacity
Zones 5–8
With design temperatures near or below 0°F, the ΔT is 60–100°F and heating loads are large. The furnace output must cover the design-cold morning — this is not the place to skimp, but it is also not a reason to grossly oversize.
Regional Examples:
Mild and hot climates need small furnaces
Zones 1–3
In warm climates the heating load is small, and an oversized furnace is especially wasteful and uncomfortable. A modest furnace — or a heat pump doing double duty — is usually the right call.
Regional Examples:
Altitude derates gas furnaces
Mountain West
Above 2,000 ft, gas furnaces lose roughly 4% of input capacity per 1,000 ft, so the required input rating rises. A high-altitude orifice/conversion kit is often required, and venting rules differ.
Regional Examples:
Fuel choice changes the math
All zones
Gas furnaces are cheap to run where gas is available; electric resistance is simple but expensive to operate. In most climates a heat pump (with electric or gas backup) beats a straight electric furnace on running cost — worth comparing before you buy.
Regional Examples:
Air sealing beats a bigger furnace
Older homes, all zones
In leaky older homes, infiltration can be 30–40% of the heating load. Air sealing and duct sealing shrink the load directly and are usually cheaper than upsizing equipment — a smaller, right-sized furnace in a tighter house outperforms a big one in a leaky house.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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How to Use This Calculator
- Enter your conditioned floor area, ceiling height, stories, and climate zone (the zone sets the winter design temperature).
- Choose your insulation level, window type, foundation, air tightness, and where the ducts run — these set how fast the house loses heat.
- Pick the fuel: a gas/propane furnace (then choose the AFUE efficiency) or an electric resistance furnace.
- For a gas furnace at altitude, enter your elevation so the capacity derate is applied.
- Click Calculate to get the heating load, the recommended furnace input and output (or kilowatts), and the blower airflow.
- Shop for a furnace whose OUTPUT covers your load — not the biggest input you can find — and compare against the rule-of-thumb size shown.
Input vs output, AFUE, and why bigger is worse
A furnace has two BTU numbers. Input is how much fuel energy it consumes; output is how much heat it actually delivers to your home, and output equals input times AFUE (Annual Fuel Utilization Efficiency). A 100,000 BTU input furnace at 80% AFUE delivers 80,000 BTU of heat; at 96% AFUE it delivers 96,000. Because the box quotes input, you have to convert to output to size correctly — this calculator does that for you and picks the smallest standard input whose output covers your heating load. The reason not to just buy big is short-cycling: an oversized furnace heats the air fast, satisfies the thermostat, and shuts off before the house evens out, so you get drafts, temperature swings, cooler-feeling supply air, and more wear on the heat exchanger and igniter. ACCA Manual S caps heating equipment at roughly 140% of the load for exactly this reason. Electric resistance furnaces are 100% efficient at the unit (1 kW = 3,412 BTU/hour) but expensive to run, so in most climates a heat pump with backup heat beats a straight electric furnace on operating cost. And remember this is a heating-only screening estimate: get a full ACCA Manual J and Manual S before you buy.
Frequently Asked Questions
What size furnace do I need?
Match the furnace's output to your home's heating load, not to its square footage. A typical 2,000-square-foot home ranges from roughly 40,000 BTU/hour of output in a mild climate to 60,000 or more in a cold, leaky one. The 'input' number on the furnace box is not the heat you get — output equals input times the AFUE efficiency, so an 80,000 BTU input furnace at 95% AFUE delivers about 76,000. Enter your home's details here to get a load-based output, the input rating to shop for, and how it compares to the rule of thumb, then confirm with a full Manual J.
What size furnace for a 2,000 square foot house?
It depends far more on climate and insulation than on the 2,000 square feet. A well-insulated 2,000-square-foot home in a mild climate might need only 40,000 to 45,000 BTU/hour of furnace output, while a leaky older home the same size in Minnesota could need 70,000 or more. The old '40 to 50 BTU per square foot' rule would suggest an 80,000 to 100,000 BTU furnace, which is usually one to two sizes too big. This calculator estimates the actual load from your insulation, windows, air tightness, and climate so you don't oversize.
What's the difference between furnace input and output BTU?
Input is the fuel energy the furnace consumes; output is the heat it actually delivers to your rooms, and output equals input times AFUE. Furnaces are advertised by input — the '100,000 BTU furnace' burns 100,000 BTU/hour of gas — but at 95% AFUE it only delivers 95,000 BTU/hour of heat, and at 80% AFUE just 80,000. Always size by output: figure your heating load, then pick a furnace whose output covers it. Buying by the input number is how people end up with a furnace that's far larger than they need.
Is it bad to have an oversized furnace?
Yes. An oversized furnace short-cycles — it heats the air quickly, satisfies the thermostat, and shuts off before the house evens out. The result is uncomfortable temperature swings, drafts, supply air that feels cooler because runs are shorter, more wear on the heat exchanger and igniter, and lower real-world efficiency. ACCA Manual S caps heating equipment at about 140% of the calculated load for exactly this reason. Bigger is not a safety margin here; a right-sized furnace that runs longer, gentler cycles is more comfortable and lasts longer.
How many BTU furnace do I need per square foot?
Rules of thumb put it at roughly 30 to 35 BTU per square foot in warm climates and 50 to 60 in cold ones, but those numbers are deliberately conservative and usually oversize by two to three times because they ignore insulation, windows, air leakage, and the actual design temperature. A tight, well-insulated home can need half the rule-of-thumb figure. That's why this calculator estimates the real load from the envelope instead of multiplying by a fixed BTU-per-square-foot number — and shows you both so you can see the gap.
What size electric furnace do I need?
Electric resistance furnaces are rated in kilowatts, and one kilowatt delivers 3,412 BTU/hour at 100% efficiency (input equals output). Divide your heating load by 3,412 to get the kilowatts: a 22,000 BTU/hour load needs about 6.5 kW, so a 10 kW electric furnace is the smallest standard size. Electric furnaces are simple and cheap to install but expensive to run, so in most climates a heat pump with electric backup costs far less to operate — worth comparing before you commit to resistance-only heat.
Does altitude affect furnace sizing?
Yes, for gas furnaces. Above about 2,000 feet, a gas furnace loses roughly 4% of its input capacity per 1,000 feet of elevation because there's less oxygen to burn the fuel, so you need a larger input rating — or a high-altitude conversion kit — to deliver the same output. At Denver's 5,300 feet that's about a 13% derate; at 7,000 feet it's over 20%. Enter your elevation and this calculator bumps the required input accordingly. Electric furnaces are unaffected by altitude. Always confirm the specific derate and any required kit with the manufacturer.