How many BTU do I need per square foot?

Between 30 and 60 BTU per square foot, set mostly by climate: about 30–35 in the hot South, 35–45 in the lower-middle states, 45–50 in the upper-middle, and 50–60 in the cold North. Multiply by your heated square footage for a ballpark, then treat it as the number you sanity-check a contractor against — not the number you buy from.

Is it better to oversize or undersize a furnace?

Neither, but oversizing is the more common and more expensive mistake. An oversized furnace short-cycles: it heats the thermostat's location quickly, shuts off, and repeats. That gives you uneven temperatures, more wear on the ignitor and blower, worse humidity control, and lower efficiency than the AFUE label implies. Undersizing leaves you cold on the coldest days. Correct sizing beats both, and it is what Manual J produces.

Does furnace size depend on AFUE?

Yes, and people miss this. Furnace capacity is usually advertised as input BTU, but what heats your house is output BTU — input multiplied by AFUE. An 80,000 BTU furnace at 80% AFUE delivers 64,000 BTU of heat; at 96% AFUE it delivers 76,800 BTU. If you replace an old 80% unit with a high-efficiency model, you often need a smaller input rating to get the same heat.

What Size Furnace Do I Need? BTU Sizing by Home and Climate

Updated 2026-09-15 · 8 min read

Jump to a section▾

Most homes need 30 to 60 BTU per square foot, set mainly by climate zone. That puts a 2,000 sq ft house somewhere between 60,000 and 120,000 BTU — a range wide enough to tell you the rule of thumb is a starting point, not an answer.

Floor area is the weakest of the six inputs that actually set heating load. A tight, well-insulated 2,000 sq ft house in Ohio and a leaky 1970s 2,000 sq ft house in Minnesota need completely different furnaces. Use the table below to know whether a contractor's number is sane, then get the calculation that produces the real one.

How many BTU per square foot by climate zone?

Region	Climate	BTU per sq ft	1,500 sq ft	2,000 sq ft	2,500 sq ft
Hot South (FL, TX Gulf, AZ)	Zone 1–2	30–35	45,000–52,500	60,000–70,000	75,000–87,500
Lower middle (GA, TN, NC, OK)	Zone 3	35–45	52,500–67,500	70,000–90,000	87,500–112,500
Upper middle (OH, PA, MO, CO)	Zone 4–5	45–50	67,500–75,000	90,000–100,000	112,500–125,000
Cold North (MN, WI, ND, ME)	Zone 6–7	50–60	75,000–90,000	100,000–120,000	125,000–150,000

These are input BTU figures, which is how furnaces are sold. What actually reaches your rooms is smaller — see the AFUE section below.

Input BTU vs output BTU — the mistake that changes the size

Furnace capacity is advertised as input BTU: the fuel energy going in. What heats the house is output BTU: input multiplied by the furnace's AFUE efficiency rating.

Input rating	At 80% AFUE	At 96% AFUE
60,000 BTU	48,000 BTU out	57,600 BTU out
80,000 BTU	64,000 BTU out	76,800 BTU out
100,000 BTU	80,000 BTU out	96,000 BTU out
120,000 BTU	96,000 BTU out	115,200 BTU out

The practical consequence: replacing an old 80% furnace with a 96% model usually means a smaller input rating, not the same one. Matching input-for-input is how homes end up meaningfully oversized after an efficiency upgrade. More on the rating itself in AFUE and furnace efficiency explained.

What actually sets your heating load

Square footage is one input of six, and usually not the dominant one:

Climate zone — the single biggest factor, and the reason the table above spans a 2× range.
Insulation levels — attic, wall and floor R-values. A deep-attic retrofit can cut heating load more than any equipment change.
Air leakage — an old leaky house can need 30–40% more heat than an identical tight one. This is why weatherstripping and draft proofing pays before equipment does.
Window area, type and orientation — single vs double pane, and how much glass faces north.
Ceiling height — heating load follows volume, not floor area. Nine-foot or vaulted ceilings break the per-square-foot rule outright.
Home layout and exposure — a detached house loses heat on four walls; a mid-row townhouse loses it on two.

Get a Manual J calculation

The industry-standard method is an ACCA Manual J load calculation. It accounts for every factor above, room by room, and produces a design heating load in BTU for your specific house at your local design temperature.

A contractor who sizes your furnace from square footage alone, or by matching the rating on the old unit, is guessing. The old unit was very likely oversized too — that is the norm in existing housing stock — so matching it perpetuates the error.

Ask for the Manual J output. A reputable HVAC contractor will run one and show you the result; several offer it free with a replacement quote. If you also want ductwork sized properly, that is Manual D, and equipment selection against the Manual J result is Manual S.

Why oversizing is the expensive mistake

An oversized furnace does not heat your house better. It short-cycles: it fires, satisfies the thermostat's immediate area quickly, shuts off, and repeats — often many times an hour.

The consequences compound:

Uneven temperatures. Rooms far from the thermostat never catch up before the burner shuts off.
Component wear. Ignition and blower start-up is the hardest part of a cycle; more cycles means shorter equipment life.
Worse real efficiency. Cycle losses mean you do not get the AFUE on the label.
Poor humidity control and more noticeable temperature swings.
More money spent up front on a bigger unit, and sometimes on the bigger ductwork it demands.

Undersizing is real too — the furnace runs continuously and still loses ground on the coldest nights — but it is far rarer, because the industry's habit is to round up. See oversized HVAC problems for the full picture.

Does a furnace sizing rule work for heat pumps?

No, and do not reuse this table for one. Heat pump capacity is rated in tons or BTU of heat-pump output, and it varies with outdoor temperature — capacity falls as it gets colder, exactly when you need more. Heat pumps are also usually sized against the cooling load and then checked against heating, with supplemental heat covering the gap.

That is a different calculation: see heat pump sizing in BTU, and heat pump vs furnace if you are deciding between them.

The bottom line

Use 30–60 BTU per square foot by climate to get a ballpark and to sanity-check a quote. Then insist on a Manual J calculation before buying, and remember that output BTU is input × AFUE — so a high-efficiency replacement usually needs a smaller input rating than the unit it replaces.

If a contractor sizes from floor area alone or matches the old unit's badge, get another quote.

Next: AFUE and furnace efficiency, what replacement costs in furnace installation cost, the cooling-side equivalent in what size air conditioner do I need, or browse all heating and cooling guides.

Frequently asked questions

As a starting estimate, 60,000 to 120,000 BTU depending on your climate zone — roughly 60,000–80,000 BTU in the mild South, 80,000–100,000 BTU in the middle of the country, and 100,000–120,000 BTU in the cold North. That range is deliberately wide because square footage is the weakest of the six inputs that set heating load. A Manual J calculation on a well-insulated 2,000 sq ft home often comes back far lower than the rule of thumb suggests.

Ask AI about this

Open an AI assistant with a question grounded in this page.

ChatGPT Claude Perplexity Google AI Grok