Furnace Size Calculator
Ballpark the furnace a house needs. Enter heated square footage, pick your US climate zone and insulation quality, choose the furnace efficiency (AFUE), and get required heat output in BTU/h plus the matching nominal furnace size.
Example: with Heated floor area (sq ft) 1800 · Climate zone Zone 3 — moderate (Mid-Atlantic, lower Midwest): 40–45 · Insulation / air sealing Average · Furnace efficiency (AFUE) 95% — high efficiency → Required heat output: 75,600 BTU/h.
- Furnace input rating79,579 BTU/h at 95% AFUE
- Suggested nominal size80,000 BTU input furnace
Computed by the calculator below using its default values. Change any input to see your own numbers.
Rule-of-thumb sizing: square footage × 30–60 BTU depending on climate. Furnaces are sold by input BTU, so divide output by AFUE.
How the rule-of-thumb sizing works
The screening method HVAC contractors quote from is BTU per square foot, scaled by climate: about 30–35 BTU/sq ft along the Gulf Coast, 40–45 in the moderate middle of the country, and 50–60 in the coldest northern states. An 1,800 sq ft house in a moderate zone needs roughly 1,800 × 42 = 75,600 BTU/h of heat output. Insulation shifts that meaningfully — tight new construction can run 10% less, a drafty older house 10% more.
Furnaces are sold by input BTU, not output. A 95% AFUE unit turns 80,000 BTU of gas input into about 76,000 BTU of delivered heat, so the required output divides by the AFUE to pick the box on the shelf. Nominal sizes step in 20,000 BTU increments: 40, 60, 80, 100, 120k.
Why oversizing backfires
A furnace that is too big short-cycles: it slams the house up to temperature, shuts off, and repeats. That means bigger temperature swings, more wear on the igniter and blower, worse dehumidification, and lower real-world efficiency. If your estimate lands between sizes, the smaller unit with good insulation upgrades usually beats the bigger one. For a real installation, have a contractor run a Manual J load calculation — it accounts for windows, orientation, infiltration, and duct losses that no square-footage rule can see.
How it’s calculated
Required output = area × zone factor × insulation modifier, with zone factors of 32/38/42/48/55 BTU per sq ft (midpoints of the widely published 30–60 BTU/sq ft climate bands) and insulation modifiers 0.9/1.0/1.1. Input rating = output ÷ AFUE. Nominal size rounds input up to the next 20,000 BTU step.
A square-footage rule is a screening estimate only — ceiling height, windows, air leakage, and ductwork move real loads ±25%, so confirm with a Manual J calculation before buying equipment.
Heating BTU per square foot by climate zone
| Zone | Example areas | BTU per sq ft |
|---|---|---|
| 1 — Hot | Florida, Gulf Coast, south Texas | 30–35 |
| 2 — Warm | Georgia, coastal Carolinas, Arizona | 35–40 |
| 3 — Moderate | Virginia, Missouri, Kentucky | 40–45 |
| 4 — Cool | New York, Chicago, Denver | 45–50 |
| 5 — Cold | Minnesota, North Dakota, Montana | 50–60 |
Industry rule-of-thumb sizing bands published in HVAC trade guidance; Manual J (ACCA) is the code-recognized method.
Common mistakes
- Matching the old furnace's input BTU when jumping from 80% to 95% AFUE — you get 19% more heat than the house needs.
- Using total square footage including unheated garage or basement space.
- Oversizing 'to be safe', which causes short cycling, temperature swings, and premature wear.
- Ignoring ceiling height: these factors assume 8-foot ceilings, so a great room with 12-foot ceilings carries roughly 50% more volume to heat.
Frequently asked questions
What is the furnace sizing formula?
Required output BTU/h = square footage × climate factor (30–60 BTU/sq ft) × insulation modifier. Then furnace input = output ÷ AFUE. An 1,800 sq ft moderate-climate house: 1,800 × 42 ≈ 75,600 output, which is an 80,000 BTU input unit at 95% AFUE.
What size furnace do I need for 2,000 square feet?
Roughly 60,000–70,000 BTU output in a warm climate, 84,000 in a moderate one, and up to 110,000 in the coldest zones — before adjusting for insulation. That maps to 80k–120k BTU input furnaces depending on efficiency.
What does AFUE mean?
Annual Fuel Utilization Efficiency — the share of fuel energy that becomes usable heat over a season. A 95% AFUE furnace wastes 5% up the flue. Furnaces are labeled by input BTU, so output = input × AFUE.
Is bigger safer for a furnace?
No. Oversized furnaces short-cycle, which wears parts, swings temperatures, and can shorten heat-exchanger life. Size to the load, not above it.
Should I trust this over a contractor quote?
Use it to sanity-check quotes, not replace them. A proper Manual J load calculation measures your actual windows, insulation, and air leakage; if a contractor sizes only by square footage, ask for the Manual J.