Furnaces are rated by input (the fuel they burn) and output (the heat they deliver); the ratio is the AFUE efficiency. A quick sizing method multiplies the floor area by a heat factor that grows from about 30 BTU/ft² in the hottest zones to 100+ BTU/ft² in the coldest. A Manual J heat-load calculation gives the definitive number.
How to use the Furnace Size Calculator
- Heated floor area
- Climate zone
- Insulation
- Furnace efficiency (AFUE)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the heated floor area, choose the climate zone and insulation level, and the furnace AFUE. The calculator gives the range of output needed, a mid-point and the matching input rating.
Heat factor by climate zone
| Zone | BTU/ft² |
|---|---|
| 1 | 30–35 |
| 2 | 35–40 |
| 3 | 40–45 |
| 4 | 45–55 |
| 5 | 55–70 |
| 6 | 70–80 |
| 7 | 80–90 |
| 8 | 90–100 |
| 9 | 100–110 |
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Heated floor area | Units: ft², m², yd², acre |
| Climate zone | Choose one: Zone 1 (30–35 BTU/ft²), Zone 2 (35–40 BTU/ft²), Zone 3 (40–45 BTU/ft²), Zone 4 (45–55 BTU/ft²) … |
| Insulation | Choose one: Average, Well insulated (−10%), Poor / drafty (+15%) |
| Furnace efficiency (AFUE) | In % |
What each result means
- Recommended output = ([Output needed – low] + [Output needed – high])÷2
- Furnace input rating = [Recommended output]÷([Furnace efficiency (AFUE)]÷100)
Formula
Output (BTU/h) = floor area × BTU per ft² for the climate zone × insulation factorInput rating = output ÷ AFUEWorked example
Inputs — Heated floor area: 2,000 ft²; Climate zone: Zone 5 (55–70 BTU/ft²); Insulation: Average; Furnace efficiency (AFUE): 95 %.
Results — Output needed – low = 110,000 BTU/h; Output needed – high = 140,000 BTU/h; Recommended output = 125,000 BTU/h; Furnace input rating = 131,579 BTU/h.
Frequently asked questions
What size furnace for a 2,000 ft² house?
Roughly 60,000–100,000 BTU/h output depending on climate and insulation.
What is AFUE?
Annual fuel utilisation efficiency – the share of fuel energy delivered as heat. A 95% furnace with 80,000 BTU/h input delivers 76,000 BTU/h.
Is a bigger furnace better?
No – oversized furnaces short-cycle, cause temperature swings and wear out sooner.
What formula does the Furnace Size Calculator use?
Output (BTU/h) = floor area × BTU per ft² for the climate zone × insulation factor.