# Furnace Size Calculator

URL: https://speedycalculator.com/calculators/construction/furnace-size/
Last updated: 2026-10-25

> The furnace size calculator estimates the heating output a furnace needs from the home's floor area and climate zone, adjusted for insulation, and converts it to the input rating for the furnace efficiency (AFUE).

## Inputs

- Heated floor area (ft², m², yd², acre)
- Climate zone
- Insulation
- Furnace efficiency (AFUE) (%)

## Results

- Output needed – low
- Output needed – high
- Recommended output
- Furnace input rating

## Formula

```
Output (BTU/h) = floor area × BTU per ft² for the climate zone × insulation factor
Input rating = output ÷ AFUE
```

## Explanation

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

ZoneBTU/ft²130–35235–40340–45445–55555–70670–80780–90890–1009100–110

### 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

FieldWhat to enterHeated floor areaUnits: ft², m², yd², acreClimate zoneChoose 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) …InsulationChoose 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)

## Worked 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.

## FAQ

**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.


## Sources

- [ACCA Manual J / Manual S](https://www.acca.org/standards/technical-manuals)