← Home & Construction Calculators
HEATING LOADBTU/hAFUE
Estimate furnace input capacity from conditioned floor area, climate, ceiling height, insulation, exposure, efficiency and design margin.
Furnace sizing symbols
Heat the building needs, measured in BTU/h.
Heated floor area in square feet.
Planning BTU/h per square foot.
Fraction of fuel input delivered as seasonal useful heat.
Ceiling height compared with an 8-foot baseline.
Furnace size formulas
Base heat load = A × climate BTU/ft²
Adjusted load = base × (H ÷ 8) × insulation × exposure
Design load = adjusted load × (1 + margin ÷ 100)
Required furnace input = design load ÷ AFUE
The heat load is the useful output needed by the home. Furnace nameplates often list both input and output. A 90% AFUE furnace with 90,000 BTU/h input provides about 81,000 BTU/h of seasonal rated output.
Where a home loses heat
Heat escapes through ceilings, walls, windows, floors, doors and uncontrolled air leakage. A professional calculation uses local outdoor design temperature, indoor target temperature, insulation R-values, window U-factors, infiltration and each surface area.
Worked example
For a 2,000 ft² home in a cold climate, 8-foot ceilings, average insulation, typical exposure, 90% AFUE and a 10% margin:
Base = 2,000 × 35 = 70,000 BTU/h
Design load = 70,000 × 1.10 = 77,000 BTU/h
Required input = 77,000 ÷ 0.90 = 85,556 BTU/h
The next common input size is approximately 90,000 BTU/h, producing about 81,000 BTU/h at 90% AFUE. A contractor must confirm available models and the actual load.
Climate-factor planning ranges
| Winter climate | Planning factor |
|---|---|
| Mild | 20 BTU/h per ft² |
| Moderate | 30 BTU/h per ft² |
| Cold | 35 BTU/h per ft² |
| Very cold | 45 BTU/h per ft² |
| Severe | 55 BTU/h per ft² |
These broad ranges are for early planning only. Two homes with the same floor area can have very different heat loss because of windows, insulation, leakage and design temperature.
Furnace-sizing checklist
- Use heated floor area, excluding unconditioned garages and porches.
- Confirm ceiling heights and open stairwells.
- Check attic, wall, floor and basement insulation.
- Record window type, size and orientation.
- Identify air leakage and duct losses outside conditioned space.
- Use local winter design temperature, not an annual average.
- Compare furnace output—not only input—with the design load.
- Confirm blower airflow and duct capacity.
Frequently asked questions
What is AFUE?
Annual Fuel Utilization Efficiency estimates the share of annual fuel energy converted into useful heat. It does not include all electrical use or distribution losses.
Should furnace input or output match the load?
The useful output should meet the design heat load. Input is higher because the furnace is not 100% efficient.
Why is an oversized furnace a problem?
It may heat quickly, shut off frequently, create temperature swings, increase noise and reduce efficiency or equipment life.
Can I size a furnace from square footage alone?
No. Square-foot methods are preliminary. Accurate sizing requires envelope areas, R-values, windows, leakage and local design temperatures.
Does this calculator size a heat pump?
No. Heat pumps require heating and cooling load checks, capacity at the local winter design temperature and backup-heat planning.
Common mistakes
- Using total building area instead of heated area.
- Ignoring high ceilings and large windows.
- Comparing furnace input directly with the home load.
- Adding an excessive safety factor.
- Replacing an old oversized furnace with the same capacity automatically.