How to use this calculator
- Enter heated floor area. Use the floor area served by the furnace or zone, not the whole building if equipment is split.
- Pick a load factor. Choose a BTU/h per ft² value that reflects climate and envelope quality.
- Set design temperature difference. Use indoor design setpoint minus outdoor winter design temperature.
- Check output, input and airflow. Read the required output capacity, AFUE-adjusted input BTU/h and estimated supply CFM.
How it works
The heat-loss screen starts with floor area and a rough load factor:
q_loss = A × LF × ΔT_design / 70°F
where LF is entered as Btu/h per square foot at a 70°F design
temperature difference. The calculator then applies the sizing margin to estimate
required furnace output.
Furnace input rating is higher than delivered output because:
q_input = q_output / AFUE
The airflow check uses the standard-air heating shortcut
BTU/h ≈ 1.08 × CFM × Δ°F, rearranged to solve CFM from output BTU/h
and selected supply-air temperature rise.
After this screen, carry the supply airflow into the duct size calculator, compare load and airflow in the HVAC airflow and BTU load calculator, and check blower pressure with the HVAC static pressure calculator.
Worked example
Verified against the live calculator
For 1,800 ft², a 35 Btu/h·ft² load factor, 70°F design temperature difference, 95% AFUE, 10% margin and 50°F air rise, the screen gives 63,000 Btu/h design heat loss. With margin, output capacity is 69,300 Btu/h. At 95% AFUE, input rating is about 72,900 Btu/h, and supply airflow is about 1,280 CFM.
Reference data
Load-factor ranges are rough screening inputs, not design rules.
| Condition | Btu/h·ft² | Use |
|---|---|---|
| Tight / efficient envelope | 15-25 | Mild climate or well-insulated construction |
| Average existing home | 25-40 | Common early screening range |
| Cold / leaky / older envelope | 40-60+ | Needs a proper heat-loss check before equipment selection |
Source: Screening ranges only. Verify final heating loads with ACCA Manual J / Manual S, ASHRAE methods, local design weather and equipment data.
Frequently asked questions
How many BTU does my furnace need?
A quick screen is heated floor area multiplied by a BTU/h per square foot load factor, corrected for your indoor-outdoor design temperature difference. This page then adds a modest sizing margin and converts output capacity to input BTU/h using AFUE.
Is BTU per square foot enough for final furnace sizing?
No. Area-based BTU rules are useful for early screening only. Final equipment sizing should use ACCA Manual J / Manual S, ASHRAE or the applicable local method with insulation, windows, infiltration, duct location, zoning and equipment data.
What load factor should I use?
A tight efficient home may screen near 15-25 Btu/h per ft² at a 70°F design difference, an average home around 25-40, and a cold or leaky home around 40-60+. Local climate and envelope quality matter more than the floor area alone.
Why is furnace input BTU/h higher than output BTU/h?
AFUE accounts for combustion and seasonal efficiency. A 95% AFUE furnace with a 69,300 Btu/h output requirement needs about 72,900 Btu/h input before selecting the nearest available equipment size.
Why does the calculator show supply airflow?
Airflow checks whether the furnace heat output can be carried at the selected temperature rise. The common heating relation is BTU/h ≈ 1.08 × CFM × Δ°F for standard air.
Method & assumptions
- Area-based heat-loss screen normalized to a 70°F indoor-outdoor design temperature difference.
- AFUE converts delivered output BTU/h to nominal fuel input BTU/h; equipment comes in discrete sizes.
- Supply airflow uses standard-air sensible heating, BTU/h ≈ 1.08 × CFM × Δ°F.
- Oversized furnaces can short-cycle, run noisy duct systems and reduce comfort. Final sizing needs room-by-room load calculation, duct design, combustion/venting and local-code review.