OnlineTool.me

Furnace Size Calculator

Enter floor area, climate, insulation quality, and AFUE to get a preliminary furnace input range and output-load estimate.

Get a Preliminary Furnace Capacity Range

Enter conditioned area, climate severity, envelope quality, and AFUE to get a rough input-capacity range for early planning.

Your preliminary input-capacity range and estimated output load will appear here.

How to Use Furnace Size Calculator

1

Enter Conditioned Area

Use the floor area actually served by the proposed heating system.

2

Choose Broad Conditions

Select winter severity, envelope quality, and the furnace AFUE.

3

Use the Result Only for Planning

Take the rough range to an HVAC professional and request Manual J and Manual S work.

Furnace Size Calculator Features

  • Broad preliminary BTU/h range
  • Climate and envelope adjustments
  • AFUE input-to-output conversion
  • Prominent Manual J and Manual S limitation

Common Use Cases

Prepare for an HVAC conversation

Understand the order of magnitude before requesting a room-by-room heating-load calculation.

See why AFUE matters

Compare required furnace input with the estimated delivered output load.

Limits and Important Notes

Result type: Planning estimate

Calculation Details

  • Apply a transparent 25, 35, 45, or 55 BTU/h per square-foot climate rate.
  • Multiply by 0.8, 1.0, or 1.2 for envelope quality.
  • Divide output load by AFUE as a decimal.
  • Display a broad ±15% preliminary input range.

Formula

Estimated output load = area × climate rate × envelope factor; input capacity = output load ÷ AFUE

Rounding

BTU/h outputs display to the nearest 1,000 because this model is intentionally approximate.

Default Assumptions

  • Conditioned floor area is used as a coarse proxy for heat loss.
  • Climate and envelope selections are broad user judgments.
  • AFUE converts estimated delivered output into nominal furnace input.

Verified Example

Inputs: 2,000 sq ft, moderate winter, average envelope, 90% AFUE.

Calculation: 2,000 × 35 × 1.0 = 70,000 BTU/h output; ÷ 0.90 = 77,778 BTU/h input.

Result: Preliminary input range ≈ 66,000–89,000 BTU/h.

Sources and Version

Version: OnlineTool.me preliminary rule-of-thumb model, August 2026

Last verified: 2026-08-12

Update responsibility: OnlineTool.me

  • Do not purchase or specify equipment from this result; it is not an ACCA Manual J load calculation or Manual S selection.
  • Actual loads depend on design temperatures, surfaces, insulation, infiltration, ducts, windows, orientation, ventilation, and room-by-room conditions.
  • Oversizing can increase cost and cycling; a qualified HVAC professional should calculate load and select equipment.

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Frequently Asked Questions