Methodology

Furnace size calculator

This is a preliminary rule-of-thumb estimator. It is not an ACCA Manual J load calculation and does not use Manual J tables. ACCA Manual J is the recognized residential load-calculation methodology, and ENERGY STAR advises that equipment be sized from a home's actual characteristics rather than rules of thumb. Our calculator is intentionally simpler:

estimated heating load = sq ft × climate starting factor × insulation × ceiling × air leakage × windows

The result is rounded to the nearest 100 BTU/hr. Valid inputs: 200–10,000 sq ft and 7–20 ft ceilings.

1. Climate starting factors

These four values are not taken from ACCA, DOE, ENERGY STAR or the IECC. The categories are broad descriptions informed by differences in U.S. winters — not official climate-zone calculations. A real professional heating-load calculation uses your local winter outdoor design temperature rather than four national categories.

2. Adjustments

ENERGY STAR explains that insulation and air sealing affect heat loss (Seal and Insulate, Why Seal and Insulate?). Those sources support the direction of each adjustment only; the exact multiplier values are ours. The ceiling formula assumes about 70% of a typical home's heat loss scales with wall height.

ACCA Manual S (educational context)

ACCA Manual S provides equipment-selection procedures that are used after a proper design heating load has been calculated (for example with Manual J). Traditional furnace sizing limits under Manual S may permit furnace output somewhat above the calculated heating load. FurnaceCalculator.com does not apply those limits numerically to its simplified estimate, because our number is not a design load.

AFUE illustration

AFUE is the ratio of useful heat output to fuel energy input over an entire heating season (DOE; ENERGY STAR). It is not simply the instantaneous ratio of a furnace's rated input to rated output.

efficiency-equivalent fuel input estimate = estimated heating load ÷ AFUE

This is a simplified efficiency illustration, not the rated input capacity of a furnace. Actual furnace input and output ratings come from the manufacturer's specifications; compare manufacturer output capacity with a heating load.

Worked example: 2,000 sq ft, cold climate

2,000 sq ft, cold climate, average insulation, average air leakage, 8-ft ceilings, typical windows:

• Base estimate: 2,000 sq ft × 50 BTU/sq ft (cold climate) = 100,000 BTU/hr
• Insulation: Average insulation × 1 = 100,000 BTU/hr
• Ceiling height: 8 ft ceilings × 1 = 100,000 BTU/hr
• Air leakage: Average × 1 = 100,000 BTU/hr
• Windows: Typical × 1 = 100,000 BTU/hr
• Estimated heating load: 100,000 BTU/hr
• Efficiency-equivalent fuel input estimate at 96% AFUE: 100,000 ÷ 0.96 ≈ 104,200 BTU/hr

What we deliberately left out

• ZIP code lookup. We found no free, reliable, publishable ZIP-to-design-temperature dataset and will not invent one.
• Basement / crawlspace adjustment. The effect depends heavily on whether the space is heated, insulated and vented.
• Duct losses, orientation, internal gains. These are Manual J territory.

Furnace replacement cost calculator

Data year 2026. Assumptions last reviewed October 2026. The calculator does not invent equipment and labor prices. It starts from published 2026 national installed-cost ranges by furnace type and places your estimate within that range.

For oil, sources disagree (Modernize ≈ $4,500–$9,000); we use the Angi/This Old House range and note the difference.

Positioning within the range (our interpolation)

Each choice gets a position score from 0 (low end) to 1 (high end). These scores are FurnaceCalculator.com assumptions, not industry-standard percentages:

• Size: Smaller 0, Medium 0.5, Larger 1
• Efficiency: Standard 0, High efficiency 0.5, Premium 1 (oil: standard 0, high 1; electric has one tier and is not scored)
• Installation: Simple swap 0, Standard 0.5, Difficult 1

position = average of scores
low = range low + (range high − range low) × position × 0.6
high = low + (range high − range low) × 0.4

So the installed estimate always covers 40% of the published range and stays inside it. Values round to the nearest $50.

Possible additional work (planning allowances)

Published guides vary widely on these, and many installed quotes already include removal and permits. These are wide FurnaceCalculator.com planning allowances, added on top and allowed to push the total beyond the published range: ductwork minor $500–$2,000, major $2,000–$8,000; removal if quoted separately $0–$500; permit if quoted separately $50–$500. Costs vary locally.

No location adjustment. We haven't found defensible public data for local furnace labor, so we don't pretend to have it. National estimates are not a contractor quote. See all sources.

Example: standard gas, medium size, standard installation, no extras → $5,050–$7,500.