Gas Furnace Sizing Calculator

Size a gas furnace based on heating degree days, building envelope, and design temperatures. Follows simplified Manual J methodology.

Related Calculators

How to Size a Gas Furnace

Why Furnace Sizing Matters

An oversized furnace heats the home too quickly and shuts off (short-cycling), which causes temperature swings, uneven heating, premature component wear, and wasted energy. An undersized furnace runs continuously and cannot maintain comfort on the coldest days. Correct sizing follows ACCA Manual J heat loss calculations.

Heat Loss Factors

Base Heat Loss = Area × Heat Loss Factor × ΔT × Ceiling Factor

ΔT = Indoor Design Temp − Winter Design Temp (99%)

Ceiling Factor = Actual Height / 8 ft

Window Loss = Windows × 250 BTU/hr each

Wall Loss = Exposed Walls × ΔT × 50 BTU/hr per wall

Infiltration Loss = Area × 0.05 × ΔT

The heat loss factor ranges from 0.22 BTU/hr/sqft/°F (excellent insulation) to 0.55 BTU/hr/sqft/°F (poor insulation). The winter design temperature is the 99% annual extreme — the temperature exceeded 99% of the time in winter.

Worked Example

Scenario: 2,000 sq ft home, 8 ft ceilings, 10°F winter design, 70°F indoor, average insulation, 10 windows, 4 exposed walls.

ΔT = 70 − 10 = 60°F
Base loss = 2,000 × 0.40 × 60 × 1.0 = 48,000 BTU/hr
Window loss = 10 × 250 = 2,500 BTU/hr
Wall loss = 4 × 60 × 50 = 12,000 BTU/hr
Infiltration = 2,000 × 0.05 × 60 = 6,000 BTU/hr
Total heat loss = 48,000 + 2,500 + 12,000 + 6,000 = 68,500 BTU/hr
Furnace input (80% AFUE) = 68,500 / 0.80 = 85,625 BTU/hr
Recommended furnace size = 100,000 BTU/hr

Practical Tips

A typical residential furnace needs 30–60 BTU/hr per sq ft depending on climate and insulation quality.
Upgrading from 80% AFUE to 95% AFUE furnace can reduce gas consumption by about 16% for the same heat output.
Always match the furnace output (not input) to your heat loss calculation — a 100k input at 80% AFUE produces only 80k output.
Get a professional Manual J load calculation before replacing your furnace — existing equipment is often oversized.

Code References

ACCA Manual J (heating), ASHRAE Fundamentals

Frequently Asked Questions

How many BTU per square foot do I need for a furnace?

As a general rule: 30 BTU/hr per sq ft for well-insulated homes in mild climates, 40 BTU/hr per sq ft for average insulation in moderate climates, and 50–60 BTU/hr per sq ft for poorly insulated homes in cold climates. These are rough estimates — actual sizing requires a Manual J calculation accounting for windows, doors, ceiling height, and infiltration.

What does AFUE mean?

AFUE (Annual Fuel Utilization Efficiency) measures how efficiently a furnace converts gas energy into usable heat over a full heating season. An 80% AFUE furnace delivers 80% of the gas energy as heat; the rest is lost up the flue. Modern high-efficiency furnaces achieve 92–98% AFUE. When sizing, divide your heat loss by the AFUE to get the required furnace input BTU.

What is a winter design temperature?

The winter design temperature (99%) is the outdoor temperature that your location exceeds 99% of hours during winter. It represents the near-worst-case condition your furnace must handle. For example, 10°F means your furnace should maintain indoor comfort when it is 10°F outside. Using the 99% design temp means the furnace may run continuously for a few hours on the 1% coldest days — which is acceptable.

What happens if my furnace is oversized?

An oversized furnace heats the space too quickly, causing the thermostat to shut it off before the house has had time to distribute heat evenly. This short-cycling leads to hot and cold spots, increased on/off wear on components, reduced efficiency (each startup wastes a small burst of gas), and lower comfort. Properly sized furnaces run longer cycles, maintaining even temperatures.