FURNACE SIZE PLANNING

Furnace size calculator

Estimate a heating-capacity range from the basic details of your home, then use it to ask for a room-by-room load calculation before choosing a furnace.

HOME HEATING DETAILS

Start with the house

This is a planning estimate. It does not replace a Manual J calculation or tell you which furnace will fit your ductwork.

ROUGH HEATING CAPACITY RANGE
38,700–52,300 BTU/h

Use this as a conversation starter. The right furnace output depends on the home's calculated heat loss and the equipment's output at the selected efficiency.

Nearest common furnace output50,000 BTU/hCheck output BTU, not just the furnace input rating

Do not order from this number alone. Ask for the Manual J load, the furnace output rating, the blower table and the duct assessment before signing a quote.

WHAT THE RANGE MEANS

Size the heat loss, then choose the furnace

Input versus output

A furnace's input rating is the gas energy it consumes. Its output is the heat delivered to the home after efficiency losses. Compare the output to the calculated heat loss.

Why square footage is not enough

Climate, insulation, windows, air leakage, ceiling height and room-by-room exposures all change the result. A single whole-house shortcut cannot see those details.

Before you buy

Ask the installer to show the load calculation, selected output, duct match, venting plan, controls, permits and commissioning scope.