Furnace size calculator

Input against output

A furnace nameplate carries two BTU/h numbers. Input is how much fuel it burns per hour. Output is how much of that heat reaches the air. AFUE is the ratio, averaged over a heating season: an 80,000 BTU/h input furnace at 80% AFUE delivers about 64,000 BTU/h; the same input at 96% delivers about 76,800.

Size against the load in output, then divide by AFUE to get the input you have to order. Doing it the other way round — matching the load to the number on the box — buys a furnace about 20% too big every time.

The altitude derate

Above roughly 2,000 feet the air is thin enough that a gas appliance cannot burn its rated input, and manufacturers derate about 4% per 1,000 feet of elevation. Denver at 5,280 feet costs a furnace about 13% of its nameplate input. The tool applies that automatically — put your elevation in and watch the required nominal size step up.

Airflow and temperature rise

Every furnace nameplate carries a rise range — typically 40 to 70 °F. Airflow, rise and output are locked together: CFM = output ÷ (1.08 × rise). At 68,400 BTU/h output and a 55 °F rise you need about 1,150 CFM. If the duct cannot move that, the rise climbs, the limit switch trips and the furnace short-cycles. Check the reading against the static pressure calculator before blaming the furnace.

What the standard requires

Non-weatherized gas furnaces manufactured today must be at least 80% AFUE. Furnaces manufactured on or after 2028-12-18 must be at least 95% AFUE, which means condensing — a second heat exchanger, PVC venting and a condensate drain. If you are quoting a replacement now in a house with a masonry chimney, that date is worth mentioning: the venting work is easier to do once.

• Zone 1
• Zone 2
• Zone 3
• Zone 4
• Zone 5
• Zone 6
• Zone 7

Sources: 10 CFR 430.32(e)(1)(iii) (read 2026-09-04)