Furnace Sizing Calculator
Estimate the heating load for your ZIP code and house, then see the furnace output range that fits it, the input rating at your AFUE and the common input size to shop for. Estimate only; confirm with a room-by-room Manual J before you order.
Furnace sizing calculator
Northeast US (default)Zone 5A · 5°F heating / 88°F cooling design
Estimate only. Not a substitute for a room-by-room Manual J.
Estimated heating load
36,700 BTU/h
31,200 BTU/h – 42,200 BTU/h at 5°F outdoor
Output range (100–140%)
36,700 – 51,400
BTU/h heating output (bonnet capacity)
Common input size to shop for
40,000 BTU/h
about 38,000 BTU/h output at 95% AFUE
Input for 115% of load
44,400 BTU/h
42,200 BTU/h output ÷ 95% AFUE
Furnaces are rated on input. What heats the house is output: input × AFUE (strictly, steady-state efficiency, which is close to AFUE for condensing units). If the furnace also carries an AC coil, the blower has to move the cooling airflow too, so the AC tonnage can set the cabinet size.
Assumptions behind this estimate
- Climate
- Northeast US (default) (zone 5A)
- Design temperature
- 5°F outdoor / 70°F indoor
- Insulation level
- average
- Wall / ceiling / window U
- 0.085 / 0.035 / 0.55
- Infiltration
- 0.5 ACH natural
- Output limits
- 100% to 140% of heating load (Manual S furnace range)
- Common inputs considered
- 40k, 60k, 80k, 100k, 120k, 140k
- Duct losses
- Not included
Do this room by room in Tradewright
Real equipment matches, a 15-year cost comparison and a signed proposal, all from one visit.
Why “same size as the old one” is usually wrong
The most common furnace sizing method is reading the old nameplate. That repeats whatever guess was made decades ago, and ignores every window, attic and air-sealing upgrade since. Many replacement furnaces end up far larger than the house needs.
An oversized furnace runs short cycles. It heats the air near the thermostat quickly and shuts off before the far rooms catch up, and it can run hotter than the ductwork was designed for. Bigger also means more airflow that the existing ducts may not carry quietly.
Mind the input versus output trap. Furnaces are sold by input BTU/h, but the house only sees output, which is input times efficiency. An 80,000 BTU/h input furnace at 80% AFUE delivers about 64,000 BTU/h. Manual S sizes on output, allowing a furnace between 100% and 140% of the heating load (ACCA Manual S).
Sources
How the estimate works
Step 1: Design temperature
Your ZIP sets an approximate 99% heating design temperature. That is the cold the furnace must handle, not the record low.
Step 2: Heat loss
Walls, windows, ceiling, floor and air leakage are each calculated as U-factor × area × temperature difference from your size, year and insulation.
Step 3: Output range
The furnace output should land between 100% and 140% of that heat loss. We use 115% as a middle target.
Step 4: Input at AFUE
Output is divided by AFUE to get input, then matched to the smallest common input size whose output still covers the load.
Questions
What size furnace do I need for my house?
Enough output to cover the heating load at your design temperature, and not more than about 140% of it. The calculator estimates that load from your ZIP, size and insulation and suggests a common input size.
What is the difference between furnace input and output?
Input is the fuel the furnace burns per hour. Output is the heat that reaches the house, roughly input times AFUE. A 60,000 BTU/h input furnace at 95% AFUE puts out about 57,000 BTU/h.
Does AFUE change the size I need?
It changes the input rating, not the output you need. A higher AFUE furnace needs less input to deliver the same heat, so a 95% furnace can carry the same house on a smaller input than an 80% unit.
Is it bad to oversize a furnace?
Yes, past a point. Short cycling, uneven rooms, noise and extra wear are common results. Manual S allows furnace output up to 140% of the heating load; beyond that, choose a smaller or modulating unit.
How many BTU per square foot for a furnace?
It varies too much to use as a rule. Climate, insulation, air leakage and windows can change the heating load per square foot several times over between homes.
Does the AC affect furnace size?
It can. The furnace blower moves the cooling airflow too, so a large AC can require a larger cabinet or blower even when the heating load is small.
Is this a Manual J?
No. It is a whole-house estimate with generalized assumptions. A Manual J calculates each room from measured walls, windows and orientation and accounts for duct location.
Get a room-by-room Manual J and real equipment matches in Tradewright.
A 20-minute demo on a real home in your territory.