Furnace Size Calculator
Estimate your whole-home heating load (BTU/h) and get a recommended furnace input size using climate zone, insulation, ceilings, air sealing, and duct condition.
Updated for 2026 IECC climate zones and current AFUE standards.
On this page: Calculator · How to use · Next steps · Equipment & tools · Related calculators · FAQ
Need cooling too? Use the AC size calculator or the advanced BTU calculator.
Estimate heating load & furnace BTU size
Enter heated square footage (exclude unconditioned garages/attics).
Colder zones have larger heating loads.
Improving insulation often reduces furnace size needs.
Higher ceilings = more air volume to heat.
More exposed surfaces increase load.
Drafty homes need substantially more BTUs.
Leaky ducts can waste 15–30% of heat.
AFUE converts heat load → furnace input size.
Used for context only (no pricing here). Try the energy cost calculator for operating costs.
For code-grade sizing, request an ACCA Manual J calculation, or get a free HVAC quote from local contractors.
How to use this furnace calculator
- Enter your conditioned square footage (heated area only).
- Select your climate zone and insulation level.
- Adjust ceilings, stories, air sealing, and ducts to match your home.
- Pick an AFUE to translate heating load into a recommended furnace input BTU.
If you’re not sure about drafts or duct condition, leave them on “Average” for a reasonable baseline.
Next steps (avoid common sizing mistakes)
Compare alternatives
In many climates, a heat pump may reduce operating cost compared to a gas furnace. In Zones 1–4, heat pumps can often serve as primary heat. In Zones 5–7, dual-fuel setups (heat pump + furnace backup) are increasingly popular.
Check cooling size too
If you’re replacing a furnace bundled with AC, use the AC size calculator for central AC tonnage and the SEER savings calculator to compare efficiency upgrades.
Estimate operating costs
Run the energy cost calculator to compare monthly costs between gas, propane, oil, and electric heating options before buying.
Get room-by-room detail
This calculator estimates whole-home load. For room-by-room breakdowns including windows, sun exposure, and occupants, use the advanced BTU calculator.
Equipment & tools
Furnace maintenance essentials
HVAC air filters ($5–30) — replace every 1–3 months for airflow and efficiency. Size yours with the air filter calculator. Smart thermostats ($25–250) — scheduling + setback saves 10–15%. Energy monitors ($20–30) — verify actual savings.
Reduce your heating load (and furnace size)
Weatherstripping ($5–15), door sweeps ($8–20), spray foam ($10–40), and outlet gaskets ($5–10) reduce infiltration. A thermal leak detector ($30–50) finds the worst spots.
ACCA Manual J — official load calculation reference. Energy.gov weatherization guide — federal tips for reducing heating load. ENERGY STAR federal tax credits — rebates for high-efficiency equipment. ElectricityLocal — check local energy rates for heat pump comparisons.
Browse all equipment: heating & cooling equipment by BTU range.
Frequently asked questions
What furnace size do I need for my home?
Furnace size depends on your heating load (BTU/h), driven by climate zone, insulation, air sealing, ceiling height, and duct losses. This calculator estimates the load and recommends a furnace input BTU size based on AFUE.
Should I oversize my furnace to be safe?
Not usually. Oversizing can cause short cycling and comfort issues. If you’re between sizes, consider improving air sealing and insulation or confirming with a Manual J rather than jumping to a much larger unit.
What is AFUE and why does it matter?
AFUE (Annual Fuel Utilization Efficiency) is a furnace efficiency rating. Higher AFUE means more usable heat from the same fuel. This calculator converts your required heat output into the furnace input needed using AFUE.
Is a Manual J required?
A full Manual J is the standard for code-grade HVAC design and some permit situations. This calculator is a planning estimate. If you need a code-grade result, ask an HVAC professional for a Manual J, or get a free quote.
Should I consider a heat pump instead of a furnace?
In mild-to-moderate climates (Zones 1–4), a heat pump can often handle most heating hours more efficiently than a gas furnace. In colder climates (Zones 5–7), a dual-fuel setup (heat pump + furnace backup) is increasingly popular. Use the heat pump calculator to compare, and the energy cost calculator for monthly cost differences.