Heating Size Calculator
See the heating BTU per hour and heat pump tonnage a home your size needs in your winter climate, before a pro runs the real load calculation.
A real person answers. Free estimate, no obligation.
Data: DOE and ASHRAE heating load method (UA x design temperature difference); Daymet (NASA/ORNL) January lows 2015 to 2024 · Updated September 2026
How heating size works
Heat loss is floor area times the temperature gap
On the coldest design night a home loses heat through its walls, roof, windows and air leaks at a rate that scales with floor area and with the gap between 70°F indoors and the outdoor design temperature. This tool uses your January average low to place that design temperature about 15°F lower, then multiplies by a factor for insulation and ceiling height.
Insulation moves the number more than anything else
A drafty older home can need almost twice the heat of a sealed, well insulated one of the same size. That is why the same 2,000 sq ft house sizes very differently in the three insulation classes here, and why air sealing before replacement often means a smaller system.
Furnaces and heat pumps read the result differently
A furnace is picked by output, so a 95% AFUE furnace with a 100,000 BTU input delivers 95,000. A heat pump is rated at 47°F outdoors and loses capacity as the temperature drops, so in a cold climate a pro sizes it on its low-temperature capacity table and decides whether backup heat rides along.
Why a pro still runs a Manual J
A real load calculation counts each window, wall and duct run, the orientation of the house and how leaky it is. Rules of thumb like this one land in the right neighborhood, but an oversized system short-cycles and an undersized one runs flat out on the coldest nights. The calculation is what a good installer does before naming a size.
Common questions
Roughly 40,000 to 45,000 BTU per hour in a mild-winter climate and 55,000 to 70,000 in a cold one for an average-insulation home. The spread is the climate; the insulation class then moves it up or down by about a third. Old rules of thumb that say 50 BTU per square foot run well above what a load calculation returns, which is how furnaces end up oversized.
Divide the heating load by 12,000 to get tons, then round to the nearest half ton. In cold climates the heating load is bigger than the cooling load, so a pro checks the unit against both and often picks a variable-speed model so it is not oversized in summer.
No. An oversized furnace heats the house in short bursts, which wears parts, leaves rooms uneven and wastes fuel. Sizing close to the calculated load with a modest margin is the standard practice.
Yes. Nine and ten foot ceilings add air volume and wall area, so this tool adds 10% to 20% to the load. Vaulted rooms with big windows can add more, which is one reason a load calculation beats a rule of thumb.
The outdoor temperature your area stays above about 99% of winter hours. It sits well below the January average low, which is why this tool subtracts 15°F from that average. Sizing to the design temperature, not the record low, is the industry standard.
Get an exact answer from a local pro
Free to call — get connected with a local pro in your area.
Keep going
disclaimer: 781 Heating is a free service to assist homeowners in connecting with local service providers. All contractors/providers are independent and 781 Heating does not warrant or guarantee any work performed. It is the responsibility of the homeowner to verify that the hired contractor furnishes the necessary license and insurance required for the work being performed. All persons depicted in a photo or video are actors or models and not contractors listed on 781 Heating.
Same-day and 24/7 emergency services are subject to provider participation, location, technician availability, and demand. Availability is not guaranteed and may vary by market and appointment capacity.
We use cookies and session-based analytics, including session replays (with form inputs masked), to improve this site. See our Privacy Policy.
About · Contact · Privacy · Terms · Disclosure · Sitemap
© 2026 781 Heating. All rights reserved.