Free home sizing tools

What size furnace does your home actually need?

Skip the guesswork and the sales pitch. Enter your square footage, climate, and insulation, and get a BTU range with the math shown. No sign-up, no email.

Shows the full formula
7 climate zones modeled
$0 no sign-up

Furnace Size Calculator

Manual-J lite
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Home energy calculators

Single-purpose tools that answer one question well. Each one shows its formula and sources, so you can check the work.

How this estimate works

This is a fast estimate to get you in the right range before you call a contractor. It is not a substitute for a full Manual-J load calculation on a sizing-critical install.

  • Base load: heated square footage multiplied by a climate-zone factor (32 to 60 BTU per square foot, hot to subarctic), the standard rule-of-thumb band used in residential HVAC sizing.
  • Insulation adjustment: the factor is scaled up for older, leaky homes and down for tight, well-insulated ones.
  • Range, not a single number: we show a band because real load also depends on windows, ceiling height, air sealing, and orientation, which a full Manual-J captures.
  • Why not just go bigger: an oversized furnace short-cycles, costs more, and heats unevenly. Right-sizing matters.