# Furnace Size Calculator

> Furnace size calculator: compute BTU output and tonnage needed from square footage, climate zone, and insulation. Editable rates, free, in your browser.

- **Canonical URL:** https://boringtoolskit.com/furnace-size-calculator/
- **Category:** Energy & Electrical
- **Evidence level:** catalogue
- **Reviewed:** July 26, 2026
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

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*Local planning estimate. Verify current terms, rates, and policies before acting.*