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Greenhouse Heating BTU Calculator

Estimate the heater size needed to keep a greenhouse warm, based on glazing type and temperature difference.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.1.0

Market
Imperial · sales tax

Recommended heater size

10332.32 BTU/hr

Check your inputs

This estimates glazing heat loss only — wind exposure, air leaks around vents and doors, and the greenhouse's surface-area-to-volume ratio (smaller structures lose heat faster per sq ft) can all push real requirements higher.

Greenhouse area
215.28 sq ft

For the dimensions entered, expect a recommended heater size of 10332 BTU/hr. Note that greenhouse floor area sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Add the equipment this sizes

This result is a specification — 10,332.321 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Commonly cited greenhouse heat-loss factors (BTU/hr per sq ft per °F of temperature difference): single-layer poly film ~1.2, double-layer inflated poly ~0.7, glass ~1.1

Your workspace

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Frequently asked questions

Why does double-layer poly need so much less heat than single-layer?
The trapped air gap between the two layers acts as an insulating buffer, similar to how double-pane windows outperform single-pane — it roughly cuts heat loss per square foot compared to a single layer of the same material.
Does this account for the sun's free heat during the day?
No — this sizes for worst-case heat loss (typically overnight, no sun), which is the standard conservative approach for heater sizing since you need heat exactly when there's no solar gain to help.
Should I heat the whole greenhouse or just a section?
For overwintering only sensitive plants, some growers heat just a smaller enclosed section within the greenhouse rather than the whole structure, which can significantly reduce the heater size needed — recalculate using just that section's area if so.