From this site

One project, every trade

Each calculator adds its lines to a single estimate — consolidated BOM, schedule and cash-flow included.

Open My Project →

HVAC Thermal Load (U-Value) Calculator

Compute heat transmission load through a building surface using the physics-based Q = U x A x ΔT formula.

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

Heat transmission load

924.04 BTU/hr

Check your inputs

This computes conductive transmission loss through one surface using standard physics — a complete Manual J (or Manual N/CIBSE-equivalent) whole-building load calculation also accounts for air infiltration, internal gains, solar gain, and every surface combined.

U-value
0.08 BTU/(hr·ft²·°F)
Equivalent in watts
270.82 W

Running these inputs gives 924 BTU/hr as the heat transmission load. Note that surface area sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 924.04 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

  • Standard heat transmission formula: Q = U-value x surface area x temperature difference (ΔT), where U-value = 1 / R-value

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why convert R-value to U-value first?
U-value (the reciprocal of R-value) represents how much heat flows THROUGH an assembly, which is what the transmission formula needs — R-value represents resistance TO heat flow, the inverse concept.
Should I use average outdoor temperature or a design temperature?
Always use a design temperature (a published extreme, like the 99% winter design condition for your area) rather than an average — HVAC equipment needs to handle the coldest/hottest realistic days, not a typical day, or it will be undersized when you need it most.
How is this different from the simpler BTU Cooling Calculator?
The BTU Cooling Calculator uses a fast rule-of-thumb (BTU per square foot) for a quick planning estimate. This calculator uses the actual physics formula with your assembly's real R-value and design temperatures, which is more accurate but requires knowing (or estimating) those values for each surface.
Do I need to run this for every wall, window, and roof section separately?
Yes, for a full building load — different surfaces almost always have different R-values (walls vs. windows vs. roof), so each needs its own calculation before summing them into a total heating or cooling load.