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R-Value to U-Value Calculator

Convert thermal resistance (R-value) into thermal transmittance (U-value) for heat-loss calculations.

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 2026-08-26 · v1.0.0

Market
Imperial · sales tax

U-value

0.05 BTU/hr·ft²·°F

High confidence

U = 1 ÷ R. The two are exact reciprocals of one another. R-values add across layers; U-values do not. Sum R first, then take the reciprocal once.

R and U describe the same physical property from opposite ends: resistance to heat flow, and the rate of heat flow itself. Insulation is marketed in R because bigger sounds better, while heat-loss calculations run on U because it multiplies straight through the Q equals U-A-delta-T relationship. The conversion is a pure reciprocal, and the mistake that ruins calculations is doing it too early. Resistances add in series — batt plus sheathing plus cladding plus air films — so the correct order is to total every layer's R and invert once at the end. Summing the individual U-values of each layer produces a number that is not merely imprecise but wrong, typically by a large margin.

Add the equipment this sizes

This result is a specification — 0.053 BTU/hr·ft²·°F — 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

  • Thermal transmittance is the reciprocal of thermal resistance: U = 1 / R
  • Heat transfer through an assembly: Q = U x A x deltaT

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Frequently asked questions

Why do heat-loss formulas use U rather than R?
Because heat loss is proportional to transmittance, so Q equals U times area times temperature difference. Resistance is the more intuitive number for comparing insulation products, but transmittance is the one that multiplies cleanly through a heat-loss calculation.
Can I add U-values across layers?
No, and this is the classic error. R-values add in series; U-values do not. Sum the resistances of every layer first, then take the reciprocal once at the end. Adding U-values gives a badly wrong answer.
Do air films count?
Yes, and they are not trivial. Interior and exterior surface films contribute meaningful resistance, and an enclosed cavity adds more. Omitting them makes an assembly look worse than it performs.