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U-Value to R-Value Calculator

Convert a U-value from a specification or datasheet into the R-value used on insulation labelling.

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

Equivalent R-value

20 hr·ft²·°F/BTU

High confidence

R = 1 ÷ U, the exact inverse relationship. The result is the whole assembly's resistance — subtract the other layers before choosing an insulation thickness.

Building regulations and window schedules are written as maximum U-values, while insulation on a merchant's shelf is labelled in R, so specifying to a regulation means crossing between them. The arithmetic is a reciprocal and takes a second. The two traps are unit systems and scope. A metric U-value in watts per square metre kelvin is roughly 5.68 times its imperial counterpart, and mixing them produces an assembly that appears five times better insulated than it is. And the converted R covers the entire build-up, not the insulation alone: sheathing, cladding, internal linings and surface films all contribute, so the product you buy only needs to close the remaining gap.

Add the equipment this sizes

This result is a specification — 20 hr·ft²·°F/BTU — 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

  • R = 1 / U, the reciprocal of thermal transmittance
  • Imperial U (BTU/hr-ft2-F) and metric U (W/m2K) differ by a factor of about 5.678

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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

My specification is in W/m²K. Can I use it here?
Not directly — that is the metric U-value and is about 5.678 times the imperial figure. Divide by 5.678 first, or you will calculate an R-value roughly five times too optimistic.
Does the resulting R-value match the insulation I need to buy?
Not on its own. The converted figure is the whole assembly's resistance, and the insulation only has to make up the difference between that and what the other layers already provide. Subtract the existing layers before specifying a product.
Why do regulations specify U rather than R?
Because a maximum U-value sets a performance ceiling on the finished assembly regardless of how it is built up, which is what a regulator wants. R-value labelling describes a single product, which is what a manufacturer wants.