SettingsSettings for this calculationUS
The R-value printed on the product or called for by the specification.
Use the number the label gives, not one you have derived. A batt sold as R-30 is R-30 across its declared thickness; a board sold as R-6 per inch is R-6 only when you state the inches. If the figure you have is per inch, multiply by the thickness first and enter that — RSI per inch and RSI for an assembly are different quantities and nothing downstream can tell them apart.
Thermal resistance (RSI)
5.283 m²·K/W
1 RSI = 5.678263 R, so R ÷ 5.678263 = RSI. Exact by definition. Carry the unconverted figure on the record and the converted one in brackets.
- R-value entered
- 30 h·ft²·°F/BTU
- Metric U-value (the reciprocal)
- 0.19 W/m²·K
- IP U-factor (the reciprocal in IP)
- 0.03 BTU/h·ft²·°F
They open the calculator with your figures already in it
R-Value to RSI Calculator: 5.28 m²·K/W — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- NIST SP 811, Appendix B — thermal insulance: 1 h·ft²·°F/BTU = 0.176110 m²·K/W
- ASTM C168, Standard Terminology Relating to Thermal Insulation (defines thermal resistance and its IP/SI forms)
Inputs used
- R-value (h·ft²·°F/BTU)
- 30
Intermediate steps
- R-value entered
- 30 h·ft²·°F/BTU
- Metric U-value (the reciprocal)
- 0.19 W/m²·K
- IP U-factor (the reciprocal in IP)
- 0.03 BTU/h·ft²·°F
Confidence note: 1 RSI = 5.678263 R, so R ÷ 5.678263 = RSI. Exact by definition. Carry the unconverted figure on the record and the converted one in brackets.
What this calculation does not cover
- Converts a declared resistance, not an installed one. Compression, gaps and wind-washing all reduce what a batt delivers and none of them is visible here.
- Says nothing about thermal bridging. A framed wall's effective RSI is below the sum of its layers, often by a fifth or more.
- Ignores surface films and air spaces, which some published assembly figures include and others do not. Compare like with like before comparing numbers.
Add the equipment this sizes
This result is a specification — 5.283 m²·K/W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-12 · v1.0.0
Regulatory standards & verification citations2
- NIST SP 811, Appendix B — thermal insulance: 1 h·ft²·°F/BTU = 0.176110 m²·K/W
- ASTM C168, Standard Terminology Relating to Thermal Insulation (defines thermal resistance and its IP/SI forms)
Which documents these citations point at
Standards referenced: ASTM C168 (ASTM International, United States).
Cite this page
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.
This is the conversion that gets skipped, and skipping it is expensive in a specific way. An RSI 3.5 wall and an R-3.5 wall sound alike and differ by a factor of nearly six, so a European product data sheet read as though it were American turns a code-compliant wall into a third of one — or, in the other direction, makes a perfectly ordinary batt look like an aerogel. The site's own unit tests already recorded readers making exactly this mistake in the U-factor fields, silently, with the wrong figure clamped rather than questioned. What the page does about it is narrow and worth stating: it converts the claim, prints both reciprocals so the U-value side is there when the specification is written that way, and then says plainly that the number it produced is a label figure and not a wall.