US R-value tools
SI U-value to US R-value calculator
Use this page when the input is a European or SI U-value in W/m²K and the output needs to be understood as a US R-value. The conversion is not just 1 / U-value. First, the SI U-value corresponds to a US U-factor equal to U-value / 5.678263337. Then R-US is the inverse of that US U-factor, which simplifies to R-US = 5.678263337 / U-value. A U-value of 0.25 W/m²K is about R-22.7, and a U-value of 0.18 W/m²K is about R-31.5.
Calculations keep US R-value, U-factor, SI R, and SI U-value separate so an inverted value is not mislabeled.
Quick answer
SI U-value to US R-value calculator
Formula: R-US = 5.678263337 / U-value SI. U-value 0.25 W/m²K converts to R-22.7 and US U-factor 0.044. U-value 0.18 W/m²K converts to R-31.5 and US U-factor 0.032.
- Formula
R_US = 5.678263337 / U_SI- Example
- Example: U-value 0.18 W/m²K converts to about US R-31.5.
Working calculator
Enter the core dimensions and check the result directly on this page.
SI U-value to US R-value table
Metric U-values converted to US U-factor and US R-value.
| SI U-value | US U-factor | US R-value | Practical reading |
|---|---|---|---|
| 0.25 W/m²K | 0.044 | R-22.7 | typical wall check |
| 0.18 W/m²K | 0.032 | R-31.5 | high resistance |
| 0.30 W/m²K | 0.053 | R-18.9 | common cavity insulation |
| 0.20 W/m²K | 0.035 | R-28.4 | lower heat transfer |
Worked examples
Example 1
Inputs
U_SI = 0.25 W/m²K
Formula
R_US = 5.678263337 / U_SI
Steps
5.678263337 / 0.25 = 22.7
Result
R_US = R-22.7
Related calculator
U-value to R-valueExample 2
Inputs
U_SI = 0.18 W/m²K
Formula
R_US = 5.678263337 / U_SI
Steps
5.678263337 / 0.18 = 31.5
Result
R_US = R-31.5
Related calculator
U-value to R-valueSI U-value to US R-value calculator
Example: U-value 0.18 W/m²K converts to about US R-31.5.
Zero and negative values are not meaningful for these conversions. The calculator clamps inputs above zero and the text explains which unit system each value belongs to.
Practical note
Before you use the result
SI U-value to US R-value calculator is intended for quick option checks and technical discussion before detailed execution. The result depends on the selected units, declared material values, and chosen surface resistances, so each change in layer or thickness should be treated as a separate variant.
The calculator does not automatically verify every local rule, thermal bridge, moisture condition, structural connection, or installation tolerance. If the result is close to a requirement, treat it as a reason for deeper verification rather than a final decision.
For better comparisons, test several realistic thicknesses, check current product data sheets, and review the complete assembly. A calculated value is most useful when the assumptions are clear: material, thickness, layer order, units, and data source.
For insulation or U-value tools, layer order and correct units are especially important. For concrete, electrical, plumbing, or heating tools, the result should be read as a quick quantity or plausibility check before standards and execution conditions are reviewed.
Save the result with the date, material name, and assumptions. If the product, diameter, cable section, or thickness changes later, do not compare the numbers alone without checking which inputs changed.
For calculator pages, clear separation between inputs and result is essential. If a value looks surprising, check units and default fields first, then review the project assumptions.
U-value to R-value Calculator
U-value to R-value Calculator
Convert a European U-value in W/m²K to US R-value using R-US = 5.678263337 / U-value.
Assembly thickness
Assembly thickness
Use thickness in inches with k-value, or switch to metric mode for millimetres and lambda. The calculator labels each unit before showing the result.
U-value
U-value
R-value and U-factor move in opposite directions: higher R-value is better, while lower U-factor or lower U-value is better.
CategoryLambdaLayerMaterialThickness
How it works
Unit assumptions
The input is SI U-value. If the input is already a US U-factor in Btu/(h·ft²·°F), use the U-factor to R-value calculator instead.
How to use the result
The equivalent R-value helps translate terminology. It does not turn a full European assembly report into a US code rating by itself.
U-value to R-value Calculator
Formula: R-US = 5.678263337 / U-value SI. U-value 0.25 W/m²K converts to R-22.7 and US U-factor 0.044. U-value 0.18 W/m²K converts to R-31.5 and US U-factor 0.032.
Use this for European wall, roof, floor, and product data when you need a US comparison number. Use the lambda to R-value calculator when the datasheet gives conductivity and thickness instead of final U-value.
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Assembly nameThickness unitInternal Rsi (m2K/W)External Rse (m2K/W)
Calculation assumptions
The calculators use visible formulas and explicit unit conversions. Treat the result as a preliminary check, not a complete building design.
- SI and US units are converted separately; R, RSI, U-value, and U-factor are not mixed without the unit factor.
- Enter positive values and compare the result with the selected product datasheet.
- Local codes, thermal bridges, fasteners, and installation quality can change the requirement.
- Last formula review: 2026-04-27.
See how formulas, unit conversions, rounding, and limitations are handled. Methodology details.
US R-value mini-hub
Move between R-value calculators, material tables, insulation comparisons, and assembly calculators without scanning the whole navigation.
R-value calculators
Material R-values
Insulation comparisons
Assembly calculators
Reference tables
Using European units?
Open the U-value, lambda, and metric insulation thickness tools.
Materials and comparisons
Check lambda values, insulation comparisons, and reference pages before choosing layers.
R-value conversion FAQ
No. Zero resistance or zero transmittance would make the inverse formula impossible or physically meaningless.
They describe heat transfer in different unit systems. US U-factor uses Btu/(h·ft²·°F), while SI U-value uses W/m²K.
Higher is better for R-value because it is resistance. Lower is better for U-factor and U-value because they measure heat transfer.
Use them for checks and comparisons, then confirm manufacturer declarations, assembly corrections, and local code rules.
Yes. It is designed for layered assemblies such as external walls, flat roofs, pitched roofs, floors, and slabs. For unusual assemblies, add every relevant layer and treat the result as a planning check before formal verification.
Yes. It is intended for fast concept-stage calculations, insulation comparison, and envelope optimisation before detailed design. It is best used to narrow choices, not to replace a code check or project-specific thermal bridge assessment.
Yes. You can switch between millimeters, centimeters, and inches, and the calculator keeps the values consistent. For fewer mistakes, choose one unit system at the start of a project and review converted thicknesses before export.
Use SI U-value to US R-value calculator as a first-pass reference. Before specifying anything, compare the result with the actual thickness, actual project dimensions, product data sheet, and local requirements.