R-value and U-factor
Lambda to R-value calculator for metric datasheets
First calculate R-SI as thickness in metres divided by lambda; then multiply by 5.678263337 for R-US. This page is built for searchers who need a practical answer first and a transparent formula immediately after it. Use it for early estimating, product comparison, and checking whether a result belongs in metric or imperial notation.
The formulas use explicit SI and US conversions, so λ, k-value, R-value, U-value, and U-factor are not mixed silently.
Quick answer
Lambda to R-value calculator for metric datasheets
First calculate R-SI as thickness in metres divided by lambda; then multiply by 5.678263337 for R-US. A 100 mm board at lambda 0.035 W/mK gives R-SI 2.86 and roughly R-16.2 in US notation.
- Formula
R_SI = thickness_m / lambda; R_US = R_SI * 5.678263337- Example
- A 100 mm board at lambda 0.035 W/mK gives R-SI 2.86 and roughly R-16.2 in US notation.
Working calculator
Enter the core dimensions and check the result directly on this page.
Worked examples
Example 1
Inputs
d = 100 mm = 0.100 mlambda = 0.035 W/mK
Formula
R_SI = d / lambda; R_US = R_SI * 5.678263337
Steps
0.100 / 0.035 = 2.86 m²K/W2.86 * 5.678263337 = 16.2
Result
R_SI = 2.86 m²K/W; R_US = R-16.2
Related calculator
Lambda to R-valueExample 2
Inputs
d = 140 mm = 0.140 mlambda = 0.040 W/mK
Formula
R_SI = d / lambda; R_US = R_SI * 5.678263337
Steps
0.140 / 0.040 = 3.50 m²K/W3.50 * 5.678263337 = 19.9
Result
R_SI = 3.50 m²K/W; R_US = R-19.9
Related calculator
Lambda to R-valueLambda to R-value calculator for metric datasheets
A 100 mm board at lambda 0.035 W/mK gives R-SI 2.86 and roughly R-16.2 in US notation.
translate W/mK and millimetres into a US R-value for imported or international products. For compliance work, confirm product data, framing fractions, air films, and local code requirements before using the number in a final report.
Practical note
Before you use the result
Lambda to R-value calculator for metric datasheets 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.
Lambda to R-value Calculator
Lambda to R-value Calculator
First calculate R-SI as thickness in metres divided by lambda; then multiply by 5.678263337 for R-US. It keeps the calculation small enough for a quick check while still linking to deeper U-value, lambda, and material pages for complete assembly work.
Assembly thickness
Assembly thickness
Thickness can be handled in millimetres, inches, or feet depending on the task, but the conversion path is always explicit.
U-value
U-value
U-value and U-factor are shown only after the relevant R-value has been calculated, which prevents a common unit-system error.
CategoryLambdaLayerMaterialThickness
How it works
Unit discipline
R-value belongs to the US convention and rises as performance improves. U-value and U-factor move in the opposite direction, so the calculator labels each output before showing the number. translate W/mK and millimetres into a US R-value for imported or international products.
Practical use
Use these pages to translate product sheets, compare assemblies, and decide whether a detailed layer-by-layer calculation is needed. First calculate R-SI as thickness in metres divided by lambda; then multiply by 5.678263337 for R-US.
Lambda to R-value Calculator
First calculate R-SI as thickness in metres divided by lambda; then multiply by 5.678263337 for R-US. A 100 mm board at lambda 0.035 W/mK gives R-SI 2.86 and roughly R-16.2 in US notation.
This page is built for searchers who need a practical answer first and a transparent formula immediately after it. Use it for early estimating, product comparison, and checking whether a result belongs in metric or imperial notation. For compliance work, confirm product data, framing fractions, air films, and local code requirements before using the number in a final report.
Add layerRestore default setupRemove
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.
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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 and U-factor FAQ
R-value is thermal resistance in US notation. A higher R-value means the layer resists heat flow better, but it must be tied to the actual installed thickness.
U-factor is the US heat transfer rate through an assembly. It is the inverse of total R-value in US units, so a lower U-factor means better thermal performance.
For US units, divide 1 by U-factor to get R-value. For SI U-value, first convert to US U-factor by dividing by 5.678263337, then invert it.
Calculate R-SI as thickness in metres divided by lambda in W/mK. Multiply the SI resistance by 5.678263337 to get US R-value.
US product labels and energy codes commonly express insulation performance as R-value because it is easy to add layer resistances and compare product thicknesses.
Yes. Metric mode keeps λ, U-value, W/mK, and m²K/W visible. Imperial mode switches the interface to R-value, U-factor, k-value, inches, feet, and Btu units.
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 Lambda to R-value calculator for metric datasheets as a first-pass reference. Before specifying anything, compare the result with the layer continuity, actual project dimensions, product data sheet, and local requirements.