US R-value tools
U-factor to R-value calculator
Use this page when a window, door, skylight, or assembly report gives a U-factor such as 0.50, 0.35, 0.30, or 0.25 and you need the equivalent R-value for comparison with insulation language. Lower U-factor is better because less heat passes through each square foot for each degree of temperature difference. Higher R-value is better because more resistance is being added. The two numbers describe the same direction only after you invert them inside the same US unit system.
Calculations keep US R-value, U-factor, SI R, and SI U-value separate so an inverted value is not mislabeled.
Quick answer
U-factor to R-value calculator
Formula: R-value = 1 / U-factor. U-factor 0.50 is R-2.0, 0.35 is R-2.86, 0.30 is R-3.33, and 0.25 is R-4.0. These values are common when translating fenestration performance into a resistance-style number.
- Formula
R_US = 1 / U_factor- Example
- Example: U-factor 0.30 converts to R-3.33 and about 1.703 W/m²K as SI U-value.
Working calculator
Enter the core dimensions and check the result directly on this page.
U-factor to R-value table
Common window and assembly U-factors with their equivalent R-values.
| US U-factor | R-value | SI U-value | Practical reading |
|---|---|---|---|
| 0.50 | R-2.00 | 2.839 W/m²K | window or door |
| 0.35 | R-2.86 | 1.987 W/m²K | window or door |
| 0.30 | R-3.33 | 1.703 W/m²K | lower heat transfer |
| 0.25 | R-4.00 | 1.420 W/m²K | lower heat transfer |
Worked examples
Example 1
Inputs
U_factor = 0.30
Formula
R_US = 1 / U_factor
Steps
1 / 0.30 = 3.33
Result
R_US = R-3.33
Related calculator
U-factor to R-valueExample 2
Inputs
U_factor = 0.25
Formula
R_US = 1 / U_factor
Steps
1 / 0.25 = 4.00
Result
R_US = R-4.00
Related calculator
Window U-factorU-factor to R-value calculator
Example: U-factor 0.30 converts to R-3.33 and about 1.703 W/m²K as SI U-value.
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
How to interpret the calculator result
U-factor to 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-factor to R-value Calculator
U-factor to R-value Calculator
Convert US U-factor values for windows, doors, and assemblies into equivalent R-values with R-value = 1 / U-factor.
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 US U-factor in Btu/(h·ft²·°F). If a document gives SI U-value in W/m²K, use the SI U-value to US R-value page instead.
How to use the result
The converter is a comparison aid. Product ratings can be tested under specific standards, so use declared performance for code documentation.
U-factor to R-value Calculator
Formula: R-value = 1 / U-factor. U-factor 0.50 is R-2.0, 0.35 is R-2.86, 0.30 is R-3.33, and 0.25 is R-4.0. These values are common when translating fenestration performance into a resistance-style number.
U-factor is common for windows and doors because those products include glass, frames, spacers, coatings, and edge effects. R-value is common for insulation because thermal resistances can be added layer by layer. Do not compare a window R-value equivalent directly with a labeled insulation batt unless the assembly context is clear.
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.
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 U-factor to R-value calculator as a first-pass reference. Before specifying anything, compare the result with the margin option, actual project dimensions, product data sheet, and local requirements.