R-Value Calculator
Calculate the R-value of a wall or roof build-up from the thickness and conductivity of each layer.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
About this calculator
This estimate converts the entered thickness in inches and conductivity into imperial R-value and metric RSI. Fixed surface-air-film values are added only when selected. Results depend on the material conductivity you enter and do not replace a tested assembly or local code calculation.
Worked example
calculated with the default values below
Material thickness 3.5 in · Thermal conductivity (k) 0.04 W/m·K · Number of identical layers 1 · Include surface air films Yes (adds about R-1) R-value (imperial) 13.47 R · RSI value (metric) 2.372 m²K/W · U-factor 0.0742 Btu/h·ft²·°F · Total thickness 88.9 mm
Method and assumptions
R = t ÷ (6.9336 × k) × n + 0.85 Air films add 0.85 R (0.15 RSI) when enabled; metric RSI = t × 0.0254 ÷ k per layer; U-factor = 1 ÷ R. This estimate converts the entered thickness in inches and conductivity into imperial R-value and metric RSI. Fixed surface-air-film values are added only when selected. Results depend on the material conductivity you enter and do not replace a tested assembly or local code calculation. The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
Data sources
The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
R = t ÷ (6.9336 × k) × n + 0.85
- R
- Total R-value (imperial)
- t
- Layer thickness (in)
- k
- Thermal conductivity (W/m·K)
- n
- Number of identical layers
- RSI
- Metric R-value (m²·K/W)
Air films add 0.85 R (0.15 RSI) when enabled; metric RSI = t × 0.0254 ÷ k per layer; U-factor = 1 ÷ R.
Frequently asked questions
Why does including air films raise my calculated R-value?
The air films are the still boundary layers on each side of the wall, and the model adds a fixed 0.85 imperial R, or 0.15 metric, when you include them. A bare 3.5-inch cavity at k equals 0.04 computes to about R-12.6, rising to roughly R-13.5 once the films are counted. This is why a real wall tests higher than the material alone suggests.
Which inputs and limits apply to R-Value Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. Material thickness: 0.1–36 in; Thermal conductivity (k): 0.005–5 W/m·K; Number of identical layers: 1–20 R = t ÷ (6.9336 × k) × n + 0.85 Air films add 0.85 R (0.15 RSI) when enabled; metric RSI = t × 0.0254 ÷ k per layer; U-factor = 1 ÷ R. This estimate converts the entered thickness in inches and conductivity into imperial R-value and metric RSI. Fixed surface-air-film values are added only when selected. Results depend on the material conductivity you enter and do not replace a tested assembly or local code calculation. The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
What is a complete worked example for Material thickness = 3.5 in; Thermal conductivity (k) = 0.04 W/m·K?
Inputs: Material thickness = 3.5 in; Thermal conductivity (k) = 0.04 W/m·K; Number of identical layers = 1; Include surface air films = Yes (adds about R-1). Calculation: R = t ÷ (6.9336 × k) × n + 0.85. Results: R-value (imperial) = 13.47 R; RSI value (metric) = 2.372 m²K/W; U-factor = 0.0742 Btu/h·ft²·°F; Total thickness = 88.9 mm.