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R ↔ USI ↔ ImperialFree

R-Value ↔ U-Value
Converter

Convert between thermal resistance (R) and transmittance (U), and between metric and imperial units. Enter any one value and get the other three: R and U in both SI and US units.

Who it's for: architects and energy designers moving thermal numbers between material datasheets and code checks. Enter any one R or U value; the tool returns all four. It uses U = 1/R within a unit system and R(imperial) = R(SI) × 5.678 between systems, at construction documents.

U = 1 / R
Within a system, U is the reciprocal of R. To switch systems: R(imperial, ft²·°F·h/BTU) = R(SI, m²K/W) × 5.678263. An R-13 (US) batt ≈ 2.29 m²K/W ≈ U-0.44 W/m²K.
Convert
All four at once

How to use this calculator

1
Enter your value
Type the single thermal number you already have into the Value field — for example 2.29 from a material datasheet.
2
Say what it is
Pick from the dropdown whether that number is an R-value or a U-value, and whether it is in SI or imperial units.
3
Convert
The tool first reduces your entry to an SI R-value, then derives the other three figures from it.
4
Read all four
R-value and U-value appear together in both SI (m²K/W, W/m²K) and imperial (ft²·°F·h/BTU, BTU/h·ft²·°F).
5
Add surface resistances if needed
These are surface-to-surface values. For a whole-assembly U-value, add the ISO 6946 surface resistances (Rsi 0.13, Rse 0.04) to the layer R before inverting.

The formula

Within one unit system, U = 1 ÷ R — thermal transmittance is the reciprocal of thermal resistance. To move between metric and imperial: R(imperial, ft²·°F·h/BTU) = R(SI, m²K/W) × 5.678263. Multiply a metric R-value by 5.678 to get the imperial figure; divide to go the other way.

Worked example

Example
Enter an R-value of 2.29 m²K/W (SI) — the resistance of a typical R-13 batt. U-value (SI) = 1 ÷ 2.29 = 0.4367 W/m²K. The imperial R-value = 2.29 × 5.678263 = 13.0032 ft²·°F·h/BTU, and the imperial U-value = 1 ÷ 13.0032 = 0.0769 BTU/h·ft²·°F. All four describe the same layer in two unit systems.

When this estimate will be off

  • Surface-to-surface values only — does not include the still-air films on each face. For a whole-assembly U-value per ISO 6946, add interior surface resistance (Rsi 0.13 m²K/W) and exterior surface resistance (Rse 0.04 m²K/W) to the layer sum before inverting.
  • No thermal bridging — studs and framing carry heat at much higher U-values than the insulated cavity. Use a parallel-path or isothermal-planes method for framed wall assemblies.
  • Single-value conversion only — if you have multiple material layers, sum all layer R-values first, then convert or invert the total.
  • R-values are measured under standard lab conditions; field performance shifts slightly when insulation is compressed, wet, or exposed to extreme temperatures.

Frequently asked questions

A common mistake: Surface-to-surface values only — does not include the still-air films on each face. For a whole-assembly U-value per ISO 6946, add interior surface resistance (Rsi 0.13 m²K/W) and exterior surface resistance (Rse 0.04 m²K/W) to the layer sum before inverting.

U-value is the reciprocal of R-value within one unit system: U = 1 ÷ R. R-value measures thermal resistance, while U-value measures heat transmittance. A layer with R = 2.29 m²K/W has U = 0.437 W/m²K. Raise the R-value and the U-value falls; the two always move in opposite directions.

Divide the imperial R-value by 5.678263. Imperial R is measured in ft²·°F·h/BTU and SI R in m²K/W, and that factor carries the unit change. An R-13 batt becomes 13 ÷ 5.678263 = 2.29 m²K/W. To go the other way, multiply the SI R-value by 5.678263.

A lower U-value means less heat passes through the tested layer or assembly for each degree of temperature difference, while a higher R-value means more resistance to that flow. This converter does not select a code, certification, or project target.

These figures are surface-to-surface. A whole-assembly U-value also adds the still-air films on each face. ISO 6946 gives an interior Rsi of 0.13 m²K/W and an exterior Rse of 0.04 m²K/W for horizontal heat flow. Add both to the layer R-values before inverting, so total R = Rsi + ΣR + Rse.

Sources

  • ISO 6946:2017 — Building components and elements — thermal resistance and transmittance — calculation methods, including the standard surface resistances Rsi and Rse.
  • IECC 2021 Table R402.1.2 — US minimum insulation R-values by climate zone for ceilings, walls, floors, and windows — the code context where these conversions appear most often.

This free R-Value ↔ U-Value Converter is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.