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R-Value and U-Factor Converter

Convert insulation R-values and window or wall U-factors between US and metric units.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Result

U-factor (US)
0.077BTU/h·ft²·°F
RSI (metric R-value)
2.289m²·K/W
U-value (metric)
0.437W/m²·K

R-13 (US) = RSI 2.289 = U 0.0769 BTU/h·ft²·°F = 0.437 W/m²·K.

R-value (US)
13h·ft²·°F/BTU
Show the working
  1. U = 1 ÷ R (in the same unit system)
    U(US) = 1 ÷ 13 = 0.07692
  2. 1 h·ft²·°F/BTU = 0.1761102 m²·K/W (NIST SP 811)
    RSI = 13 × 0.1761102 = 2.2894
  3. 1 BTU/h·ft²·°F = 5.678263 W/m²·K (NIST SP 811)
    U(SI) = 0.43679

The conversions

R-value measures resistance to heat flow; U-factor measures how much heat flows. For the same assembly they are reciprocals: U = 1 ÷ R.

  • 1 US R-value (h·ft²·°F/BTU) = 0.1761102 m²·K/W (RSI)
  • 1 US U-factor (BTU/h·ft²·°F) = 5.678263 W/m²·K

Both factors are from NIST Special Publication 811.

Common pitfalls

Only take the reciprocal of a whole assembly’s value, including its air films. You cannot add U-factors of layers in series — add their R-values and then convert. Insulation labels in the US show R-values; window labels show U-factors.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

I have
R-value, US (h·ft²·°F/BTU)
Value
13

Result

R-value (US)
13 h·ft²·°F/BTU
U-factor (US)
0.077 BTU/h·ft²·°F
RSI (metric R-value)
2.289 m²·K/W
U-value (metric)
0.437 W/m²·K

R-13 (US) = RSI 2.289 = U 0.0769 BTU/h·ft²·°F = 0.437 W/m²·K.

Assumptions

  • International Table BTU; factors from NIST SP 811. U-factor and R-value are reciprocals only for the same assembly, including its air films.

Limitations

  • Does not convert per-inch values (thermal resistivity); convert the R-value of the actual thickness.

Frequently asked questions

What is R-13 in RSI?

About RSI 2.29 (13 × 0.1761102).

Is a lower U-factor better?

Yes. A lower U-factor means less heat flow; a higher R-value means the same.

What is the difference between RSI and R-value?

RSI is the metric R-value in m²·K/W. A US R-value is about 5.68 times larger than the same insulation’s RSI.

Sources

  1. NIST Guide to the SI (SP 811), Appendix B.9 — thermal insulance and heat transfer coefficient — National Institute of Standards and Technology. 1 °F·h·ft²/BTU_IT = 0.1761102 m²·K/W; 1 BTU_IT/(h·ft²·°F) = 5.678263 W/(m²·K); 1 lb/ft³ = 16.01846 kg/m³; 1 lb/h = 1.259979 × 10⁻⁴ kg/s.

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