Construction Calculator

R-Value and U-Value Conversion

Quick answer: 1 m²·K/W ≈ 5.678 ft²·°F·h/BTU (R-5.68), US R-13 ≈ 2.29 m²·K/W; U = 1/R, so R = 4 m²·K/W gives U = 0.25 W/(m²·K) ≈ 0.044 BTU/(h·ft²·°F).

R in m²·K/W and US R-value (ft²·°F·h/BTU), and U-value in W/(m²·K) and BTU/(h·ft²·°F) – U = 1/R.

How the calculation works

Enter a thermal resistance or U-value and choose the unit. The table shows the same value as R (m²·K/W and US R-value) and as U (W/(m²·K), BTU/(h·ft²·°F), kcal/(h·m²·°C)), plus clo and tog.

Formula and assumptions

The thermal resistance of a layer is R = d/λ (m²·K/W). The US R-value is in ft²·°F·h/BTU: 1 ft²·°F·h/BTU = 0.17611 m²·K/W. The U-value is the reciprocal: U = 1/R; for a whole wall U also includes surface resistances (Rsi + Rse ≈ 0.17 m²·K/W).

More worked examples

  • R-19 (US) insulation ≈ 3.35 m²·K/W.
  • A wall with U = 0.25 W/(m²·K) → R = 4 m²·K/W ≈ R-22.7.
  • A window with U = 1.1 W/(m²·K) ≈ 0.194 BTU/(h·ft²·°F).
  • 12 cm EPS, λ 0.035: R = 0.12/0.035 ≈ 3.43 m²·K/W.

R and U

R (m²·K/W)R-value (US)U (W/(m²·K))
1R-5.71.0
2R-11.40.50
3R-17.00.33
4R-22.70.25
5R-28.40.20
6R-34.10.17

Common mistakes

  • Converting U to R by multiplying instead of taking the reciprocal.
  • Comparing a US R-value with R in m²·K/W without the factor 5.678.
  • Forgetting the surface resistances Rsi and Rse when calculating a wall U-value.

Frequently asked questions

What is R-13 in m²·K/W?

R-13 ≈ 13 × 0.17611 ≈ 2.29 m²·K/W.

How do I get the U-value from R?

U = 1/R. For R = 5 m²·K/W, U = 0.2 W/(m²·K) (without surface resistances).

How do I convert a U-value in W/(m²·K) to a US U-factor?

Divide by 5.678: U = 1.4 W/(m²·K) ≈ 0.247 BTU/(h·ft²·°F).

Is a higher R better?

Yes, a higher thermal resistance means lower heat loss. For the U-value it is the opposite – lower is better.

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Publisher: Art-Visum d.o.o. · Content reviewed:
Calculations use common consumption rates and manufacturers' technical data sheets; results are estimates. Methodology · Standards · About