U-Value ⇄ R-Value Converter (SI and US)

Understanding the U-value and R-value converter — concept and principle

Insulation performance is quoted in two opposite ways. The R-value (thermal resistance) says how strongly a layer resists heat flow; the U-value (thermal transmittance, called U-factor in the US) says how much heat gets through. On top of that, Europe and Korea use SI units while the US uses imperial units, so "R-13" on an American batt is not R 13 in SI. Enter any one of the four values and this converter shows all of them, so you can compare products, windows and wall specs from different countries.

R is the resistance of the insulation layer; U is its reciprocal, the heat that still leaks through each square metre per kelvin.

How it is calculated

The calculator first converts your input to SI R-value (m²·K/W):

from U (SI): R = 1 ÷ U
from R (US): R(SI) = R(US) ÷ 5.678263
from U-factor (US): R(SI) = 1 ÷ (U(US) × 5.678263)

Then it reports U(SI) = 1 ÷ R(SI), R(US) = R(SI) × 5.678263 and U(US) = 1 ÷ R(US).

The principle

Heat flow through a wall works like electric current through a resistor. The heat flux is q = U × ΔT, and resistance is the inverse of transmittance, so U = 1 ÷ R. Layers in series add their resistances, which is why designers sum R-values and then invert the total to get U. The factor 5.678263 links the two unit systems: 1 BTU/(h·ft²·°F) equals about 5.678263 W/(m²·K), because it combines the BTU-to-joule, hour-to-second, square-foot-to-square-metre and °F-to-K conversions.

Worked example

Default inputs: R-value (US) = 13, a common wall batt.

R(SI) = 13 ÷ 5.678263 = 2.289 m²·K/W (shown 2.28943239).
U(SI) = 1 ÷ 2.28943 = 0.4368 W/(m²·K).
R(US) stays R-13, and U-factor (US) = 1 ÷ 13 = 0.0769230769 BTU/(h·ft²·°F).

How to read the result

Higher R and lower U both mean better insulation. Doubling R halves U and therefore halves heat loss through that element, but the benefit of each extra step gets smaller in absolute terms. A single-layer R-value covers only that material; whole-assembly values must also include surface air films, framing, fixings and thermal bridges, which usually make the real U-value worse than the insulation alone suggests. Window U-values normally refer to the whole unit including frame.

Common mistakes

Comparing a US R-value directly with an SI R-value; they differ by a factor of about 5.68.
Adding U-values of layers instead of adding R-values and then inverting.
Treating an insulation product's R as the wall's overall R.

Keep in mind: Building-code requirements depend on your country, climate zone and building type. For real projects, use certified product data and have the design checked by a qualified professional against local codes.

Frequently asked questions

How do I convert R-19 to SI?

19 ÷ 5.678263 ≈ 3.35 m²·K/W.

Is a lower U-value better?

Yes. A lower U-value means less heat passes through each square metre per degree of temperature difference.

How do I get R from insulation thickness?

Divide thickness in metres by thermal conductivity: 0.1 m of a 0.035 W/(m·K) material gives about 2.86 m²·K/W.

Sources: ISO 6946, Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods · NIST Special Publication 811 (2008), Guide for the Use of the SI, Appendix B

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.