Enter any u-value into the calculator to convert it into the equivalent RSI value.

U Value to RSI Formula

The following equation is used to calculate the RSI from the U-value.

RSI = 1/U

  • Where RSI is the thermal resistance per unit area
  • U is the u-value

What is a RSI?


The R-value measures thermal resistance used in building materials, including insulation. The higher the R-value, the greater the resistance to heat flow and the better the material is at keeping a structure warm in winter, cool in summer, or both.

R-value is used in building codes and standards to regulate the amount of insulation required in residential and commercial structures, especially in walls and floors.

The term R-value represents the thermal resistance of a building material or system (such as insulation) according to ASTM Standard C518 (American Society for Testing and Materials). R-value measures thermal resistance used in the building and construction industry. It represents the ability of materials to resist heat flow (thermal resistance) and is commonly used as a rating for insulation materials.

In general terms, R-value describes how well a particular material or assembly of materials resists heat flow from one side to the other. It is calculated by measuring the temperature difference across an insulating material with both sides exposed to an agreed-upon temperature, usually 110°F or 140°F.