The rate of heat flowing through a material is determined by the material's R-value or metric U-value. The R-value is measured in the SI, or System International, units of Kelvin meters squared per Watt, or in imperial units, square foot degrees Fahrenheit hours per British thermal unit. The U-value has the inverse of R-value units, Watts per Kelvin meters squared. The greater the U-value or the lower the R-value, the more conductive the material will be. In conversation, the scale of the value implies which system is being used and units are not given.

Multiply the U-value by 0.176. At this point, the units will remain the same, Watts per Kelvin meters squared. For example, begin with a U-value of 0.75 Watts per Kelvin meters squared. Multiplying through gives you (0.176)(0.75) = 0.132 Watts per Kelvin meters squared.

Divide 1 by the product of 0.176 and the U-value. The factor 0.176 indicates that the R-value is not simply the reciprocal of the U-value since there is a conversion between units as well as the value. In the example above, dividing 1 by 0.132 gives you (1/0.132) = 7.58.

## Sciencing Video Vault

Switch the units from the metric Watts per Kelvin meter squared to the imperial square foot degrees Fahrenheit hours per British thermal unit. This will distinguish the American R-value from the metric U-value. Therefore, the U-value 0.75 Watts per Kelvin meters squared translates to an R-value of 7.58 British thermal units per hour per square foot degrees Fahrenheit.