cssunits allows you to use the R units library with CSS and LaTeX widths.
Install from GitHub with:
# install.packages("devtools")
devtools::install_github("hughjonesd/cssunits")
library(cssunits)
library(units)
#> udunits system database from /usr/local/share/udunits
install_css_units()
set_units(72, "pt") + set_units(1, "in")
#> 144 [pt]