How insulation quantity is estimated
This kind of insulation estimator works from either a total square-foot area or from simple wall dimensions. It then applies the selected stud spacing and insulation format to estimate the coverage each batt or roll can provide, before adding a waste allowance for trimming and offcuts.
The R-value you choose changes the assumed product thickness and the coverage assumptions used by the estimate. That makes the result useful for comparing common batt and roll scenarios, even though final product coverage should still be checked against the specific insulation brand you intend to buy.