Insulation Calculators
R-value is a laboratory property of a material. A house is not a laboratory.
The calculators
Insulation is the one trade on this site where the number on the packaging and the number the building gets are furthest apart, and where the gap is caused by three different things that get confused with each other constantly.
Air moving through it beats resistance every time
An R-value is measured with still air on both sides. Air that leaks past the insulation, or convects inside it, carries heat by a mechanism the rating does not describe at all. That is why a sealed R-38 attic outperforms a leaky R-60 one, and why blower door testing and duct sealing sit in the same code chapter as the R-value table.
None of these calculators measure that. It is not a quantity you can take off a drawing, and a page that implied otherwise would be selling you the easy half.
Installation quality is graded, and the grades are not close
Batts compressed around wiring, cut short at the top plate, or left with a gap behind an electrical box do not deliver their label. The industry grades this I, II and III, and the difference between a Grade I install and a Grade III install of the same product is large enough to swallow an entire code upgrade.
The framing is a hole in the insulation
This one is arithmetic, so it is the one the calculator can do. A stud is roughly R-1.25 per inch against R-3 to R-4 per inch for the insulation beside it, and heat takes the easy path. Work the two paths in parallel and a wall full of batts comes out well below the number on the bag.
What is here
The R-value calculator gives the prescriptive minimum for your climate zone in every code edition still enforced, and works out what your own build-up actually delivers. Pages for blown-in, spray foam and attic quantities follow.