Estimate wall insulation from wall length and height, in square feet and bags of batts.
Bags = wall area ÷ coverage per bag, plus waste. Framing takes up part of the cavity, so gross wall area slightly overstates the batts you'll install; that margin usually covers waste.
At 88 sq ft per bag, about 12 bags before waste. Coverage varies a lot by R-value and batt width, so use the number on the bag.
For small openings most estimators don't; subtract large glazed areas.
Yes: buy batts sized for the framing (15" wide for 16" o.c., 23" wide for 24" o.c.).
Doing it from plans instead? Read Insulation takeoff from plans, or see all free takeoff calculators.