Can this fit a calculator?
Yes,the dimension of the calculator for reference is
7.3 x 1 x 3.5 inches (TI-83 Plus) / 4.8 x 5.75 x 2.01 inches (Standard Calculator)
Will the case fit bullet journal ?
Yes, it fits the bullet journal.
And also fits for A5 A6 A7 A8 notebook, 7.9 inch iPad mini , 6 inch Kindle,etc.
Will a B5 Stalogy fit inside?
Yes, it fit a 5.5" X 8.5" sketchbook with 21 colored pencils inside. The material is fairly flexible with a very strong zipper.