I used it once in financial projections at work: assuming the customer base would continue to double every six months, and that each customer generated data at

I used it once in financial projections at work: assuming the customer base would continue to double every six months, and that each customer generated data at a constant rate, and using integration to compute the total storage needed. I might have also used it when taking depreciation and inflation into account. Technically, though, I learned that calculus from a textbook, rather than by being taught.