Bra size confusion?

I've been feeling kind of blah since I had my baby so I've decided that I want to buy a nice pair of underwear for when my husband gets back from over seas in two months. I was always unsure about my true bra size even before getting pregnant, so I decided to measure myself and put that into a size calculator. When I measured my bust size was 40 inches and under my bust was 36 inches. When I put this into the first calculator it said my size was 38B. I wanted to be extra sure so I put the measurements into two more calculators. One of them said 38C and the other said 36D. I am so confused. I know I'm definitely not a C or D cup but I'm afraid if I get a 38B then the band will be too small because according to lengire companies, small or too big. I really want to buy off of the adore me website because you get your first set for $25 and I hear that the bras and panties are really good quality. Does anyone know if their size calculator is accurate or how I can find my most accurate bra size?