I have a new apple 8 watch and am wondering if a 40 mm band will fit my 41mm watch?

You're welcome. It's not the band size that's different on a 38mm, 40mm or 41mm watch. It's the size of the screen of the watch which is different. Bands sold for any of those models will fit any of those models.

Apple bands in 38mm, 40mm and 41mm, will fit a 41mm Apple Watch 8.