Is a 245 40 18 tire lower profile than a 255 40 18?

Is a 245 40 18 tire lower profile than a 255 40 18?
Yes, the sidewall will be 4mm shorter. Tire sizes work like this 245 (tread width in mm)40 (Ratio of sidewall to tread width) 18 (Bead diameter) so 245mm X 40% = 98mm, 255mm X 40% = 102mm. Hope this helps.