If the ambient humidity rises to 65%, the moisture content will rise to 11.7%, or an equivalent 4% in MC. The flooring boards will expand 0.0369" each. For a ten-foot stretch of flooring, the expansion represents between 1.7712" or 1 49/64" (4.5 cm). The boards will exert pressure against one another until they abut against the walls.

48 boards x 0.0369" = 1.7712 (4.5 cm)