A 300 mm flatsawn oak panel can move 5 mm between a damp summer and a heated winter. Glue it solid and it splits. This works out the real figure for your species, your cut and your workshop, using the dimensional change coefficients published by the USDA Forest Products Laboratory.
The coefficients are Table 13-5 of the Wood Handbook (USDA Forest Products Laboratory, General Technical Report FPL-GTR-190), a public-domain reference. Nothing here is estimated or interpolated: if a species is not in that table it is not in this list.
Change in dimension = starting width x coefficient x change in moisture content, in percentage points. That is equation 13-2 of the Wood Handbook. It is a straight-line approximation validated between 6% and 14% moisture content, which covers almost all indoor furniture.
Tangential movement - the flatsawn face - is roughly twice radial movement in most species. Quartersawing the same board can halve the problem. That is why quartersawn stock is worth its price for wide panels, door frames and anything that has to stay flat.
Wood settles at an equilibrium moisture content set by the air around it. This tool converts your relative humidity and temperature using the Hailwood-Horrobin equation as fitted by Simpson, equation 4-5 of the Wood Handbook. At 21 C the results reproduce the published table exactly: 30% RH gives 6.2%, 65% gives 12.0%, 80% gives 16.0%.
Measure it. A cheap hygrometer in the room the piece will live in, read in February and again in August, beats any table. If you cannot, a heated house in a temperate winter typically sits at 25-35% and an unconditioned summer at 60-75% - but a coastal workshop and a centrally heated flat are different worlds.
Movement along the grain is negligible and is ignored, which is correct for solid timber and wrong for nothing you are likely to build. Thickness moves like width and follows the same coefficients. Plywood and MDF barely move at all - do not use this for them. And a board that is still drying will move far more than this on its way to equilibrium.