Warp & Twist are the two characteristics used to determine the flatness of printed circuit boards.
Warp determination :
The percentage K of the deflection (tk) relative to the length (L) of the curved edge
K= tk/L*100 %
If the warp occurs both lengthwise and crosswise, the largest value counts.
Twist determination :
Twist is the percentage V of the deflection (tv) relative to the length (D) of the diagonal:
V=tv/D *100 %
More can be learned from our BLOG on warp & twist.