Its official, scientists are bored...
After combing the beaches of Britain, they came up with OW = 0.125 x S. Put simply, that means you need to use eight times as much sand as water.
They said any more water meant the walls were too soggy and any less meant the sand wouldn't bind together.
The scientists also figured out which beach had the best sand for castles, rating Torquay as number one.
Top beaches
Bridlington, Great Yarmouth and Bournemouth also did well, because of their fine grained sand.
Beaches were also judged on how clean the beach was and how good the sand looked.
Professor Matthew Bennett, who led the study, said: "If people get the water-sand combination right, they will have hours of fun."