But water turnover is not exactly equal to the requirement for drinking water, Prof Speakman said.
He said: "Even if a male in his 20s has a water turnover of 4.2 litres per day, he does not need to drink 4.2 litres of water each day.
"About 15% of this value reflects surface water exchange and water produced from metabolism.
"The actual required water intake is about 3.6 litres per day. Since most foods also contain water, a substantial amount of water is provided just by eating.
"This study shows that the common suggestion that we should all be drinking eight glasses of water is probably too high for most people in most situations and a 'one-size-fits-all policy' for water intake is not supported by this data."
He said the research represented a big step forwards in predicting future water needs.
But there can be negatives in drinking too much water.
"Clean drinking water isn't free," Prof Speakman said.
"If people on average drink half a litre more than they need and you multiply that by 40 million adults in the UK, that means that we're needlessly drinking and peeing 20 million litres of water that we have to supply.
"There is a cost in doing that."