You know that sharp odor of chlorine from the swimming pool you can recall from earliest childhood? It turns out it’s not just chlorine, but a potent brew of chemicals that form when chlorine meets sweat, body oils, and urine.

But up until now, just how much urine has been difficult to measure, says chemist Xing-Fang Li of the University of Alberta. Li and her colleagues report they can now tell roughly how much pee is in a pool by measuring the artificial sweeteners carried in most people’s urine. Certain sweeteners can be a good proxy for pee, she says, because they’re designed to “go right through you” and don’t break down readily in pool water.

The scientists calculated that one 220,000-gallon, commercial-size swimming pool contained almost 20 gallons of urine. In a residential pool (20-by-40-foot, five-feet deep), that would translate to about two gallons of pee.