Most end up in the hotel's waste system. In the US alone, 2 million soap bars are tossed every day, where they end up as landfill.
However, there are organisations that collect used soap from hotels, melt it down and distribute fresh, hygienic soap bars to disadvantaged communities around the world.
Victoria-based Soap Aid (soapaid.org) is a not-for-profit that does just that.
To date, Soap Aid has distributed more than a million soap bars around the world, supplying hygienic soap to more than a quarter of a million people and saving more than 100 tonnes of soap from ending up in landfill.
Clean the World (cleantheworld.org), which began in the US a decade ago, is a soap recycling powerhouse, distributing close to 50 million recycled soap bars in more than 127 countries.
Almost 1.5 million children under five die each year due to preventable childhood infectious diseases.
Diarrhoea alone kills close to a million, and something as simple as washing their hands with antibacterial soap can go a long way toward keeping them infection free.