Before you start having nightmares about invisible creepy-crawlies all over your hands, take a deep breath. Liquid hand soap is still effective after a splash of water—phew—with one caveat. “As long as there’s surfactant, you will be able to remove [germs], although you may need more as dilution increases,” explains Tetro. In other words, the surfactant, aka the soap, will work as long as there’s enough of it in the mix.
We know what you’re thinking: how do I know it’s enough? There’s no exact science about how much water is OK to add—we asked. “Honestly, I don’t believe anyone has done that experiment…It’s not supposed to be diluted, but everyone does it,” says Tetro. “The ratio of soap to water will determine the level of removing power that occurs. So, if you dilute a little, then it’s not a large problem.”
Instead of keeping a Pyrex under the sink for measuring, don’t stress. You can make an educated guess based on how many suds bubble up while you wash, regardless of the type of hand soap (foam, bar, liquid) you’re using. “As long as you are able to coat your hands with the active ingredient, usually shown by lather, then there really is no better or effective type of soap,” Tetro reassures us.