How much water is lost when boiling a gallon of water?

If you boil water long enough, all the water will evaporate. However, since you posted your question in the Homebrewing category, let's answer the question with that in mind. If you're making a 5-gallon batch, you can expect to lose at least a gallon during the 90-minute (or longer) boil. When I'm making five gallons, I don't actually boil the whole five gallons. I usually boil three to four gallons and then add enough sterilzed water after the boil to equal five gallons. This helps in the rapid cooling process, which is essential in causing the "cold break," an important step in beer making.