Question: how much roundup in 15 gallons of water?
Answer: Mix 1.5 ounces (3 tablespoons) of herbicide to 1 gallon of water to renovate an entire lawn, or to eradicate weeds and grasses that are easy to kill. For tougher jobs, such as vines or perennials weeds, mix 2.5 ounces (5 tablespoons) to 1 gallon of water.