Your weight: The first step to knowing how much water to drink everyday is to know your weight. The amount of water a person should drink varies on their weight, which makes sense because the more someone weighs the more water they need to drink. A two hundred pound man and 100 pound woman require different amounts of water every day.
Multiply by 2/3: Next you want to multiple your weight by 2/3 (or 67%) to determine how much water to drink daily. For example, if you weighed 175 pounds you would multiple that by 2/3 and learn you should be drinking about 117 ounces of water every day.
Activity Level: Finally you will want to adjust that number based on how often you work out, since you are expelling water when you sweat. You should add 12 ounces of water to your daily total for every 30 minutes that you work out. So if you work out for 45 minutes daily, you would add 18 ounces of water to your daily intake.
NOTE: Says Trent Nessler, PT, DPT, MPT, managing director of Baptist Sports Medicine in Nashville (WebMD), how much water you need “depends on your size and weight, and also on your activity level and where you live. In general, you should try to drink between half an ounce and an ounce of water for each pound you weigh, every day.” This gives you a broader range, but as you can see, supports the "Multiply by 2/3" rule of thumb.