You hear it all the time: “Drink eight, eight-ounce glasses of water per day!” But does that one-size-fits-all advice really work for everyone? How much hydration do we really need?

Those numbers are actually averages. Yes, the average person needs 64 ounces of water per day, but there are many other factors that come into play, that might cause you to need even more fluids. For example, we can’t really expect a 250-pound man to be properly hydrated by the same amount of water that a 100-pound woman is drinking. And if you exercise a lot, or live in a hot climate, those factors will obviously create more thirst.

Follow a hydration schedule. By the time you actually feel thirsty, your body is already dehydrated. That’s why many experts recommend that you carry a refillable water bottle, and sip water regularly throughout the day. During exercise in particular, 4-6 sips every 15 to 30 minutes is a good rule of thumb. By keeping your body hydrated during exercise, you can protect your muscles from becoming overheated, and keep your body temperature in a safe range.

But also listen to your thirst. The above recommendations are a good starting place. But depending upon your activity level, some experts recommend following your body’s natural cues. If you’re urinating frequently during exercise, you might be overdoing the hydration schedule. Long distance runners, in particular, can be prone to over-hydration and subsequently low blood sodium levels. In this case, many experts recommend simply following your thirst.

Try this strategy. Weigh yourself before and after exercise, and if you lose more than 2 percent of your body weight during the workout, you might be dehydrated. On the other hand, if you gain weight, you’re over-hydrating.

Be flexible. As the weather changes, your hydration needs might change as well. Increase your fluid intake on extra hot days. If you travel to a cold climate and take your workout regimen with you, be aware that cold weather can mask symptoms of dehydration.

Most people find that combining a schedule with other methods of monitoring hydration will work best. But always keep in mind the symptoms of severe dehydration (headache, fatigue, dry mouth, rapid heartbeat, rapid breathing and dizziness), just in case. If you experience these signs, slow down on the exercise until you’ve properly rehydrated yourself.