How much water should you drink when hiking in the heat?

Use the quart an hour to decide how much to pack: for a three-hour hike, that's about three quarts.¹ What you actually need can run higher or lower, because it changes with the heat, how hard you're going, and your size.² So keep an eye on your bottles, and turn back once half your water is gone.¹

Start the hike well hydrated, then drink whenever you feel thirsty, in small sips rather than big gulps.³⁴ Sweat carries salt as well as water, so on a hike of more than a couple of hours, eat salty snacks like pretzels or crackers along the way.²⁵ Salt helps replace what you sweat out, but it won't protect you if you drink far more than your body needs.² That much water, or even sports drink, can dilute the sodium in your blood to a dangerous level, a condition called hyponatremia.²⁶ Thirst makes a better guide than a drinking schedule, because it usually keeps you from drinking too little or too much.²

If you start to feel unwell, how much you've been drinking tells you which problem it probably is.² Dark urine, peeing less than usual, dizziness, headache or weakness can mean you've fallen behind or the heat is getting to you, so rest in the shade for 30 to 45 minutes and sip water, ideally with electrolytes.³⁷ Feeling nauseated or bloated after drinking a lot suggests the opposite problem, too much water, so stop drinking and eat something salty while you rest.² If you don't feel better after resting, get help.⁷ Confusion is an emergency whatever the cause, so if someone in your group becomes confused, call 911 and start cooling them while you wait.²