How much water to drink in Phoenix heat

There are two kinds of bad answer to the question of how much water to drink in a Phoenix summer. The first is a rounded number with no source: eight glasses a day, or a gallon, or “just stay hydrated”. The second is a shrug, on the grounds that nobody publishes a figure.

Nobody is wrong about the first. NWS Phoenix publishes real numbers, and they are larger than most people carry. The shrug is wrong, and this page exists to close it.

Everything below is the two agencies that publish hydration guidance for this desert (the National Weather Service’s Phoenix office and the CDC’s National Institute for Occupational Safety and Health) read together, with the places they differ left visible rather than averaged. Health and safety pages on this site cite official sources only and give no individual medical advice; where a decision turns on a person’s own condition or medication, the answer is a clinician, and where somebody is confused, fainting or fitting, the answer is 911.

The claim: eight glasses a day

The eight-glasses figure is not on any Arizona, county or federal page this site could open, and the reason is not that the agencies are being coy. Hydration requirement is not a property of a person. It is a property of a person doing something, in a particular temperature and humidity, for a particular length of time.

So the useful guidance is a rate rather than a total, and that is exactly the shape of what the agencies publish.

What the agencies actually publish

The figures, in units you can carry

The agencies publish in litres, and this site’s readers are more likely to own a quart bottle than a measuring jug, so the conversions belong here rather than in a footnote. The arithmetic is this site’s; the litres are the agencies’.

• 2 litres is about 68 US fluid ounces, a little over half a US gallon, or just over two quarts.
• 1 litre is about 34 US fluid ounces, a shade over a quart.
• 1 to 2 litres an hour outdoors is therefore 34 to 68 ounces an hour, and it is a rate per hour outdoors, not per hour of daylight.
• 4 litres is about 135 US fluid ounces, a little over a US gallon: the weather service’s figure for what strenuous activity can take out of a body in an hour at the top end.

Set beside those, Salt River Project’s one gallon per person per day reads as what it is: a household emergency supply for an outage or a storm, sized for drinking and basic sanitation through a disruption, not a rate for working outside.

The gap between the weather service’s figures and the household one is the whole point. One is a rate for a person doing something in the heat; the other is a stock for a house that has lost its water. They are not competing numbers, and neither is a target for a trail.

Why the thermometer is the wrong guide

Water loss tracks exertion and humidity, not the dry-bulb reading on the wall, and Phoenix is the city where that gap is easiest to see. June is the hottest month by temperature. July and August give back a degree or two and add the monsoon dew point, which is where the heat index separates from the thermometer and where the hours above the first National Weather Service category multiply.

The chart is the argument for drinking on a schedule rather than on thirst. In April, seven hours of the day reach the first category and none reach the second. In July, the day never leaves the first category at all: eight hours are Caution and sixteen are Extreme Caution, which the National Weather Service defines as the point where heat stroke, heat cramps or heat exhaustion are possible with prolonged exposure or physical activity.

Same city, same activity, same person. The number of hours in which the answer is “drink on a clock” has more than tripled, and the litres-per-hour figure applies to all of them.

Water alone is not always the answer

Both agencies say the same unusual thing about salt, and it is the opposite of what most people reach for. The CDC says to avoid salt tablets. NWS Phoenix says to avoid them unless a physician directs otherwise.

What they recommend instead is a drink or a snack that replaces carbohydrates and electrolytes, taken alongside water, repeated every 15 to 20 minutes during hard work. A litre of water an hour with nothing else in it replaces volume and not much else, and cramps are the classic sign that the deficit has two halves.

The other half of that guidance is the stopping rule, and it is more specific than most people expect. If cramps have not subsided within an hour of fluids and rest, or the person has heart problems or is on a low-sodium diet, that is medical help rather than more water.

When it stops being about drinking more

Rhabdomyolysis is the reason a hydration article needs a limit clause. It is muscle breakdown releasing proteins and electrolytes into the blood, it can cause dangerous heart rhythms, seizures, kidney damage or death, and the CDC is blunt that symptoms alone cannot tell you whether you have it.

The signs it lists are dark urine the colour of tea or cola, muscle aches or cramps worse than you would expect, weakness, and being unable to finish a workout you could do before. The instruction that follows is not to drink more and see how it goes. It is to stop, drink more liquids with water preferred, and seek immediate care at the nearest medical facility.

Dark urine after hard work in the heat is a reason to go, not a reason to keep sipping.

Where the numbers leave a gap

Three gaps are worth naming rather than smoothing over.

The first is indoor water. NWS Phoenix’s 2-litres-a-day figure is for people who stay mostly indoors, which in a Phoenix July is most people, most days, in air conditioning, and air conditioning dehydrates less obviously than the sun. The second is that no agency publishes a figure for the hours between: the person who is outdoors for twenty minutes at a time, several times a day, is not covered by a per-hour-outdoors rate in any useful way. The third is that all of this is a rate for a healthy adult. The CDC’s own framing puts people with chronic conditions, people on medications that affect fluid balance, older adults and young children in a different category, and the specific answer for a specific person is a question for a clinician.

The weather service also makes one point that undercuts any reliance on alerts: it reports that heat-related illness has been observed even on days it classifies as low risk. The guidance is not conditional on the warning.

A rule you can carry

Keep a bottle in the car and another at the desk. Drink on a clock rather than on thirst, which NWS Phoenix notes arrives only once the body is already mildly dehydrated. Add something with electrolytes and carbohydrates for long, hard work, and skip the salt tablets. Treat the litres-per-hour figure as a rate per hour outdoors and do the sum before you leave, not halfway through. Set the day’s activity against the Heat-Safe Hours Planner, which puts the ceiling for hard effort at a heat index of 85 °F, below the first National Weather Service category, on the grounds that thirst arrives after the deficit does. And treat dark urine or cramps that will not settle as a reason to stop and seek care.

If somebody is confused, has stopped sweating or is sweating heavily with hot skin, has slurred speech, has fainted or is fitting, call 911. The Heat-Illness Triage Guide works through the symptoms as a decision path, in the CDC’s own wording.