Does coffee count as water? What the evidence says

Mostly, yes. For people who drink coffee regularly, a normal cup adds more fluid than the caffeine in it takes away. Caffeine does make you pee a little more, especially in large amounts or if you are not used to it, but at everyday levels the water in the drink wins. The NHS counts tea and coffee toward your daily fluids, and so do other health bodies.

So the more useful questions are about something else: how much caffeine is sensible in a day, whether coffee is crowding water out of your afternoon, and whether the last cup lands too close to bedtime. This post covers all three, starting with where the old "coffee dehydrates you" idea came from.

Where the "coffee dehydrates you" idea came from

Caffeine is a mild diuretic, which means it nudges the kidneys to make a bit more urine for a while after you have it. That part is true, and it is why the idea stuck.

The effect is strongest with large doses and in people who don't usually have caffeine, which is not how most of us drink coffee at a desk. Regular drinkers seem to become partly tolerant to the effect, so for them a normal cup makes only a small difference to how much they pee.

Put simply: a mug of coffee is mostly water. The caffeine takes back a small part of it, and for regular drinkers a very small part.

What the research suggests

The evidence here is not huge, but it points the same way.

• A small 2014 study had regular coffee drinkers, all men, spend a few days drinking moderate amounts of coffee and a few days drinking the same amount of water. Their hydration measures came out much the same either way. It is one small study in one group, but it tested something close to real life.
• Reviews of the research generally conclude that moderate caffeine intake does not lead to dehydration in people who are used to it.
• Health bodies agree. The NHS includes tea and coffee in its 6 to 8 glasses a day, and the US National Academies' report on water reached a similar view: caffeinated drinks contribute to your daily water. How much water should you drink at work sets the official figures side by side.

What the research does not say is that coffee is as good as water in every way. It hydrates you fine, but it also brings caffeine, and often milk, sugar or syrup. Those are the parts worth watching.

What counts, drink by drink

Nearly everything you drink adds to your fluids. What differs is what comes along with the water.

How much caffeine is sensible

Health bodies give a fairly consistent upper guide for healthy adults.

• Up to about 400 mg a day. The US Food and Drug Administration says this amount is not generally linked with dangerous effects in healthy adults, and the European Food Safety Authority reached a similar figure. EFSA also suggests keeping a single dose to about 200 mg.
• Less in pregnancy. The NHS and EFSA suggest no more than 200 mg a day if you are pregnant. If you are pregnant or breastfeeding, ask your doctor or midwife.
• Less if it doesn't suit you. Some people feel jittery, anxious or notice a racing heart on far less. Some medicines and health conditions change how caffeine affects you, so if you are unsure, ask a doctor or pharmacist.

The tricky part is knowing what is in your cup. The FDA gives roughly 80 to 100 mg for an 8 ounce cup of coffee and 30 to 50 mg for the same amount of black or green tea, but real drinks vary a lot. A large café coffee is often two or more times that size, and strength depends on the beans and how it is made. Four "cups" a day can mean very different things depending on the cups.

Swapping the third coffee for water

If you drink three or four coffees a day and feel fine, there is no fluid reason to stop. But for many people the third cup isn't about the coffee. It is about the break: the walk to the kitchen, something warm to hold, a pause between tasks. You can keep the break and change the drink.

• Water first, then decide. Before each coffee, have a glass of water. Sometimes that is all you wanted.
• Make the afternoon cup decaf or herbal. Same mug, same ritual, much less caffeine.
• Have something hot that isn't coffee. Hot water with a slice of lemon, or a herbal tea, still feels like a proper break.
• Go sparkling. A cold glass of sparkling water has enough bite to feel like a treat.

This is also where a nudge from outside helps, because the third coffee is usually automatic. Notchpals has two Water characters, Melo the watermelon and Puddle the seal pup, and whichever one is in your crew gets a single afternoon visit, somewhere between 2 and 5 PM, about when the third cup tends to happen. Puddle, for the record, takes a harder line than the evidence does.

PuddleNot coffee, please! Coffee makes me dizzy. Water makes me clap.

Notchpals doesn't know what is in your mug and doesn't count coffees, so what Done means is up to you. A glass of water before the coffee counts. So does swapping the coffee for a herbal tea. If the mug in your hand is still half full, press Later: it comes back once, 20 minutes on, by which point the mug is usually empty and the kitchen is the natural next stop.

Timing caffeine for sleep

Coffee's bigger effect on a working day is often at night. Caffeine stays in your body for hours after the cup is empty, and how long varies a great deal from person to person. A coffee at 4 PM can still be working when you are trying to fall asleep, even if you don't feel wired.

A practical rule many people use is to have the last caffeinated drink in the early afternoon and switch to water, decaf or herbal tea after that. If you sleep badly, try moving your cutoff earlier for a week and see what changes. What time to stop drinking coffee for better sleep goes through how to work out your own cutoff.

The short version

• Coffee and tea count toward your fluids. Caffeine's diuretic effect is real but small for regular drinkers.
• Keep caffeine to about 400 mg a day if you are a healthy adult, and less if you are pregnant or it doesn't agree with you.
• Café sizes hold more than you think, so go by the size of the cup, not the number of them.
• If coffee has pushed water out of your day, keep the break and swap the drink, especially for the afternoon cup.
• Stop caffeine early enough that it is gone by bedtime.