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By Gunal M
You have probably heard that coffee dries you out, so each cup needs a glass of water to make up for it. The research does not back that up for ordinary amounts. For most people, a normal cup of coffee counts toward your fluids instead of cancelling them out.
Here is what two studies actually found, where the real limits are, and an easy way to count coffee in your day. This is general information, not medical advice.
Quick answer: moderate coffee, a few cups a day, hydrates about as well as water for regular coffee drinkers. Very large doses of caffeine in one go can make you pee more, and water is still the best default.
Does coffee dehydrate you?
Caffeine is a mild diuretic, which means it can make you urinate more. That is where the myth comes from. But how much it matters depends on the dose and on whether you drink coffee regularly, and in studies of everyday amounts the effect on hydration is small to none.
What the research found
Four cups a day vs water
In a 2014 study, 50 men who normally drank three to six cups of coffee a day were given either coffee or water for three days each, with food, fluids and activity controlled. The researchers found no meaningful differences between the two on a range of blood and urine hydration measures (Killer et al., PLoS ONE).
The authors' conclusion: moderate daily coffee in regular drinkers hydrates about as well as water. A caveat is that all the participants were men who already drank coffee, so it may not apply to everyone.
A very strong dose
A smaller 2017 study looked at ten healthy adults who drank one to three cups of coffee a day. Each had 200 mL of water, a lower-caffeine coffee (about 269 mg of caffeine on average), or a higher-caffeine coffee (about 537 mg). Only the high-caffeine coffee increased urine output: about 613 mL over three hours, versus 356 mL after water. The lower-caffeine coffee did not differ from water (Frontiers in Nutrition).
So there is a point where caffeine does make you pee more, but 537 mg is far more than a typical cup holds.
How much caffeine is in your drink?
The FDA says about 400 mg of caffeine a day is not generally associated with negative effects for most healthy adults, which is roughly two to three 12-ounce cups of brewed coffee. For a 12-ounce drink, it lists (FDA):
- Brewed coffee: about 113 to 247 mg.
- Black tea: about 71 mg.
- Green tea: about 37 mg.
- Caffeinated soft drink: about 23 to 83 mg.
- Decaf coffee: about 2 to 15 mg per 8-ounce cup.
Sensitivity varies. Pregnancy, breastfeeding, some medicines and some health conditions can call for a lower limit, so ask your doctor if any of those apply to you.
Does coffee count toward your daily water?
Yes, in the sense that the water in your coffee is fluid your body can use. Tea, milk and juice count too, and so do water-rich foods. But water is still the best baseline because it has no sugar, calories or caffeine.
A sensible approach is to treat coffee as part of your fluids but not the only part. If most of what you drink is coffee, add water on top, and drink more water on hot days and when you exercise.
If you are not sure how much you need in total, see how much water you should drink a day in ounces or in litres.
Tips for coffee drinkers
- Have a glass of water with your coffee if you like, but you do not need to match each cup.
- Watch your total caffeine, including tea, energy drinks and soda, and stay near the 400 mg limit.
- Go easy late in the day, since caffeine can affect sleep.
- Check your urine. Pale yellow means you are well hydrated. Dark means drink more.
- Be careful with sugar. Sweetened coffee drinks add calories and sugar that plain coffee does not.
Count coffee without the math
In Hydrabit you can choose Water, Coffee, Tea, Juice, Soda or Other before you log. Water counts in full. The other drinks count toward your goal at a slightly lower typical value, so a coffee helps but is not scored like a glass of water.
- Log a glass in a tap, on your iPhone or Apple Watch.
- See your day at a glance as a ring.
- Get gentle reminders that stop once you reach your goal.
- Set your own goal in millilitres or ounces.
Download Hydrabit on the App Store. It is free to use, has no account, and keeps your data on your device.
Frequently Asked Questions
The bottom line
Coffee does not dehydrate you at normal amounts. Count it toward your fluids, keep water as your base, stay near 400 mg of caffeine, and let your urine colour tell you if you need more.
Pick your goal, log your drinks and let reminders do the remembering. Download Hydrabit to get started.
