The short answer
At normal amounts, no. Caffeine has a mild diuretic effect — it makes your kidneys pass a little more fluid — but the water in the drink itself far outweighs it. Controlled studies have found that moderate coffee drinking hydrates you about as well as plain water, and major health bodies count tea and coffee toward your daily fluid intake.
Where the myth comes from
Old studies gave people large doses of pure caffeine, often after a caffeine-free period, and measured more urine. That's a real diuretic effect — but it's not what happens when a regular coffee drinker has their usual cup. Your body also adapts: people who drink caffeine routinely barely show a diuretic response at all.
When caffeine's diuretic effect actually matters
- Very large single doses — roughly 300 mg or more at once (3+ coffees, a strong pre-workout) can noticeably increase urine output.
- If you rarely have caffeine — your body hasn't adapted, so the effect is stronger.
- Around hard exercise or heat — when you're already losing fluid through sweat, it's worth drinking extra water regardless.
The practical takeaway
Your morning coffee is keeping you hydrated, not drying you out. Just keep your total caffeine sensible — check your limit with the intake calculator — and have a glass of water alongside coffee if you tend to skip plain water in the morning. (That water habit also helps with the post-coffee slump.)
Put it into practice: see how much caffeine you're actually getting from each drink in the caffeine chart.