The first sign of mild dehydration is rarely thirst. It is usually foggy attention, a dull headache and an afternoon energy dip. By the time thirst actually shows up, you are already running a small deficit.
A calculation that is good enough
At ordinary activity levels, start with bodyweight in kilograms times 30-35ml. A 60kg person lands at roughly 1.8-2.1 litres. Add 400-600ml per hour for intense sessions over 40 minutes or for hot conditions.
Pace it rather than chugging it
Split the total into six to eight servings of 200-300ml. Absorption is steadier and you avoid the load of a large volume in a short window. Attach servings to fixed anchors: on waking, two across the morning, one either side of lunch, two in the afternoon, a small amount in the evening.
How to tell you are on target
The simplest marker is urine colour: pale straw or clear means you are hydrated, dark amber means catch up. A second check is timing — if an afternoon slump lifts within 20 minutes of a glass of water, mild dehydration was probably part of it.
Frequently asked questions
Do tea and coffee count?
Yes. Their diuretic effect is widely overstated; the bulk of a normal serving still counts toward total fluid. Just mind the caffeine and your sleep.
Can drinking too much be a problem?
A very large volume in a short time can dilute blood sodium. After heavy sweating, rehydrate in portions and include some electrolytes.