Hydration reminder calculator
This tool turns your body weight and activity into a daily fluid target, then spreads it across your waking hours so you know roughly how often to take a drink. It helps you build a steady drinking habit instead of gulping a lot at once or forgetting entirely.
How it works
The estimate uses two common general guidelines:
- Base = body weight (kg) × 35 ml
- Exercise add-on = exercise hours × 500 ml
- Daily target = (base + add-on), shown in litres
It then converts the target into the number of glasses of your chosen size (target ÷ glass size) and spreads them evenly across your waking hours to suggest a reminder interval: interval = (waking hours × 60) ÷ glasses. Smaller glasses or more fluid mean more frequent sips; more waking hours stretch them out.
Example
For a 70 kg person, 1 hour of exercise, 16 waking hours, 250 ml glass:
- Base = 70 × 35 = 2,450 ml; exercise = 500 ml → target = 2,950 ml ≈ 2.95 L
- Glasses = 2,950 ÷ 250 = 11.8 glasses
- Interval = (16 × 60) ÷ 11.8 ≈ a drink every 81 minutes
| Weight | Exercise | Daily target |
|---|---|---|
| 60 kg | 0 h | 2.10 L |
| 70 kg | 1 h | 2.95 L |
| 90 kg | 1.5 h | 3.90 L |
Why spacing matters more than the total
Research consistently shows that drinking water steadily throughout the day is more effective than consuming the same total in a few large sittings. A steady pace keeps plasma osmolality more stable, reduces the risk of dilutional hyponatraemia (dangerous especially for endurance athletes who drink very large quantities at once), and avoids the discomfort of stomach bloating from a large bolus.
The reminder interval this calculator produces is designed precisely to create that even spacing. Setting a timer or phone alarm at the suggested interval is more reliable than relying on thirst alone — by the time you feel thirsty, mild dehydration has already begun.
What counts as fluid
The 35 ml/kg baseline is a total fluid target, not a plain-water target. Roughly 20% of typical daily fluid intake comes from food (fruits, vegetables, soups), and beverages such as tea, coffee, milk, and diluted juice all contribute. Caffeinated drinks have a mild diuretic effect but still provide net hydration in normal quantities — the idea that coffee “doesn’t count” toward hydration is a persistent myth.
Signs you may need to adjust
The calculator gives a starting point. Adjust your target if:
- Urine colour is consistently dark yellow or amber — drink more
- Urine is consistently very pale and you are urinating frequently — you may be overdrinking
- You are exercising in heat — sweat rate can reach 1–2 litres per hour; the 500 ml/hour preset may underestimate loss during intense outdoor activity
- You are ill with fever or diarrhoea — fluid loss rises sharply and targets should increase significantly
This is a general guideline, not medical advice. Calculated entirely in your browser; nothing is uploaded.