Dehydration is the silent performance killer. A 2% drop in body weight through fluid loss increases heart rate by 5-10 beats per minute at the same power output. Your blood gets thicker, your heart works harder, and your cooling system starts failing. Most cyclists know they should drink more. Very few actually drink enough.
How Much to Drink
The general framework:
| Conditions | Fluid per Hour | |---|---| | Cool (< 15°C) | 400-500ml | | Moderate (15-25°C) | 500-700ml | | Hot (25°C+) | 700-900ml | | Extreme heat (35°C+) | 900ml+ |
These are starting points. Individual sweat rates vary enormously — some riders lose 500ml/hour, others lose 1.5 litres. The best way to calibrate: weigh yourself before and after a 1-hour ride (no drinking, no bathroom). Every kg lost equals roughly 1 litre of sweat.
When to Start
Start hydrating before you clip in. 500ml in the hour before the ride. Then drink consistently from the first 15 minutes — don't wait until you're thirsty. By the time you feel thirst, you're already 1-2% dehydrated.
On rides under 60 minutes in cool conditions, water is fine. Over 60 minutes or in warm conditions, you need electrolytes.
Electrolytes
Sweat isn't just water — it contains sodium, potassium, magnesium, and chloride. Sodium is the most important for cyclists because it's lost in the highest quantities (500-1500mg per litre of sweat).
Target: 500-700mg sodium per hour on rides over 2 hours or in hot conditions.
How to get it: Electrolyte tablets (Nuun, SiS, Precision Hydration) in one bottle, energy drink or plain water in the other. This gives you flexibility to adjust based on how you feel.
Signs You're Getting It Wrong
Under-hydrating:
- Heart rate creeping up at the same power
- Headache developing mid-ride
- Dark urine post-ride
- Feeling foggy or irritable
- Cramps in the final third of a long ride
Over-hydrating (hyponatremia — rare but serious):
- Nausea and bloating
- Drinking far more than you're sweating
- Only drinking plain water on very long rides without electrolytes
Practical Tips
Two-bottle system. One bottle with electrolytes, one with energy drink or plain water. This lets you adjust your sodium and carb intake independently.
Set a timer. On long rides, set a recurring alarm every 15-20 minutes to remind you to drink. Most cyclists forget, especially when the pace is hard.
Practice in training. Just like fuelling, hydration is a skill. Your gut adapts to higher fluid intake over time. Don't try a new hydration strategy on race day.
Use our Fuelling Calculator alongside your hydration plan — carbohydrate and fluid intake work together.
Key Takeaways
- 2% dehydration increases heart rate by 5-10 BPM at the same power
- Drink 500-700ml per hour in moderate conditions, more in heat
- Start hydrating before the ride — 500ml in the hour before
- 500-700mg sodium per hour on rides over 2 hours or in heat
- Two-bottle system: electrolytes in one, energy drink in the other
- Don't wait until you're thirsty — you're already dehydrated
- Weigh yourself before and after a ride to calibrate your personal sweat rate
- For the complete fuelling strategy, read our in-ride nutrition guide
- Dehydration is a common cause of elevated heart rate on the bike
- Race day nutrition requires a hydration plan alongside your fuelling plan
- Indoor training demands even more hydration — 750ml+ per hour
