How Much Water to Drink During Exercise (The Real Numbers)

This used to be a back-of-the-envelope job. Now it is one tap, but the logic has not changed. Most people guess at exercise hydration. The actual math is simple: sweat rate, replace it, add electrolytes when it counts.

Calculate your sweat rate

Weigh yourself before and after a 1-hour workout (no fluids). Each kg lost ≈ 1L sweat.

Replace, don't over-drink

Aim to replace 80–100% of fluid lost during long sessions. More can dilute sodium.

When electrolytes matter

Workouts over 60 minutes, hot weather, heavy sweaters. Plain water alone can backfire.

Signs of dehydration vs over-hydration

Dehydration: dark urine, dizziness, cramps. Over-hydration: clear urine all day, nausea, confusion.

Pro tips

• Pre-hydrate the morning of a hard workout (500ml on waking).
• Salt the night before endurance events. Sodium drives fluid retention.

Frequently asked questions

Are sports drinks worth it? Yes for sessions over 60 minutes or in heat. For shorter workouts, water is fine.

Why do I cramp despite drinking? Cramps are usually electrolyte or fatigue-driven, not dehydration. Add sodium and pacing.

If you only remember one thing from this post, make it that the inputs matter more than the calculator.