Hydration & Performance: How Much Water Do You Really Need?

As a registered dietitian who is obsessed with sports, whether it’s BJJ, HYROX or running, I had to learn the hard way how important hydration is. Unfortunately, I have been injured a lot throughout my athletic career, the worst being an ACL rupture and multiple concussions.

I started to think back if there were any patterns, and almost every time, I could link the incident to being under-fueled or under hydrated. Even though several factors can influence the likelihood of injury (lack of sleep, training partner etc.), hydrating properly was an easy change that ended up having a huge impact.

Once I spotted the pattern, I started making significant changes to my hydration protocol. I started making significant changes to my hydration protocol. Once I figured out how much I personally needed for my activity level, my performance skyrocketed, sleep improved and injuries decreased.

So let’s clear it up for you too: how much water do you really need, and why does it matter?

Why hydration actually matters for performance

Even a 2% drop in body water is enough to measurably reduce strength, endurance, and reaction time. For context, that’s the amount of fluid most people lose in a single hard training session. You don’t need to be visibly dehydrated to be underperforming — by the time you’re thirsty, you’re already behind.

I notice it on the mats or in any intense session: when I’ve under-hydrated, my rounds feel heavier, my grips fatigue faster, my legs feel tired and my mind is less brain sharp. In clinic, I see the same pattern in clients who feel gassed out during sessions or aren’t recovering well. Quite often, it’s not a calorie or sleep issue, it’s a hydration issue.

The “8 glasses a day” rule is outdated

It’s a decent starting point, but it ignores body size, climate, activity, and the fact that you get roughly 20% of your daily water from food. A 90 kg athlete training twice a day does not have the same needs as a 55 kg office worker.

How much you actually need

A simple starting formula I use with clients: aim for 30–35 ml of water per kilogram of body weight per day as a baseline. For a 70 kg person, that’s around 2.1–2.5 litres.

Then add fluid for training:

• +500 ml in the 2 hours before a hard session
• +500–750 ml per hour of training, depending on sweat rate and heat
• +500 ml post-session, ideally with some sodium

The easiest way to check it: your urine should be pale straw yellow, not bright yellow and not clear. Clear urine usually means you’re flushing electrolytes unnecessarily.

When water isn’t enough: electrolytes

This is the piece most people miss. If you’re training hard, sweating heavily, or competing in hot conditions for over 2 hours? You’re losing electrolytes, mainly sodium through sweat. Replacing the water without the sodium, actually dilutes what’s left in your blood, which is why you can drink litres and still feel flat, crampy or dizzy.

For most training sessions under 60 minutes, food + water is fine. For longer or sweatier sessions, competitions or hot-room training, add an electrolyte tablet or a pinch of salt with a little carbohydrate. You don’t need any fancy brands 😉

Post training hydration hack

A hack I always share with my clients, is to drink a glass of chocolate milk post training. It contains carbohydrates, proteins and natural electrolytes – which all aid in getting you fully hydrated again. And if you aren’t a fan of fancy electrolytes, sports drinks or chocolate milk – simply add an extra pinch of salt to your post training meal.

Quick checklist

• Drink ~30–35 ml per kg body weight per day as a baseline
• Add 500 ml before, 500–750 ml per hour during, and 500 ml after hard sessions
• Don’t wait until you’re thirsty — thirst is a lagging indicator
• Pale straw urine = good; clear = overdoing it; dark = catch up
• Add electrolytes when sessions go over an hour or you’re sweating heavily
• Drink chocolate milk or have extra salt in your meal post training

If you want help dialling in hydration, fuelling, or recovery for your sport, that’s what I do day to day. Book a free intro call or check out my nutrition services.