Hydration isn't “drink more water.”
It's drink the right amount, for you.

Mistake one: not drinking enough, especially in the heat or over long sessions, and paying for it with cramping, GI distress, and performance falling off a cliff.

Mistake two — more common than people think, especially in longer endurance events: drinking too much. Overdrinking dilutes sodium in the blood and can cause exercise-associated hyponatraemia, a genuine medical emergency. "Just drink heaps" is not good advice.

The fix for both is the same: know your own sweat rate and build a plan around it, rather than following a generic rule that was never written for your body.

Know your numbers: the sweat rate test

Sweat rates vary hugely between people — anywhere from 0.5 to over 2.5 litres an hour — and shift again with heat, humidity, and intensity. This test takes one training session and tells you more than any generic hydration chart ever will.

1. Empty your bladder, then weigh in nude Do this right before your session starts. Note the number in kg.
2. Weigh your fluids before you start Weigh your bottles or hydration pack before the session (1g = 1ml, so this converts directly to volume).
3. Train as normal for 60 minutes Same intensity, same conditions you're trying to plan for. Don't hold back on drinking — drink as you normally would.
4. Weigh yourself again, plus your leftover fluid Towel off first — sweat on skin adds error. Note any urine losses if you went during the session.

Worth knowing: repeat this test across different conditions — a hot, humid day and a cool one will give very different numbers. If you're chasing conditions abroad, retest closer to the time rather than relying on one result from a Tāmaki Makaurau winter.

Before, during, after

Before

• ~5–7ml/kg of fluid roughly 4 hours before training or competing.
• If urine is still dark 2 hours out, add a further 3–5ml/kg, sipped rather than skulled.
• Aim for pale straw-coloured urine before you start — not completely clear. Clear urine can be a sign of overhydration.
• Don't chug a big volume in the final 15 minutes — it won't be absorbed in time and just means an early toilet stop.

During

• Sessions under 60 minutes: plain water is genuinely fine.
• Sessions over 60–90 minutes, high intensity, or in the heat: match intake to your tested sweat rate — commonly 400–800ml/hour, adjusted from your own number.
• The gut can only absorb roughly 1–1.2L/hour. High sweat rate athletes usually can't fully replace losses during exercise, and that's fine — you finish the gap afterwards.
• Sip regularly rather than drinking in big, infrequent gulps.

After

• Rehydrate at roughly 125–150% of the body mass you lost — for every 1kg lost, aim for 1.25–1.5L of fluid over the following few hours.
• Pair fluid with sodium — a normal meal with salt does the job, or add a sodium-containing drink if you're not eating straight away.
• Spread it out over several hours rather than smashing it all at once.
• Check urine colour again the next morning as a simple gut-check.

Electrolytes: the sodium conversation

Sodium is the electrolyte that actually matters most during exercise. The others get a lot of marketing attention for less practical reason.

Potassium, magnesium, and calcium are usually covered by normal diet — losses during a single session are small relative to what a normal diet provides. Don't overthink these unless you're deep into multi-day or extreme-heat territory.

Which drink actually works

DIY option: a standard isotonic sports drink can be made more hydrating by diluting it — roughly a third to a half a cup of water added per litre — plus about 0.5–0.8g of table salt per litre.