How to Build Your Ironman Hydration Plan From Your Sweat Test Results

You've got your sweat test results. You know your sodium concentration. Now what?

This article walks through how to translate your test data into a practical, leg-by-leg hydration plan for an Ironman or 70.3 — specifically in Australian conditions where heat is a real variable.

Start with your numbers

You need two figures from your sweat test: your sweat sodium concentration (e.g. 1,200mg/L) and your sweat rate (e.g. 800ml/hr at moderate intensity).

These two numbers let you calculate your hourly sodium loss: sweat sodium concentration × sweat rate = hourly sodium loss. For example, 1,200mg/L × 0.8L/hr = 960mg sodium per hour. This is your baseline replacement target for moderate-intensity, mild-condition exercise. From here, you adjust up for heat and intensity.

Swim leg

Most athletes don't lose meaningful amounts of sweat during the swim — the water temperature keeps the body cool, and the effort level is typically below the threshold where heavy sweating occurs. Hydration during the swim isn't practical anyway.

Focus instead on being well-hydrated before you start. A sodium preload 60-90 minutes before race start — particularly on warm days — helps you begin with good fluid retention. Aim to feel comfortably hydrated, not bloated.

Bike leg

This is where the majority of your hydration happens and where mistakes are most common.

Calculate your expected bike duration, apply your hourly sodium target, and work backward to figure out what products and volumes will deliver that sodium. Multiply your hourly sodium loss by the expected bike duration in hours — e.g. 960mg/hr × 2.5hr = 2,400mg sodium needed on the bike. Check the sodium content of your chosen electrolyte products per serving, and distribute your sodium intake across the bike leg rather than front-loading or leaving it all for the end.

In heat above 25°C, increase your fluid intake target proportionally. A rough guide: add 10-15% per 5°C above your baseline testing conditions. And eat — sodium intake needs to be paired with carbohydrate to support both fuelling and gut absorption.

Run leg

Intensity typically rises on the run relative to the bike, and heat has often built through the race morning by this point. This makes the run leg the highest-risk segment for hydration issues — and the one where cramping most often occurs.

Apply the same calculation, adjusted for your expected run duration and conditions. In a 70.3, a two-hour run at race pace might require 1,500-2,000mg of sodium for an athlete in the mid-to-high concentration range.

Use aid station opportunities strategically. Rather than taking a small amount at every station, plan your approach based on what the products at each station contain and whether they match your target.

The most common mistakes

1. Relying on course nutrition. On-course electrolytes vary enormously by race and aren't calibrated to your individual needs. Know what will be on course, calculate whether it meets your targets, and carry supplementary sodium if needed.

2. Drinking to schedule, not to target. Drinking every 20 minutes is a fine rough guide, but if you don't know what you're trying to achieve with each drink, the schedule alone won't get you there.

3. Neglecting the swim-to-bike transition. This is a gap where dehydration can begin to accumulate before you even start your main nutrition window. Drinking immediately on the bike matters.

4. Panic-eating salt tablets late in the race. Late-race sodium is useful but not a rescue strategy. By the time you're cramping, depletion has been building for hours. Prevention through front-loaded, consistent intake is far more effective.