Light~200–700 mg/L
Sodium is less of a limiter for you. Sports drinks alone usually cover it, and some light salters may even need to avoid overly concentrated drinks and salt tabs to prevent GI upset.
Hydration & Electrolytes
Carbs scale predictably with output and duration. Hydration and sodium don't. Two athletes riding side by side at the same power can lose wildly different amounts. This is the framework for getting yours dialled in.
Why it matters
Get hydration right and it stabilises heart rate, supports fuelling tolerance, and protects performance late in the race. Get it wrong and you're dealing with GI distress, cramping, and fading power. Errors compound: small mistakes in the first half become race-ending problems in the second.
Hourly fluid target
The goal is to limit excessive dehydration without overhydrating. Use the conditions to find a general Ironman starting range, then refine it against your own sweat rate.
Cool
500–650ml/hr
Cool, low-sweat conditions. Drink closer to thirst and avoid overhydrating early in the race.
Moderate
650–800ml/hr
Typical race-day conditions. Steady intake that keeps thirst manageable and heart rate stable for the effort.
Hot
800–1000+ml/hr
Heat and humidity push losses up. Increase fluids before increasing carbs, and make cooling a priority.
Haven't tested your sweat rate?
These ranges are general guidelines, not your personal numbers. Two athletes in the same conditions can need very different amounts. Treat them as a starting point and dial them in with the Sweat Rate Lab.
More fluid is not always better. Overhydration can dilute blood sodium and create real health risks. Signs it's working: stable HR for the effort, manageable thirst, no stomach sloshing.
Find your sodium cohort
Rather than chasing an exact number, find your cohort and work from there. Sodium loss is measured as a concentration, milligrams per litre of sweat (mg/L). Some athletes sweat lightly but lose lots of sodium; others sweat buckets and lose little.
Medium~700–1100 mg/L
A standard sports drink is typically sufficient. Supplemental sodium (a tab or stronger mix) may help at the higher end of the range or in the heat.
Heavy~1100–2200+ mg/L
Requires consistent electrolyte intake: usually both a sports drink and dedicated salt supplements. If you skip this, you'll likely cramp or fade late no matter how well you fuelled carbs.
If you're untested
Start in the medium range and adjust from what you observe. Cramping late? You're probably undershooting sodium. GI issues with every electrolyte product? You might be overshooting. Median sodium loss sits around 800 mg/L, a reasonable opening estimate to refine through real training and racing.
Principles that don't change
The numbers are personal, but the rules behind them hold for everyone.
Variability is the rule
You can't predict sweat rate or sodium loss from appearance, fitness, or body size. Visible salt rings reflect concentration, not total loss. They aren't reliable data.
Front-load, stay flexible
Sweat rate runs higher early and eases over long events. Slightly front-load sodium, then adjust on feel, temperature, and gut tolerance. Rigid plans with no flexibility tend to break late.
More isn't better
Drinking past your needs can dilute blood sodium and create health risks, not gains. Aim for stable HR, manageable thirst, and no bloating, not the fullest possible stomach.
Rehearse, then re-tune
Long, hard sessions are your test bed: practise bottle mixing, sodium dosing, and aid-station execution there. Needs shift with heat acclimation, fitness, and season, so nothing new on race day.
Don't know your sweat rate yet?
The Sweat Rate Lab turns a weigh-in/weigh-out test into your personal fluid loss per hour, the number this guide builds on.