Ironman Nutrition Plan Calculator
Calculate hourly calorie, carb, sodium, and fluid intake targets for Ironman and long-course triathlon racing.
About this calculator
Ironman-distance racing turns nutrition into a pacing problem in its own right: burn roughly 8-10 calories per kilogram of body weight per hour at Ironman effort (the upper end applying in hot conditions), but your gut can only absorb a fraction of that while working. This calculator assumes a 35% absorption rate on the bike — the leg with the least jostling and the most time to digest — and then discounts run-leg intake to 65% of the bike figure, because impact and blood-flow redirection away from the gut make solid or dense fueling much harder to tolerate on foot. Carbohydrate targets fall out directly from calories at 4 calories per gram, which — because both calorie burn and absorption scale with body weight — means the output swings widely across the input range, from roughly 28g/hr for a lighter 40kg athlete in cool conditions up to around 118g/hr for a 150kg athlete in the heat; even the defaults (75kg, moderate heat) land in the mid-50s, below the 60-90g/hr range commonly cited in modern research, so heavier or hot-weather athletes should expect this calculator's number to run ahead of that range and lighter, cool-weather athletes should expect it to run behind (dual-transport glucose+fructose blends can push tolerance higher still, which this calculator doesn't distinguish between).
Sodium and fluid needs scale together off a 500mg / 500mL per hour baseline, with flat step increases for hotter conditions and heavier sweat rates — a simplification of what's really an individual, sweat-test-driven number, so treat these as a starting point rather than gospel. The race is split roughly 10% swim, 53% bike, 35% run, 2% transitions to compute total nutrition needs, deliberately excluding the swim since fueling underwater isn't realistic. The gel-equivalent figure (based on ~100 calories per gel) is there to translate the math into an actual shopping list and special-needs-bag count — useful for logistics, but your gut's real-world tolerance on race day should always override any calculator's absorption assumptions.
Inputs
Results
Bike calories/hr
223
How to Use This Calculator
- Enter your Body Weight in kilograms.
- Set your Expected Race Time in hours.
- Choose Heat Conditions (Cool, Moderate, or Hot) and your Sweat Rate.
- Review Bike Calories/Hour, Run Calories/Hour, Carbs/Hour, Sodium/Hour, and Fluid/Hour targets.
- Use Total Race Calories, Total Carbs, Total Sodium, and Gel Equivalents to fill your special needs bags and aid station strategy.
How the result changes with Body weight (kg)
| Body weight (kg) | Bike calories/hr |
|---|---|
| 40 | 119 |
| 56 | 167 |
| 113 | 336 |
| 150 | 446 |
What each input means
- Body weight (kg)
- Your body weight in kilograms. Used to estimate calorie expenditure and absorption rates.
- Expected race time (hours)
- Estimated total race time in hours. Ironman finishes typically range from 8 to 17 hours.
- Heat Conditions
- Expected race day temperature. Affects sodium and fluid needs.
- Sweat Rate
- Your personal sweat rate. Affects sodium and fluid recommendations.
What each result means
- Bike calories/hr
- Target calorie intake per hour on the bike leg.
- Run calories/hr
- Target calorie intake per hour on the run (reduced due to GI stress).
- Bike carbs/hr (g)
- Target carbohydrate grams per hour on the bike.
- Run carbs/hr (g)
- Target carbohydrate grams per hour on the run.
- Sodium/hr (mg)
- Target sodium intake per hour in milligrams.
- Fluid/hr (mL)
- Target fluid intake per hour in milliliters.
- Total race calories
- Total calories needed across the entire bike and run.
- Total race carbs (g)
- Total grams of carbohydrates needed for the race.
- Total sodium (mg)
- Total sodium needed across the race.
- Total fluid (L)
- Total fluid needed in liters.
- Gel equivalents
- Approximate number of energy gels (~100cal each) equivalent to total calorie needs.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersBody weight (kg) = 75, Expected race time (hours) = 12, Heat Conditions = 2, Sweat Rate = 2 = 4 input(s) provided
- Calculate Bike calories/hrBike calories/hr = round(calBurnPerHr * absorptionRate)223 = 223
- Calculate Run calories/hrRun calories/hr = round(bikeCalPerHr * 0.65)145 = 145
- Calculate Bike carbs/hrBike carbs/hr = round(bikeCalPerHr / 4)56 = 56
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is my run calorie target lower than my bike calorie target, even though I'm burning calories at a similar rate on both?
The calculator discounts run-leg intake to 65% of the bike figure to model gut tolerance, not calorie burn — running's impact and the blood flow redirected to your legs make your digestive system far less able to process food than the smoother, lower-impact effort of cycling. Your total calorie need doesn't change much between disciplines, but how much you can safely absorb does.
Why does the calculator only compute nutrition needs for the bike and run, not the swim?
Total race nutrition is calculated as bikeCalPerHr × bikeHours + runCalPerHr × runHours only — swim hours are estimated (about 10% of total race time) for reference, but no calories are attributed to that segment, since eating or drinking mid-swim isn't practically realistic.
My carb target came out well above (or below) the commonly cited 60-90g/hr range — is that a bug?
No, it's a direct consequence of the formula scaling with body weight and heat: calorie burn is set at 8-10 cal/kg/hr, 35% of that is assumed absorbable, then converted to grams at 4 calories per gram. A 40kg athlete in cool conditions lands around 28g/hr while a 150kg athlete in heat lands near 118g/hr — both are correct outputs, they just fall outside the 'typical' range because that range is built around average-sized athletes.
How are sodium and fluid targets adjusted for heat and sweat rate?
Both start from a 500 (mg or mL) per hour baseline and add flat step increases: +200 per heat level above cool, and +150 per sweat-rate level above light sweater. A heavy sweater racing in heat gets 900 mg/hr sodium and 900 mL/hr fluid, while a light sweater in cool conditions stays at the 500 baseline — a simplified linear model that your own sweat-test data will always beat.
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