Triathlon Nutrition Calculator (carbohydrate and fluid per hour on race day)

Understanding the triathlon nutrition calculator — concept and principle

This calculator plans carbohydrate and fluid intake for race day in a triathlon or other long endurance event. It uses the sports nutrition consensus on carbohydrate per hour and your sweat rate to estimate how much to eat and drink, without drinking more than you sweat.

How it is calculated

Carbohydrate target (g/h, upper): under 1 h 0; 1–2 h 30; 2–2.5 h 60; over 2.5 h 90

Carbohydrate for the race = target × hours

Expected sweat loss (L) = sweat rate (L/h) × hours

Fluid to drink (L) = sweat loss − body weight × accepted loss % ÷ 100 (never below 0)

Fluid per hour = fluid ÷ hours

The principle

Muscles burn carbohydrate quickly during hard exercise, and stores run low after about 90 minutes. The gut can absorb about 60 g of glucose an hour; adding fructose, which uses a different transporter, raises total uptake to about 90 g per hour. Fluid replaces sweat, but drinking more than you lose risks low blood sodium (hyponatraemia), so the plan allows a modest, accepted loss of body weight.

Worked example

Default inputs: 5.5 hours, 72 kg, sweat rate 0.9 L/h, accepted loss 2%.

1. Over 2.5 h → up to 90 g/h (glucose + fructose); race total = 90 × 5.5 = 495 g.
2. Sweat loss = 0.9 × 5.5 = 4.95 → 5 L.
3. Accepted loss = 72 × 2 ÷ 100 = 1.44 L; fluid to drink = 4.95 − 1.44 = 3.5 L.
4. Per hour = 3,510 ÷ 5.5 = 638 mL.

How to read the result

The targets are upper limits that must be practised in training, because the gut adapts with use. Many athletes do well with less. Sweat rates change with heat, pace and each leg of the race; measure yours by weighing before and after training. Sodium in drinks or gels helps in long, hot races.

Common mistakes

• Trying 90 g/h for the first time on race day.
• Drinking to a fixed schedule regardless of thirst and heat.
• Forgetting that little can be drunk during the swim.

Frequently asked questions

How many grams of carbohydrate per hour in an Ironman?

Up to about 90 g per hour from a glucose–fructose mix, if practised in training.

How do I measure sweat rate?

Weigh yourself before and after an hour of training, add fluid drunk, and subtract urine.

Is losing 2% of body weight safe?

A loss up to about 2% is generally tolerated; more can impair performance.

Sources: Thomas DT, Erdman KA, Burke LM, "American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance", Med Sci Sports Exerc 2016;48(3):543–568 · Sawka MN, Burke LM, Eichner ER, et al., "American College of Sports Medicine position stand. Exercise and fluid replacement", Med Sci Sports Exerc 2007;39(2):377–390