How this calculator works
The race distance sets the length of each leg. If you enter a target finish time, the page divides it using typical triathlon proportions: swim about 16 percent, bike about 49 percent and run about 32 percent for an Olympic race, with the bike and run taking larger shares over longer distances. If you enter your own splits, those are used directly. Each leg is then costed with a metabolic equivalent, or MET, that reflects race effort: 8 for swimming, 10 for cycling and 11 for running. One MET is 3.5 millilitres of oxygen per kilogram per minute, and a litre of oxygen releases about five kilocalories. The nutrition targets scale with race duration rather than distance, because the gut works per hour, not per kilometre.
Formula
Leg energy (kcal) = MET x 3.5 x body mass in kg / 200 x minutes. Total energy is the sum of the three legs. Carbohydrate target = 45 g per hour x race hours. Fluid target = 600 mL per hour x race hours. Sodium target = 450 mg per hour x race hours.
Worked example
A 75 kg athlete targeting 2:30:00 for an Olympic distance, or 9,000 seconds, gets split times of 1,440 seconds swimming, 4,410 seconds cycling and 2,880 seconds running, with the remainder as transition. Swim energy is 8 x 3.5 x 75 / 200 x 24 = 252 kcal. Bike energy at 10 MET for 73.5 minutes is 965 kcal, and run energy at 11 MET for 48 minutes is 693 kcal. The total is about 1,910 kcal. A 2.5 hour race calls for about 113 g of carbohydrate, 1,500 mL of fluid and 1,125 mg of sodium.
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Frequently asked questions
How much carbohydrate should I eat during a race?
For events over about 90 minutes, 30 to 60 g per hour is the traditional range, and higher intakes up to 90 g per hour are used by trained athletes with a mix of glucose and fructose. Start below your limit and practise the plan in training.
Why is fluid per hour rather than per kilometre?
Sweat loss depends on time, heat, humidity and effort. Two athletes covering the same course at different speeds lose fluid at similar rates per hour but very different totals. Drinking to thirst and to a tested sweat rate is more reliable than a fixed volume.
Do I need to replace all the calories I burn?
No. The body stores carbohydrate and fat, so the goal is to delay depletion, not match expenditure. Most athletes can absorb only a fraction of their hourly burn, and eating too much causes stomach problems.
Does the calculator work for a relay or a shorter race?
The maths works for any distance you can enter, but fuelling matters mainly beyond 60 to 90 minutes. For a sprint or a relay leg, water and a small carbohydrate intake are usually enough.