Carbohydrate: 30 to 90 g per hour, set by race length
The carb target uses the same duration bands as our carb intake calculator, from Asker Jeukendrup's 2014 review. Races under an hour need no carbohydrate beyond breakfast. From one to 2.5 hours the target is 60 g per hour of a single sugar such as glucose. From 2.5 hours up, the gut can absorb about 90 g per hour, but only from a glucose and fructose mix, because the two use different transporters.
That 90 g ceiling assumes a trained gut. Jeukendrup's 2017 review on training the gut shows that absorption and comfort improve when you practise high carb intakes in training. If you have not, the calculator caps you at 60 g per hour, which most runners and riders tolerate on race day without practice.
Fluid: stay under 2 % body-mass loss, never drink more than you sweat
The ACSM 2007 position stand on fluid replacement sets the goal as preventing a body-mass loss greater than 2 % from water deficit, without drinking so much that you gain weight. The calculator takes your total sweat loss for the race, subtracts the 2 % you can afford to lose, and spreads the rest across the hours. It never plans more fluid than your sweat rate, because drinking beyond sweat losses is the main cause of exercise-associated hyponatremia.
Sweat rate matters more than any other input. If you have not measured it, the default is 1.28 L per hour, the mean for endurance athletes in Barnes and colleagues' 2019 data from 1,303 athletes. In Baker's 2016 data individual rates ran from 0.26 to 5.73 L per hour, so a weigh-in test with our sweat rate calculator makes the plan far more accurate.
Sodium: matched to your sweat, inside the ACSM range
Sweat sodium differs a lot between athletes. Baker and colleagues measured a predicted whole-body concentration of 35.9 ± 10.4 mmol/L across 506 athletes. The calculator's low, typical and salty presets are that mean and one standard deviation either side: about 590, 825 and 1,065 mg of sodium per litre of sweat. A lab or patch test result can be entered directly.
ACSM recommends drinks with 20 to 30 mmol/L of sodium, which is 460 to 690 mg per litre. The plan uses your sweat concentration clamped into that range and never plans more sodium than you lose. It also shows the sodium per bottle you need, so you can pick a drink mix or add capsules to match.
Before the race: carb loading and the pre-race meal
For events longer than 90 minutes, Burke and colleagues (2011) recommend 10 to 12 g of carbohydrate per kg of body mass per day for the 36 to 48 hours before the race. For a 70 kg athlete that is 700 to 840 g a day, which means cutting fibre and fat to make room. Shorter races do not need a load.
On race morning, eat 1 to 4 g of carbohydrate per kg in the 1 to 4 hours before the start, as in the 2016 joint position of the ACSM, the Academy of Nutrition and Dietetics and Dietitians of Canada (Thomas, Erdman and Burke). Use the small end close to the start and the large end when you have three or four hours.
How it works
- 1Enter the race. Give the expected finish time, or the distance and your goal pace. Duration sets the carb band and how many feeds you need.
- 2Enter body mass and sweat rate. Body mass sets the 2 % loss budget. Use a measured sweat rate if you have one, or the endurance-athlete average.
- 3Pick your sweat sodium. Choose low, typical or salty (white crust on your kit after long sessions), or type a measured value in mg/L.
- 4Set your products. Enter grams of carbohydrate per gel, the carb content of your drink and your bottle size. Carbs from the drink are counted first.
- 5Follow the timeline. The plan lists every feed by race clock and totals the gels and bottles to carry. Rehearse it in a long training session first.
Frequently asked questions
How many gels do I need for a marathon?
For a 3:30 marathon with water only and 25 g gels, the plan uses one gel every 15 minutes, 13 gels in total, to hit 90 g per hour. With an untrained gut the target drops to 60 g per hour, about one 25 g gel every 25 minutes. A carb drink reduces the gel count.
Is 90 g of carbs per hour too much?
Not for races over 2.5 hours, if the carbs are a glucose and fructose mix and you have practised high intakes in training. Without gut training, 60 g per hour is the safer ceiling, and that is what the calculator uses when you say your gut is untrained.
How much should I drink per hour in a race?
Enough to keep total body-mass loss under 2 %, and never more than your sweat rate. For a 70 kg athlete sweating 1.28 L per hour over 3.5 hours that is about 900 ml per hour. In a short race you may need almost nothing.
How much sodium per hour do I need?
It depends on how much and how salty you sweat. At 900 ml of fluid per hour and ACSM's 20 to 30 mmol/L drink range, that is roughly 410 to 620 mg per hour. Typical and salty sweaters both land at the top of that range, because their sweat is saltier than 30 mmol/L. The calculator also shows how much you lose, so you can see the gap.
Do I need to carb load for a half marathon?
Only if you expect to run longer than 90 minutes. Burke and colleagues set the carb-loading threshold at events over 90 minutes, so a 1:45 half qualifies and a 1:25 half does not. Everyone benefits from a carb-rich pre-race meal.
Sources
- Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine 44(Suppl 1): S25-S33.
- Jeukendrup, A. (2017). Training the gut for athletes. Sports Medicine 47(Suppl 1): 101-110.
- Sawka, M. et al. (2007). ACSM position stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise 39(2): 377-390.
- Baker, L. et al. (2016). Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences 34(4): 358-368.
- Barnes, K. et al. (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes. Journal of Sports Sciences 37(20): 2356-2366.
- Burke, L. et al. (2011). Carbohydrates for training and competition. Journal of Sports Sciences 29(Suppl 1): S17-S27.
- Thomas, D., Erdman, K. & Burke, L. (2016). Nutrition and athletic performance. Joint position statement. Medicine & Science in Sports & Exercise 48(3): 543-568.
Last updated 6 September 2026.