Race Fueling Calculator: Carbs, Fluid and Sodium for Race Day
A race fueling calculator turns three hourly targets, carbohydrate, fluid and sodium, into the totals you need to carry or find on course. Enter your expected finish time and your hourly numbers below; if you are not sure what those numbers should be, the sections underneath explain where to start for races in Malaysian heat.
Race fueling calculator
The calculator multiplies your hourly targets by the race duration. Product lines use the label figures: 42.2 g carbohydrate and 221 mg sodium per QC Stamina pouch, 220 mg sodium per Elec Stamina caplet, and each pack’s own usage directions. It is a planning aid, not a prescription.
What carbohydrate number should you enter?
Enter 30 to 60 g an hour for races of one to two and a half hours, and up to 90 g for longer ones if your gut is trained. Those bands come from the joint ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position statement (Thomas et al., 2016) and from Asker Jeukendrup’s review of carbohydrate during exercise, which adds that rates above about 60 g need a mix of sugars, such as glucose with fructose, to be absorbed (Jeukendrup, 2014).
Ultras are the exception. Pace is lower, much of the climbing is walked, and eating gets harder as the hours stack up. The International Society of Sports Nutrition’s position stand on ultras recommends 30 to 50 g of carbohydrate an hour within 150 to 400 kcal (Tiller et al., 2019). If you are entering a trail race of eight hours or more, 40 to 50 g is a realistic first number. Our guide to carbs per hour explains the ceilings and how to train your gut towards them.
What fluid number should you enter?
Enter a number below your measured sweat rate, never above it. The ACSM fluid position stand gives 0.4 to 0.8 litres an hour as a starting range and suggests keeping weight loss under about 2% of body mass (Sawka et al., 2007). The ISSN ultra stand suggests 450 to 750 ml an hour (Tiller et al., 2019). Both are averages from cooler climates. A Malaysian runner on an exposed course at 9 am can sweat well over a litre an hour, and the same runner after a rain shower can sweat half that.
So measure first. Our sweat rate calculator gives you litres an hour from a bathroom scale and one training run. If your sweat rate is 1.2 L/h, entering 700 to 900 ml here keeps you inside the 2% line on a long race without drinking more than you lose. If the calculator ever suggests a fluid total that would leave you heavier at the finish than at the start, the number is too high. Drinking beyond what you sweat is the main cause of exercise-associated hyponatraemia (Hew-Butler et al., 2015).
What sodium number should you enter?
Enter zero for races under about two hours, and 200 to 550 mg an hour for longer ones as a first guess. Sodium need rises with how long you are out, how salty your sweat is and how much of your sweat you drink back. Our sodium per hour guide walks through those three variables and where the 200 to 550 mg range comes from.
Watch the ratio between your sodium and fluid numbers. 250 mg an hour with 600 ml of fluid is about 420 mg per litre, close to many sports drinks. The ISSN ultra stand notes that ultra runners may need more than 575 mg per litre to reduce the risk of hyponatraemia (Tiller et al., 2019), but it also stresses that drinking to thirst comes first. More salt does not make overdrinking safe.
Why is one hourly rate a simplification?
Because nobody eats evenly for a whole race. In the first hour of a marathon most runners take little or nothing, and in a trail ultra intake usually drifts down as the day heats up and appetite fades. The calculator assumes a flat rate so the totals are easy to read; treat the hourly figure as an average across the race, not a promise for every hour.
For long races, it helps to run the calculator twice: once for the first half at a higher rate while your stomach is fresh, once for the second half at a lower one, then add the totals. Front-loading is not just convenience. In a study of runners at a 161 km mountain race, those who finished had eaten and drunk more over the first 48 km than those who dropped out (Stuempfle et al., 2011).
A worked example for the Penang Bridge marathon
Take a runner aiming for 3 hours 45 minutes at the Penang Bridge International Marathon in December. The race starts in the dark, and the long middle section over the bridge has no shade once the sun comes up. She enters:
- Duration: 3.75 hours
- Carbohydrate: 60 g an hour
- Fluid: 600 ml an hour, from a sweat test that gave her 1.0 L/h
- Sodium: 250 mg an hour
The calculator gives 225 g carbohydrate, 2.3 L fluid and 938 mg sodium. That is about nine 25 g gels if gels were all she used. In practice she plans six gels, takes sports drink at alternate stations for the rest, and times each gel a minute before a water point so she can wash it down.
Because her race is under four hours, the calculator skips the pouch line. For sodium it shows about four caplets, which matches the pack directions for 2.3 L of water. If she drinks mostly sports drink rather than water, she counts that sodium too and takes fewer.
Now stretch the same numbers to a 20-hour trail ultra and the totals jump to 1.2 kg of carbohydrate, 12 L of fluid and several grams of sodium. That is when the plan has to be built around aid stations, drop bags and real food rather than what fits in a pocket. Our hub page on ultra marathon nutrition from 50K to 100K covers that side.
Who needs the product lines at all?
Most people running 10Ks and half marathons need none of them. Gels, the sports drink at aid stations and water cover them. The product lines are there for runners whose races go long enough that the totals above get awkward to carry.
Our Elec Stamina caplets list 220 mg of sodium each and are meant for efforts of about two hours or more, from a long marathon to a trail ultra; the pack pairs one caplet with each 500 ml of water drunk, up to ten a day. The calculator’s QC Stamina line uses that pouch’s label figures for races of four hours or more. Neither replaces the steps that matter more: knowing your sweat rate, training your gut and rehearsing the plan in the heat you will race in.
References
- Thomas DT, Erdman KA, Burke LM (2016). American College of Sports Medicine joint position statement: nutrition and athletic performance. Medicine & Science in Sports & Exercise, 48(3), 543-568. doi:10.1249/MSS.0000000000000852
- Jeukendrup A (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25-S33. doi:10.1007/s40279-014-0148-z
- Tiller NB, Roberts JD, Beasley L, et al. (2019). International Society of Sports Nutrition position stand: nutritional considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 16(1), 50. doi:10.1186/s12970-019-0312-9
- Sawka MN, Burke LM, Eichner ER, et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390. doi:10.1249/mss.0b013e31802ca597
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. (2015). Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference. British Journal of Sports Medicine, 49(22), 1432-1446. doi:10.1136/bjsports-2015-095004
- Stuempfle KJ, Hoffman MD, Weschler LB, Rogers IR, Hew-Butler T (2011). Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace. Journal of the American College of Nutrition, 30(6), 529-535. doi:10.1080/07315724.2011.10719999
