Carbs per hour calculator for cycling

The formula

Work in kilojoules is average power times time. Only part of the energy you burn leaves the pedals as work. That share is the gross efficiency, work divided by total energy expenditure, and it differs between riders [9]. The calculator assumes 22 percent, a modelling choice rather than a measurement of you, and works out the energy burned from the work. Because 1 / (0.22 × 4.184) is close to one, a kilojoule of work is roughly a kilocalorie burned.

work (kJ)    = average power (W) × seconds ÷ 1000
energy (kcal) = work ÷ 0.22 ÷ 4.184
carbs (g/h)   = energy × carb share ÷ 4 kcal per g ÷ hours

The carbohydrate share rises with intensity, from under half of the energy at an easy pace to nearly all of it at threshold [6, 7, 8]. Those studies measure intensity against VO₂max or maximal workload rather than FTP. The calculator takes intensity as average power divided by FTP and maps it onto its own estimated share, which follows the same trend.

Worked example: 3 hours at 200 W with an FTP of 260 W is 2160 kJ of work and about 2347 kcal. At that intensity the model estimates that 73 percent of the energy comes from carbohydrate, about 143 g every hour. The default intake target for a ride that long is 90 g/h. Burn and intake are different quantities and need not match.

How much to eat, by ride length

Intake targets are based mainly on how long you ride, and are then adjusted for intensity and for what you tolerate [1, 2, 3]. At a low absolute intensity they may need to be lower [1]. The table gives simplified default targets for performance-focused rides. About 60 g/h is a rough level for how much glucose alone can be delivered and used. About 90 g/h is the level for glucose and fructose mixed, which use separate transporters. These are approximate levels, not fixed absorption limits. That is why you can burn more carbohydrate than you eat on a long hard ride.

The gut adapts: regular high intake in training raises how much carbohydrate it absorbs and oxidises and eases stomach trouble [3, 5]. Build up gradually in training before you use a high target in a race, and tick the trained-gut box only if you have. Its 120 g/h target is an advanced option to test individually. It has been studied experimentally, but not all of what is eaten is oxidised straight away [4].

When to eat

• Regularly: the ride's target in whole servings, spread evenly from about 20 minutes in to 10 minutes before the finish, and no more often than every ten minutes. Because that window is shorter than the ride, the feeds come a little more often than serving size ÷ target per hour.
• Before climbs: as a rule of thumb, eat about 15 minutes ahead of a long or steep climb, while the road still makes it easy.
• Not on descents: wait for the bottom.
• Near the finish, the planner stops reminding you 10 minutes out, to keep the run-in clear. That is a practical choice: carbohydrate can still help late in a ride, even through signals from the mouth before it is used as fuel [1].

The targets are for race and key-session days. Before prolonged exercise, a meal of 1 to 4 g of carbohydrate per kg of body weight 1 to 4 hours earlier tops up your stores [3].

The race power planner can add these reminders to your FIT course, placed on the route where you will be riding, so your head unit tells you when to eat. The model is written out on the methodology page.

Limitations

Substrate use varies between riders with fitness, diet, sex, body size and glycogen status, and efficiency varies between riders too. Fluids, sodium and heat are not covered. Average power is not normalized power, so a stop-start ride is estimated roughly. Treat the result as a starting point and test it in training.

References

1. Jeukendrup AE. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine. 2014;44(Suppl 1):S25–S33. doi:10.1007/s40279-014-0148-z
2. Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine joint position statement: nutrition and athletic performance. Medicine & Science in Sports & Exercise. 2016;48(3):543–568. doi:10.1249/MSS.0000000000000852
3. Burke LM. Fueling strategies to optimize performance: training high or training low? Scandinavian Journal of Medicine & Science in Sports. 2010;20(Suppl 2):48–58. doi:10.1111/j.1600-0838.2010.01185.x
4. Podlogar T, Wallis GA. New horizons in carbohydrate research and application for endurance athletes. Sports Medicine. 2022;52(Suppl 1):5–23. doi:10.1007/s40279-022-01757-1
5. Jeukendrup AE. Training the gut for athletes. Sports Medicine. 2017;47(Suppl 1):101–110. doi:10.1007/s40279-017-0690-6
6. Brooks GA, Mercier J. Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept. Journal of Applied Physiology. 1994;76(6):2253–2261.
7. Romijn JA, Coyle EF, Sidossis LS, et al. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology. 1993;265(3):E380–E391.
8. van Loon LJC, Greenhaff PL, Constantin-Teodosiu D, Saris WHM, Wagenmakers AJM. The effects of increasing exercise intensity on muscle fuel utilisation in humans. Journal of Physiology. 2001;536(1):295–304.
9. Coyle EF, Sidossis LS, Horowitz JF, Beltz JD. Cycling efficiency is related to the percentage of type I muscle fibers. Medicine & Science in Sports & Exercise. 1992;24(7):782–788.