Carbohydrate Fuelling Calculator
Create your personalised fuelling plan based on your physiology
Local & Convenient
Located between Guildford, Farnham & Woking
Free Parking
Easy, stress-free on-site parking
Precision Hydration
Authorised Sweat Test Centre
Exo Analytics
Partner Test Centre
Trusted By:
Athletes, Coaches and everyday performers
Use your sweat sodium concentration, sweat rate and event duration to create a personalised hydration strategy for training and racing.
Race Fuel & Hydration Strategy Calculator
Set your energy demand and target, your hydration demand and target, then build the detailed hour-by-hour product plan that nets against both.
Energy Demand & Target
What this tells you
Your total physiological CHO/fat demand is fixed by your test data — it doesn't change based on what you eat.
Set a target CHO intake rate alongside it. We show a recommended gross intake (accounting for ~80% oxidation efficiency) as a starting point — your target doesn't have to match it exactly.
The hour-by-hour table nets your target against demand to show glycogen draw-down. Once you build a Detailed Plan, this updates to reflect your actual planned intake.
Hydration Demand & Target
Uses the same event duration as Energy Demand.
What this tells you
You tell us what you're losing — sweat rate and sodium concentration. Everything else is calculated.
Your target intake is the recommended rate: your loss rate, capped by the gastric emptying assumption. If you lose more than your gut can absorb, the target reflects what's actually achievable — not an unrealistic "match your losses" number.
The Net Hydration Effect nets that target (or your actual product plan, once built) against your true total loss — so the deficit shown is the honest bottom line, even when a perfect strategy still leaves one.
Detailed Planning
Select products from your Nutrition Library, add them as rows, then enter planned units for each hour — e.g. 3 jelly babies, 1 PF30 sachet, 1.5 sports drink bottles. This nets your actual mix against your Energy Demand and Hydration targets, hour by hour.
Nutrition Library
How the library works
This is a shared library — anyone can add a product here, and it appears for every visitor immediately.
Detailed Planning uses this as a lookup table when you enter unit counts hour by hour — the Unit field tells you what "1" means for each product (a sachet, a specific bottle size, a tablet, a piece).
Different sizes = different entries. A 500ml and a 750ml bottle of the same drink should be two separate library entries, not one — that way "1 unit" is always exactly right, and you can still enter fractions (e.g. 1.5) if only part of a bottle was consumed in an hour.
Dissolvable items (electrolyte tabs, carb powders): add the tab/powder and the water as two separate entries, then log both in the same hour in Detailed Planning — the totals combine automatically.
Total electrolytes = sodium + potassium + magnesium + calcium
Products are reviewed and tidied up periodically — please only add real, accurate products.
Notes about the Calculator
About This Fuelling Calculator
This calculator uses the data from your i-Thrive performance test to create a personalised race / training fuelling plan.
- Based on current sports nutrition reserach
- Helps prevent bonking at laters stages
- Supports performance & recovery
- Guidance for training and race day
Important
Practice your nutrition and hydration strategy in training to ensure gastrointestinal comfort and tolerance on reace day.
Disclaimer: This calculator provides an estimate of your fuelling based on the information entered. Carbohydrate and Fat Oxidation varies with environmental conditions, exercise intensity and individual factors. Use these recommendations as a guide and always test your fuelling strategy during training before race day.
DON'T KNOW HOW MUCH CARBOHYDRATE & FAT YOU BURN?
We have a range of tests that provide you insights of the total grams of carbohydrate & fat that you oxidise at different intensities.
Related Services
VO2 Max & Lactate Threshold
Exo Analytics Carbohydrate Oxidation Test
Precision Hydration Sweat Test
Related Blogs
Hydration & Electrolytes for Marathon Des Sables
CHO Demand is Treated as Fixed
For the athlete:
This calculator treats your carbohydrate demand as a fixed physiological fact — determined by your exercise intensity and body, not by what you eat during the event. Eating more carbohydrate does not reduce how much your working muscles need per hour. It only affects how much of that demand is met from food versus your stored glycogen. Think of demand as the size of the hole — fuelling determines how much of it you fill from external sources.
The assumption:
CHO energy expenditure per hour (kcal/hr), derived from your metabolic test data, is constant throughout the event regardless of exogenous carbohydrate intake.
The science:
Substrate oxidation during exercise is primarily driven by exercise intensity, not nutritional status during the bout itself. At intensities above approximately 65% VO₂max — the range relevant to most endurance competition — carbohydrate becomes the dominant fuel source and this relationship is largely fixed by the intensity of effort. While carbohydrate ingestion during exercise can modestly spare muscle glycogen by maintaining blood glucose availability, the effect on total carbohydrate oxidation rate is small and does not materially alter the hour-by-hour demand picture. The foundational work establishing intensity as the primary driver of substrate use was conducted by Romijn and colleagues in 1993, who used isotope tracer methodology to precisely quantify fat and carbohydrate oxidation across a range of exercise intensities. Brooks and Mercier formalised this relationship in 1994 through the crossover concept, which describes the predictable shift from fat toward carbohydrate oxidation as intensity increases. Coyle and colleagues demonstrated in 1986 that carbohydrate feeding prolongs endurance primarily by maintaining blood glucose when glycogen is low — confirming that exogenous carbohydrate supplements rather than replaces the demand signal, it does not eliminate it.
References:
Romijn, J. A., Coyle, E. F., Sidossis, L. S., Gastaldelli, A., Horowitz, J. F., Endert, E., & Wolfe, R. R. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology — Endocrinology and Metabolism, 265(3), E380–E391. https://doi.org/10.1152/ajpendo.1993.265.3.E380
Brooks, G. A., & Mercier, J. (1994). Balance of carbohydrate and lipid utilization during exercise: The crossover concept. Journal of Applied Physiology, 76(6), 2253–2261. https://doi.org/10.1152/jappl.1994.76.6.2253
Coyle, E. F., Coggan, A. R., Hemmert, M. K., & Ivy, J. L. (1986). Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. Journal of Applied Physiology, 61(1), 165–172. https://doi.org/10.1152/jappl.1986.61.1.165
Why Glycogen Depletion Causes Fatigue
For the athlete:
Glycogen is the carbohydrate your muscles and liver store for use during exercise. When it runs low your body cannot sustain high-intensity effort — this is what athletes call bonking or hitting the wall. It is not simply a matter of willpower. The depletion of glycogen triggers real physiological changes that force you to slow down, regardless of your mental state. Understanding this is the reason the glycogen tracking in this calculator matters — it is not an abstract number, it is a direct proxy for your ability to sustain your target pace or power.
The assumption:
Glycogen remaining is a meaningful predictor of performance sustainability. Progressive depletion increases fatigue risk and the ability to sustain target intensity.
The science:
The relationship between muscle glycogen and endurance performance is one of the most replicated findings in exercise physiology, with foundational work dating back over five decades. Bergström and Hultman (1967) established through needle biopsy studies that time to exhaustion during prolonged exercise correlates directly with starting muscle glycogen concentration — athletes with higher pre-exercise glycogen consistently performed longer before exhaustion. This work also demonstrated that dietary carbohydrate manipulation in the days before exercise could substantially alter muscle glycogen stores and therefore performance capacity, which became the scientific basis for carbohydrate loading protocols.
Coyle and colleagues (1986) extended this understanding by showing that fatigue during prolonged exercise coincides with muscle glycogen depletion and that carbohydrate feeding delays fatigue by maintaining blood glucose availability when muscle glycogen falls — but cannot prevent fatigue if glycogen depletion is complete. This established the two-mechanism model: glycogen provides the primary intramuscular fuel, while blood glucose from exogenous carbohydrate becomes the secondary fuel as glycogen declines.
The picture was further refined by Noakes and colleagues, whose central governor hypothesis (2001, 2012) proposed that the brain anticipates glycogen depletion and reduces motor output — involuntarily slowing the athlete — as a protective mechanism before complete depletion occurs. This explains why performance decrements begin before glycogen reaches zero and why perceived exertion rises sharply in the later stages of glycogen-depleting exercise. More recently, Gejl and colleagues (2017) demonstrated that fatigue can occur even when total muscle glycogen appears adequate, if specific intramyofibrillar glycogen pools — those directly adjacent to the contractile machinery — are selectively depleted. This suggests glycogen’s role in fatigue is even more localised and complex than total concentration alone would imply.
Impey and colleagues (2016, 2018) added a further dimension by demonstrating that muscle glycogen concentration directly influences molecular signalling during and after exercise — specifically activation of AMPK and downstream expression of PGC-1α, which drives mitochondrial adaptation. This means glycogen is not simply a fuel tank but also a metabolic signal, with implications for both performance and training adaptation.
References:
Bergström, J., & Hultman, E. (1967). A study of the glycogen metabolism during exercise in man. Scandinavian Journal of Clinical and Laboratory Investigation, 19(3), 218–228. https://doi.org/10.3109/00365516709090629
Coyle, E. F., Coggan, A. R., Hemmert, M. K., & Ivy, J. L. (1986). Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. Journal of Applied Physiology, 61(1), 165–172. https://doi.org/10.1152/jappl.1986.61.1.165
Noakes, T. D. (2012). Fatigue is a brain-derived emotion that regulates the exercise behavior to ensure the protection of whole body homeostasis. Frontiers in Physiology, 3, 82. https://doi.org/10.3389/fphys.2012.00082
Gejl, K. D., Hvid, L. G., Frandsen, U., Jensen, K., Sahlin, K., & Ørtenblad, N. (2017). Muscle glycogen content modifies SR Ca²⁺ release rate in elite endurance athletes. Medicine & Science in Sports & Exercise, 49(4), 678–688. https://doi.org/10.1249/MSS.0000000000001132
Impey, S. G., Hearris, M. A., Hammond, K. M., Bartlett, J. D., Louis, J., Close, G. L., & Morton, J. P. (2018). Fuel for the work required: A theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Medicine, 48(5), 1031–1048. https://doi.org/10.1007/s40279-018-0867-7
The RAG Status Thresholds — What the Science Says
For the athlete:
The green, amber and red status indicators in this calculator are not arbitrary — they are anchored to what exercise physiology research tells us about the relationship between glycogen remaining and the ability to sustain effort. Green means you have sufficient reserves to maintain your target intensity. Amber means you are entering a range where performance is likely to be compromised. Red means you are at serious risk of being unable to sustain target pace or power. Depleted means your stores are functionally exhausted at your planned intake rate.
The assumption:
GREEN = glycogen remaining above 50% of starting stores. AMBER = 25–50% remaining. RED = below 25% remaining. DEPLETED = stores exhausted at planned intake rate.
The science:
The specific percentage thresholds used in this calculator do not correspond to a single published study — no research has established precise universal cut-points at exactly 25% and 50% — but they are grounded in the broader body of evidence on the glycogen-performance relationship and are deliberately set conservatively for planning purposes.
The research basis is as follows. Bergström and Hultman (1967) established that exhaustion typically occurs at near-complete glycogen depletion, but subsequent work refined this substantially. Coyle and colleagues (1986) demonstrated that measurable performance decrements — specifically the inability to maintain target power output — begin well before complete depletion, as the muscle shifts to less efficient metabolic pathways. In practical terms, studies using cycling time trials have shown that significant power reductions occur when muscle glycogen falls to approximately 150–200 mmol per kilogram of dry weight, which in a well-loaded athlete (starting concentration approximately 450–550 mmol/kg dry weight) corresponds to roughly 35–45% of starting stores remaining. This broadly supports the 50% AMBER threshold as the point at which the athlete should be aware of increasing risk.
The 25% RED threshold aligns with research showing severe performance impairment — the inability to sustain moderate intensities — typically occurring below approximately 70–100 mmol/kg dry weight, which represents approximately 15–25% of a well-loaded starting concentration. Below this level athletes in laboratory studies consistently demonstrate marked increases in perceived exertion, reduced power output, and eventual inability to continue at target intensity.
The central governor framework proposed by Noakes (2012) adds an important practical nuance: the brain begins downregulating motor output — forcing pace reduction — in anticipation of depletion, not only at depletion itself. This means the AMBER zone is not simply a warning that things might get difficult — it is the zone in which the central nervous system is likely already making involuntary adjustments to protect the athlete from complete depletion. Athletes who find themselves in this zone during a race will typically experience it as rising perceived effort and an increasing struggle to hold target pace, even if they feel they still have energy available.
It is important to note that the glycogen figures in this calculator are planning estimates derived from your intake target and your test-derived demand data — they are not real-time measurements. Individual variation in starting glycogen, glycogen resynthesis efficiency, and the accuracy of test-derived substrate data all introduce uncertainty. The RAG thresholds should therefore be interpreted as planning guides rather than precise physiological measurements.
References:
Bergström, J., & Hultman, E. (1967). A study of the glycogen metabolism during exercise in man. Scandinavian Journal of Clinical and Laboratory Investigation, 19(3), 218–228. https://doi.org/10.3109/00365516709090629
Coyle, E. F., Coggan, A. R., Hemmert, M. K., & Ivy, J. L. (1986). Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. Journal of Applied Physiology, 61(1), 165–172. https://doi.org/10.1152/jappl.1986.61.1.165
Noakes, T. D. (2012). Fatigue is a brain-derived emotion that regulates the exercise behavior to ensure the protection of whole body homeostasis. Frontiers in Physiology, 3, 82. https://doi.org/10.3389/fphys.2012.00082
Jeukendrup, A. E. (2011). Nutrition for endurance sports: Marathon, triathlon, and road cycling. Journal of Sports Sciences, 29(Suppl 1), S91–S99. https://doi.org/10.1080/02640414.2011.610348
The 80% Oxidation Efficiency Assumption and What It Means for Your Fuelling Target
For the athlete:
Not all of the carbohydrate you eat during exercise is actually used by your muscles. Some is absorbed more slowly than it can be oxidised, and some passes through without contributing to energy production. This calculator applies an 80% oxidation efficiency figure — meaning for every 10 grams you eat, approximately 8 grams contribute to meeting your energy demand. This is why your recommended gross intake is always higher than your raw CHO demand: you need to eat more than you need, to deliver what you need. The exact efficiency depends on the type of carbohydrate products you use — which is explained below.
The assumption:
80% of ingested carbohydrate is oxidised and contributes to meeting physiological CHO demand. Gross intake must therefore equal demand ÷ 0.80 to fully cover requirements from exogenous sources.
The science:
The oxidation efficiency of ingested carbohydrate has been extensively studied using stable isotope tracer methodology, primarily by Jeukendrup and colleagues over a series of studies from the late 1990s through to the 2010s. Their work identified a fundamental constraint: the intestinal transporter responsible for glucose absorption — sodium-dependent glucose transporter 1 (SGLT1) — becomes saturated at an intake of approximately 60 grams per hour. Above this rate, additional glucose or maltodextrin cannot be absorbed faster, meaning oxidation efficiency falls as intake rises beyond this ceiling. At intakes around 60 g/hr using a single carbohydrate source, measured oxidation efficiency in these studies was consistently in the range of 80–88%, which is the basis for the 80% figure used in this calculator.
The important development came from research by Wallis and colleagues (2005) and Currell and Jeukendrup (2008), who demonstrated that combining glucose with fructose — which uses a completely separate intestinal transporter (GLUT5) — allows total absorption to bypass the SGLT1 ceiling. By loading both transport pathways simultaneously, total exogenous carbohydrate oxidation can reach approximately 1.26–1.75 grams per minute, equivalent to 75–105 grams per hour. In these multi-transportable carbohydrate (MTC) conditions, oxidation efficiency rises to approximately 88–92% at intakes up to 90 g/hr, because a greater proportion of what is ingested can actually be absorbed and oxidised.
More recent work by Podlogar and Wallis (2022) has shown that in highly trained athletes with conditioned guts, intakes of up to 120 g/hr using optimised glucose-to-fructose ratios of approximately 1:0.8 can produce oxidation rates of 105–110 g/hr — though gastrointestinal tolerance becomes a significant individual limiting factor at these rates.
What this means practically:
The 80% figure used in this calculator is accurate and appropriate for single-source carbohydrate products (glucose, maltodextrin, sucrose) at moderate intakes of 40–60 g/hr. If you are using modern multi-transportable carbohydrate products at intakes of 60–90 g/hr, your effective oxidation efficiency is likely closer to 88–92%, meaning the calculator is slightly conservative in this scenario. If you are pushing single-source intake above 60 g/hr, efficiency will fall below 80% as the transporter ceiling is exceeded. The 80% figure represents a sound middle-ground assumption for applied planning across the broadest range of athletes and products.
How to tell if your product is single source or multi-transportable:
The answer is on the ingredients label — but you need to know what to look for. Single source products will list one primary carbohydrate — typically maltodextrin, glucose, glucose syrup or dextrose — as their dominant or only carbohydrate ingredient. Multi-transportable products will list both a glucose source and fructose as significant ingredients. Look for the words fructose, fruit sugar or fruit juice concentrate appearing alongside maltodextrin or glucose — and critically, appearing in a meaningful quantity, not just as a trace flavouring. A genuine MTC product will typically show fructose contributing roughly one third to one half of the total carbohydrate content. One useful shortcut is sucrose — if sucrose is the primary carbohydrate source, the product is effectively MTC because sucrose is a disaccharide that splits into exactly one glucose and one fructose molecule during digestion, naturally loading both intestinal transporters. Products listing sucrose or cane sugar as the main carbohydrate ingredient therefore behave as MTC products in the gut, even if they are not marketed that way. If a product lists only maltodextrin with no fructose source, it is single source regardless of any marketing claims about advanced formulation or superior absorption.
References:
Jeukendrup, A. E., Jentjens, R. L., & Moseley, L. (2005). Nutritional considerations in triathlon. Sports Medicine, 35(2), 163–181. https://doi.org/10.2165/00007256-200535020-00005
Jeukendrup, A. E. (2004). Carbohydrate intake during exercise and performance. Nutrition, 20(7–8), 669–677. https://doi.org/10.1016/j.nut.2004.04.017
Wallis, G. A., Rowlands, D. S., Shaw, C., Jentjens, R. L., & Jeukendrup, A. E. (2005). Oxidation of combined ingestion of maltodextrins and fructose during exercise. Medicine & Science in Sports & Exercise, 37(3), 426–432. https://doi.org/10.1249/01.MSS.0000155399.23358.82
Currell, K., & Jeukendrup, A. E. (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine & Science in Sports & Exercise, 40(2), 275–281. https://doi.org/10.1249/mss.0b013e31815adf19
Podlogar, T., & Wallis, G. A. (2022). New horizons in carbohydrate research and application for endurance sports. Sports Medicine, 52(Suppl 1), 5–23. https://doi.org/10.1007/s40279-022-01757-1