— Carbs per hour (g/h)
— Carb source type
— Fluid per hour (ml/h)
— Sodium per hour (mg/h)
— Per 20-min intake
— Product equivalent (generic)
— Total carbs (g)
— Total fluid (L)
Methodology
Carbohydrate recommendations follow the International Olympic Committee consensus statement (Thomas et al. 2016) and Jeukendrup's dose-response research (2014). For events under 75 minutes, exogenous carbohydrates provide minimal benefit. From 75–150 minutes, 30–60 g/h is recommended. Beyond 2.5 hours, 60–90 g/h using a 2:1 glucose-to-fructose blend optimises intestinal transport via SGLT1 and GLUT5 co-transport. For ultra events exceeding 4–5 hours, 80–100+ g/h is achievable with a trained gut. Carbohydrate dose is capped by gut training level to reflect the real physiological constraint of intestinal absorption. Fluid requirements are modelled on sweat rate research by Sawka et al. (ACSM Position Stand 2007), scaled by body weight, intensity, and environmental heat load. Sodium recommendations (500–1500 mg/h in heat) prevent hyponatremia and support fluid retention. Running applies a 10–15% lower fluid recommendation versus cycling due to greater mechanical efficiency reducing heat production per unit of speed.
FAQ
How is this different from Precision Hydration's planner or similar brand tools?
Brand-affiliated tools are built around their product portfolio. Recommendations are shaped by what they sell — their sodium supplement, their gel, their drink mix. This planner is built purely from peer-reviewed research (IOC 2016, Jeukendrup 2014, ACSM 2007) and outputs generic quantities — grams of carbohydrate, millilitres of fluid, milligrams of sodium — that you can meet with any product, food, or combination. There is no product recommendation built into the algorithm, no affiliate link in the output, and no nudge toward a specific brand. You get the science; you choose the execution.
What does 'product equivalent' mean?
Product equivalent translates the calculated carbohydrate and sodium targets into familiar reference units — for example, 'approximately 1.5 standard energy gels' or '500 ml of typical isotonic drink' — without naming any specific brand. A standard energy gel contains approximately 22–25 g of carbohydrate and 20–60 mg of sodium. A standard isotonic drink contains approximately 6–8 g of carbohydrate per 100 ml and 400–700 mg of sodium per litre. These are generic industry averages. Always read the label of whichever product you choose to confirm actual values.
How does the environment affect fueling needs?
Heat and humidity increase sweat rate substantially, sometimes doubling it compared to cool conditions. A runner who loses 500 ml/h in cool weather may lose 1,000–1,500 ml/h in hot, humid conditions. Sodium losses also increase with sweat rate. The planner adjusts both fluid and sodium recommendations upward as temperature and humidity rise. Carbohydrate targets are less affected by environment, though extreme heat can suppress gastric emptying — one reason gut-trained athletes handle high-carb intake better in races across all conditions.
Why does running require less fluid per hour than cycling?
This is a biomechanical and aerodynamic difference, not a physiological one. At the same absolute power output and environmental conditions, cyclists generate heat at a broadly similar rate to runners. However, cyclists travel at higher speeds, which creates substantially greater airflow over the body, enhancing convective cooling and reducing the sweat rate needed to maintain core temperature. Runners have less airflow benefit, particularly at slower paces, and can carry less fluid without aid station access. The planner applies a modest (10–15%) downward adjustment to fluid targets for running relative to cycling at equivalent intensity and environment.
How do I adjust the plan during the race?
The plan gives you targets — treat them as a starting framework, not rigid rules. Adjust based on three signals: thirst (drink when thirsty, not on a rigid schedule beyond the first 60 minutes), gut feel (back off carbohydrates if you feel nauseous or bloated — a 20–25% reduction usually resolves it), and energy level (if bonk symptoms appear, take an immediate higher-carb hit of 30–40 g rather than waiting for the next scheduled intake). In heat, prioritise fluid and sodium over carbohydrate if you must choose. In cool conditions, prioritise carbohydrate if stomach capacity is limited.
Why does the calculator separate carbohydrate strategy from hydration strategy?
Carbohydrate and fluid needs are driven by completely different physiological mechanisms. Carbohydrate fuels muscle contraction and prevents central fatigue — it is primarily governed by exercise duration and intensity. Fluid replaces sweat loss to maintain plasma volume and thermoregulation — it is primarily governed by environmental heat, body size, and sweat rate. Conflating the two leads to the common mistake of drinking for energy (over-hydrating with sugary drinks) or eating for hydration (under-fuelling with plain water). Keeping the strategies separate lets you adjust each independently — for example, increasing fluid in unexpected heat without automatically increasing carbohydrate intake.
This planner provides evidence-based estimates, not a clinical prescription. Individual sweat rates, gut tolerance, and sodium losses vary significantly between athletes. Test your fueling strategy in training — never debut a race nutrition plan on race day.
