The race fueling calculator above does three pieces of arithmetic for an endurance event. It applies a carbohydrate rate from the published band to your race duration and body weight. It converts a sweat rate you measured yourself into an hourly and total sweat volume. And it multiplies that volume by your own sweat sodium concentration to give sodium lost per hour.
Arb Digital publishes this with one design decision worth naming up front: the page reports losses and published bands, and it does not set a drinking target. Fluid strategy in long events is genuinely contested, both dehydration and overdrinking cause harm, and the correct answer depends on conditions and on the individual. A calculator that produced a confident millilitre figure would be pretending to a certainty the field does not have.
What This Race Fueling Calculator Does
It takes the carbohydrate-per-hour band published in a position stand, lets you pick a rate inside it, and scales that across the race so you know the total you would need to carry. It expresses the same rate per kilogram, which is how the figure is often compared between athletes of different sizes.
On the fluid and electrolyte side it does conversions, not prescriptions. Sweat rate is something you measure; sweat sodium concentration is something you have tested. Given both, the arithmetic is simple and the tool does it. What the numbers mean for what you carry and drink in a specific race, in specific weather, is a decision for you and, ideally, a sports dietitian.
This is a different job from the daily-intake tools on the site. Our water intake calculator covers published daily fluid references, not in-race losses. The macro calculator splits daily calories across macronutrients, and the TDEE calculator estimates daily energy expenditure. This page is about the hours of the race itself.
How to Use It
- Enter body weight and realistic race duration. Use a duration you have actually run or ridden, or a sober prediction — our marathon time predictor converts a recent race result into an estimate.
- Pick a carbohydrate rate inside the published band. The tool offers three points across the band and does not recommend one.
- Measure your sweat rate rather than guessing it. Weigh in before and after an hour of training in similar heat and at similar effort, add back the fluid you drank, and convert. One kilogram of loss is close enough to one litre.
- Use your own sweat sodium figure. Concentrations vary several-fold between people, so the default in that field is a placeholder to be replaced, not a typical value to rely on.
- Rehearse the plan in training. The numbers describe supply, not tolerance, and gut tolerance for carbohydrate during hard efforts is trainable and highly individual.
The Published Basis
The carbohydrate band comes from the International Society of Sports Nutrition position stand on nutrient timing, which states that extended bouts of high-intensity exercise beyond about 60 minutes challenge fuel supply and fluid regulation, and that carbohydrate should be consumed at a rate of approximately 30 to 60 grams per hour, in a 6 to 8 percent carbohydrate-electrolyte solution taken as 6 to 12 fluid ounces every 10 to 15 minutes throughout the bout, particularly for efforts spanning beyond 70 minutes.
Two things in that sentence are easy to skim past. The band is stated per hour, not per kilogram, so a 55-kilogram runner and a 90-kilogram runner are given the same range — carbohydrate oxidation during exercise is limited by intestinal absorption rather than body size, which is why the guidance is not scaled by weight. And the delivery pattern is part of the recommendation: small, frequent amounts rather than large infrequent ones.
On the electrolyte side, the risk that makes this worth taking seriously is set out in the StatPearls review Hyponatremia, which names marathon running among the settings for excessive free water intake, and states that in exercise-induced hyponatremia, increased water intake, solute loss through sweating and sympathetic stimulation of vasopressin can combine to produce it. That is a real and occasionally fatal condition in endurance events, and it is caused by drinking too much, not too little. For daily context on sodium rather than race losses, the Dietary Reference Intakes summary tables for sodium and potassium give the published population figures.
Worked Example
A 70-kilogram runner expecting a four-hour race, choosing 60 grams of carbohydrate per hour, with a measured sweat rate of 1.0 litres per hour and a tested sweat sodium concentration of 1,000 milligrams per litre.
Carbohydrate is 60 grams per hour, so the race total is 60 × 4 = 240 grams. Per kilogram that is 60 ÷ 70 = 0.86 grams per kilogram per hour. Sweat volume is 1.0 litre per hour, so 4.0 litres across the race. Sodium lost is 1,000 × 1.0 = 1,000 milligrams per hour, and 4,000 milligrams over the race. Two hundred and forty grams of carbohydrate is roughly six standard 40-gram gels, which is the sort of number worth knowing before the start rather than at 30 kilometres.
Why the Numbers Are Supply, Not Tolerance
The band describes what is useful to deliver. It does not describe what your gut will accept while you are working hard. Gastrointestinal distress is among the most common reasons endurance races go wrong, and the rate at which someone can absorb carbohydrate under race stress differs between people and improves with practice.
That has a practical consequence the arithmetic hides: a plan at the top of the band that has never been rehearsed is riskier than a plan at the middle of the band that has. Race day is the wrong time to discover a tolerance limit, and the tool's three selectable rates exist to make trying different points in training straightforward.
Concentration matters too. The position stand specifies a 6 to 8 percent carbohydrate-electrolyte solution, and a much more concentrated drink delivers the same grams in a form that empties from the stomach more slowly. Fuelling from gels plus plain water reaches a similar total by a different route, and the practical difference is where the fluid comes from, which is why fuel and fluid should be planned together rather than separately.
Sweat Rate Is the Number Most People Get Wrong
Sweat rate is not a fixed personal constant. It rises with heat, with humidity, with intensity and with acclimatisation state, and it can differ by a factor of two or more for the same athlete between a cool spring race and a hot summer one. A rate measured once in mild conditions will understate a hot race substantially.
Measure it more than once, in conditions matching the events you care about. The protocol is simple: weigh in with minimal clothing, train for an hour at target effort, record everything you drink, weigh out, and add the drunk volume to the weight lost. A kilogram of body mass is close enough to a litre of sweat for this purpose.
Sweat sodium concentration varies even more between individuals than sweat rate does, and it is not something you can estimate from how salty your skin feels after a session. If sodium is a real part of your plan, it is worth having measured; if it has not been measured, treat the figure in this tool as a placeholder and the resulting sodium number as illustrative. Our running calorie calculator and cycling calorie calculator cover the energy side of the same session.
Arb Digital builds calculators and content that cite their sources and state their limits plainly — the same standard as this page.
Browse All Free Tools Talk To Our TeamCommon Mistakes to Avoid
- Treating the sodium figure as an amount to drink — it is what your sweat rate and concentration imply you are losing, which is a different quantity from what to replace.
- Guessing a sweat rate instead of measuring one — it changes by a factor of two or more with heat and intensity, so a guess can be wildly wrong.
- Scaling the carbohydrate band by body weight — it is published per hour because absorption, not body size, is the limiting factor.
- Trying a new fuelling rate on race day — the band describes supply, not gut tolerance, and tolerance is individual and trainable.
- Assuming more fluid is always safer — exercise-associated hyponatremia is caused by drinking too much, and marathon running is one of the settings in which it occurs.
Related Free Tools From Arb Digital
Predict a race time with the marathon time predictor, set session paces with the training pace calculator, estimate session energy cost with the running calorie calculator or the cycling calorie calculator, look up published daily fluid references with the water intake calculator, or split daily calories with the macro calculator. The full free online tools hub lists every health tool we publish.
Frequently Asked Questions
The ISSN position stand on nutrient timing states approximately 30 to 60 grams of carbohydrate per hour for extended high-intensity bouts, in a 6 to 8 percent carbohydrate-electrolyte solution taken in small amounts every 10 to 15 minutes, particularly beyond about 70 minutes.
Because carbohydrate use during exercise is limited mainly by intestinal absorption rather than by body size. That is why the published figure is stated per hour rather than per kilogram, and why athletes of very different sizes are given the same range.
Weigh yourself with minimal clothing before an hour of training at target effort in similar conditions, record everything you drink, weigh yourself afterwards, and add the drunk volume to the weight lost. One kilogram of loss is close enough to one litre of sweat.
No, deliberately. It reports what your measured sweat rate implies you are losing. Both under-drinking and over-drinking carry risk in long events, and intake decisions belong with you, a sports dietitian or your medical team.
StatPearls describes it as arising when increased water intake, solute loss through sweating and sympathetic stimulation of vasopressin combine, and names marathon running among the settings for excessive free water intake. It is caused by drinking too much, not too little.
From a sweat test or a laboratory measurement. Concentrations vary several-fold between individuals and cannot be estimated from how salty your skin feels, so the default in this tool is a placeholder rather than a typical value.
This page does not say. It offers three points across the published band so you can test different rates in training. Gut tolerance under race stress is individual and improves with practice, and race day is the wrong time to find its limit.
The published guidance is framed around bouts extending beyond about 60 to 70 minutes. For shorter efforts the fuelling question is a different one, and the band this page applies was not written for it.
This page applies published population guidance and performs unit arithmetic on figures you supply, for general information only. It is not medical or dietary advice, it sets no intake target, and it is not a substitute for guidance from a qualified sports dietitian or doctor.
