The plan, quarter hour by quarter hour
Feeds are spread between minute 20 and the last few minutes of the session, alternating between what you carry so you do not end up eating six gels in a row. The right hand column is cumulative carbohydrate, drink included.
| Time | What you take | Drink | Cumulative |
|---|
The millilitres per row are an average: sip every five or ten minutes instead of draining the bottle when you happen to remember. If the session is a race with fixed aid stations, move the feeds to the kilometres where you can actually pick something up.
How this is worked out
The underlying arithmetic is simple: multiply the grams per hour you want by the hours the session lasts, and that is the total carbohydrate you have to get in. The hard part is not the multiplication, it is making that number come out of things that exist: 500 ml bottles, 23 g gels, bananas. That is why this calculator starts with the drink, which you are going to take anyway for the fluid, and only then fills the remaining gap with solids.
The order matters. Count the gels first and add the drink afterwards and you overshoot, ending up with a sugar concentration your stomach will not empty. Count the drink first and the solids become the fine adjustment.
From the planned session to the grams
If the session sits in your calendar with an IF, a TSS, a kJ figure or a zone breakdown, you do not have to guess the grams per hour: they come out of the workout itself. All four routes end in the same two numbers, an average intensity and an amount of energy.
On the bike, mechanical work in kilojoules and metabolic cost in kilocalories are so close that one is used for the other: at around 24 % efficiency, a kilojoule of work costs roughly a kilocalorie. IF gives you the average power (IF × FTP) and from there the kJ. TSS gives you IF, because TSS = 100 × hours × IF². And time in zones is summed segment by segment with the typical intensity of each zone.
Running has no watts, so the known cost of running is used instead: about one kilocalorie per kilo per kilometre, almost independent of pace. Your threshold pace and the IF give the speed, and the speed gives the kilometres.
Then comes the interesting part: what share of that energy comes from carbohydrate. At low intensity fat dominates and carbohydrate supplies about a third; at threshold it supplies close to 80 %; above that, nearly all of it. The calculator interpolates between those points and converts the carbohydrate kilocalories into grams at 4 kcal per gram.
That makes it possible to say something almost nobody works out: your stores hold on the order of 5.5 g of usable glycogen per kilo — about 400 g for a 72 kg athlete, muscle and liver together — so dividing the store by the grams you oxidise per hour gives the time at which you run out if you eat nothing. That is the number that explains why three hour sessions get hard somewhere after the two and a half hour mark.
Why there is a ceiling, and what it depends on
Glucose crosses the gut wall through a transporter called SGLT1, and that transporter saturates. In most people, at around 60 g per hour: it does not matter that you swallowed 90 g of maltodextrin, the rest stays in the gut and turns into trouble.
Fructose uses a different door, GLUT5, which is independent. That is why a 2:1 glucose to fructose blend raises the ceiling to about 90 g/h, and modern high fructose blends around 1:0.8 reach 120 g/h in athletes who have trained the gut for it. Outside the lab, 120 g/h is not a number you improvise: it is the end of a progression measured in weeks.
Read the label of your product before choosing the ceiling in the calculator. Many supermarket sports drinks are glucose or maltodextrin only, and with those the real ceiling is 60 g/h no matter how many gels you add.
The drink does two jobs at once
A bottle of sports drink is both carbohydrate and fluid, and each has its own target. Fluid follows your sweat rate: 400 to 500 ml per hour in the cold, around 700 in normal conditions, and above 900 in real heat. Carbohydrate follows your hourly target. The two almost never line up on their own, which is where the two classic problems come from.
The first is trying to put all the carbohydrate in the drink: 90 g/h would need a concentration close to 13 %, nearly double what empties from the stomach comfortably, and you end up bloated. The second is the opposite: bottles so dilute that you would have to drink two litres an hour to hit the grams. The practical answer is the one this calculator applies: the drink covers the fluid and part of the grams, and the rest arrives as a gel or a solid, with water on top.
Sodium is not optional in long sessions
Sweat carries sodium, and the amount varies enormously between people: from about 200 mg per litre to over 2000 in those who leave their kit white. The average this calculator uses to warn you is 500 mg per litre of sweat, which in mild conditions works out at about 350 mg per hour.
Below that, in sessions over three hours in the heat, you get cramps and that bloated feeling despite drinking plenty. Above four or five hours the risk changes name: drinking a lot of fluid without sodium dilutes blood sodium, and that is hyponatraemia, which is a serious problem and not an inconvenience. The rule is not to drink more than you sweat, and to make sure what you drink has salt in it.
The four mistakes that keep repeating
Starting late. The first gel at ninety minutes makes up for nothing: it refills a tank that is already half empty. The first feed goes between minute 20 and 30, while you are not hungry yet.
Trying a product for the first time on race day. The gut adapts to what you give it regularly. A gel you have never tried, at 90 g/h, in competition, is a bet with a terrible payout.
Counting only the gels. The sports drink at an aid station adds 20 or 30 g per bottle that almost nobody counts, which is how plenty of people overshoot without knowing it.
Copying someone else's plan. The grams per hour each person tolerates are different, and they are trainable. If 50 g/h is where you are comfortable now, raise it with a gut training plan before trying 90 in a race.
Frequently asked questions
How many grams of carbohydrate per hour should I take?
It depends on duration and intensity. Under an hour, little or nothing. One to two hours, around 30 g/h. Two to two and a half hours, 60 g/h. Above two and a half hours at race intensity, 90 g/h, and up to 120 g/h in very long events if you have trained the gut. If the session is planned by IF, TSS, kJ or zones, the calculator takes the figure from the workout itself instead of from the table.
How many calories is a kilojoule on the bike?
Practically one. The mechanical work your power meter measures is produced at roughly 24 % efficiency, and since a kilocalorie is 4.184 kilojoules, the two factors almost cancel out. That is why a 2000 kJ session is read as 2000 kcal burned without any meaningful error.
How do you get from TSS to grams of carbohydrate?
The TSS of a session is 100 × hours × IF², so TSS and duration give you IF. IF and your FTP give the average power, and that gives the kilojoules. Of that total energy, the share coming from carbohydrate depends on intensity: around 40 % at 55 % of threshold and close to 80 % at threshold. Converted at 4 kcal per gram, those are the grams you oxidise.
Can I take 90 g/h with ordinary glucose gels?
Not usefully. If the product is glucose or maltodextrin only, the SGLT1 transporter saturates at around 60 g/h and the excess stays in the gut. To go beyond that you need fructose in the blend: 2:1 to reach 90 g/h and around 1:0.8 to approach 120.
How much should I drink per hour?
Roughly what you sweat, and you measure that by weighing yourself before and after a session without a toilet break: every kilo lost is about a litre of sweat, plus whatever you drank. As a reference, 400 to 500 ml/h in the cold, around 700 in mild conditions and above 900 in the heat. Drinking far more than you sweat improves nothing and in long events it is dangerous.
When do I take the first gel?
Between minute 20 and 30. The idea is to stop the tank dropping, not to refill it once it is already low. In a three hour session a feed every twenty minutes beats three gels in a row in the last hour.
What about swimming?
The energy engine here is built for cycling and running, because the cost of swimming per 100 metres depends so heavily on technique that an average figure would mislead more than it helps. For a swim session, use the grams per hour mode with the duration: under an hour in the pool you need nothing but water, and for long open water swims the plan looks more like the bike one. If what you want is your paces and zones, they are in the CSS calculator.
Is this plan for racing or only for training?
Both, but in a race you have to fit the feeds to the real aid stations: if you can only get a bottle at kilometre 20 and 60, move there whatever the table puts elsewhere. What matters is that you have rehearsed the race plan exactly as written, with the same product, in at least two long sessions.
