Free · marathon, half marathon, 10K & ultra

Race fuelling calculator

What to eat the day before, what for breakfast, what goes in your pocket on the run and what after the finish. In grams AND in real food – vegan, vegetarian or everything.

All calculators

What are you running?
How long will you be out there?
h min

Your target time. Better honest than ambitious – duration decides almost everything.

Your weight (kg)

On the run it doesn’t matter (your gut is just as fast at 55 kg as at 95 kg). For sweat, breakfast and the day before, it does.

Weather on race day
How do you eat?

The amounts stay the same. What changes is what you use to hit them.

Fine-tuning (optional) Do you already eat while running? How salty is your sweat? What do you like on the run? Please leave out

On the run you need
0 g carbs / hour
–
Duration
–
kcal per hour
–
Sweat per hour

Your complete plan is waiting

The number above is the engine. Enter your email and confirm it with one click – then the whole gearbox appears here:

  • The day before as a meal plan (including carb-loading from 2.5 hours)
  • Breakfast and the last snack before the start
  • Your schedule on the run – by kilometre
  • Fluids and salt, matched to weather and weight
  • What helps after the finish
  • All in real food, switchable: vegan, vegetarian, omnivore

One click to go

There’s an email from alethics in your inbox right now. Click the link in it – your plan will then open here by itself, with everything you just entered.

Nothing there? Check your spam folder or wait two minutes. You can leave this page open.

And after the race?

Race day is quickly planned. So is your everyday life.

This plan gets you through one day. In the Start Plan we look at your whole week – training, food, sleep – and turn it into something that fits your life. No list of bans.

Request your Start Plan

Explained briefly

How a gel works – two doors in your gut

Picture your small intestine as a club with two entrances. At the main entrance there’s a bouncer who only lets glucose in – about 60 grams per hour, then it’s full. Fructose comes in through the small side entrance, roughly another 30 grams. That’s it, no matter how much you push in at the front.

60 g/h
Door 1 · Glucose
The main entrance. Needs sodium as company – that’s why sports drinks contain salt.
+30 g/h
Door 2 · Fructose
The side entrance. Works independently of door 1.

That’s why the upper limit is around 90 g per hour – and why good gels contain a mix of glucose and fructose. If you only eat glucose (classic gummy bears, for example), you’re queuing at a single door. Whatever doesn’t get through stays in your gut and causes exactly the stomach trouble nobody wants at km 32.

And your stomach? The more concentrated the gel, the more water it draws into the gut. So: take a few sips of water with it, unless the gel is explicitly isotonic. And very important: your gut is trainable. If you never eat in training, you’ll have an untrained bouncer on race day.

Myth check

What you can forget about race fuelling

“At kilometre 30 you hit the wall.”

The timing is surprisingly often right – your stores are roughly empty just where the race gets hard. But it isn’t a law of nature. Large analyses of marathon results show it only hits some runners, and mostly those who started too fast and refuelled too little along the way. You can control both.

“Drink as much as you can.”

The most dangerous sentence on this list. Drinking in advance dilutes your blood sodium (hyponatraemia). In a study at the Boston Marathon, 13 percent of finishers had sodium levels that were too low. That’s why the drinking recommendation in this calculator never exceeds your sweat rate. The most honest test: if you weigh more after the run than before, you drank too much.

“Carb-loading means a pasta party the night before.”

Wrong in both directions. You don’t need the old method with a depletion week – one to two days with lots of carbs and little training fill your stores almost completely. A single plate of pasta the evening before, on the other hand, is too little to make any difference. That’s why the plan contains a whole day.

“Once your fat metabolism is trained, you don’t need gels.”

Fat burning really can be increased. But race performance doesn’t improve because of it: in a controlled study with elite race walkers, the high-carb groups improved – the ketogenic group didn’t, despite better fat burning. At race pace your body wants sugar. Full stop.

“I’ll try the new gel from the race expo straight away on Sunday.”

Please don’t. The golden rule: nothing new on race day. No new gel, no new breakfast, no new shoes. If you want to test something, do it on a long training run – not at km 27 in front of thousands of spectators.

“Everyone needs salt tablets.”

Not everyone. Under about two hours, your normal diet easily covers your salt losses. After that it becomes relevant, especially in heat and if you get white marks on your shirt. Many sports drinks and some gels already contain sodium – count that in instead of adding more on top.

This calculator provides guide values for general orientation and does not replace medical or dietetic advice. It is based on the ACSM and IOC consensus on sports nutrition and Asker Jeukendrup’s work on carbohydrate intake in endurance sport. Test everything in training first. If you have a medical condition (e.g. diabetes, kidney or heart disease), please talk to your doctor.