Calories Needed During a Marathon: Fuelling Guide

Two runners line up at the same marathon. Same training, same goal time, same kit. At mile 21, one is steady and grinding through the final 5 miles. The other has stopped, hands on knees, watching strangers jog past. Their training did not separate them. Their fuelling did. Knowing the calories needed during a marathon, and matching them with the right carbohydrate intake on the day, is the single biggest race-day variable most runners get wrong.

This guide walks through the numbers: how many calories a marathon burns, how much you should take in while running, and how to build a fuelling plan that gets you to the finish line without crawling the last 10 km.

How Many Calories Does a Marathon Burn?

A useful working number: roughly 100 calories per mile for an average-sized runner. Across the full 42.2 km (26.2 miles) that lands at around 2,620 calories burned during a marathon. It is a rough average though, and the spread is wide.

The actual figure depends on three things:

• Body weight – heavier runners burn more. A 60 kg runner might burn around 2,200 calories. An 85 kg runner can burn over 3,500 calories across the same distance.
• Running economy – efficient runners spend less energy per kilometre. Years of training improve this.
• Pace and terrain – faster running burns more calories per minute, and hills compound the cost. A hilly marathon can add 5 to 10% to the total.

A simple estimate: multiply your body weight in kg by 1.05, then by the distance in km. So a 70 kg runner doing a flat marathon burns roughly 70 x 1.05 x 42.2 = about 3,100 calories. Use our weight versus pace calculator to see how body weight changes the energy cost of your goal pace.

Here is the catch though – you cannot eat 2,600 calories while you run. Your stomach will not tolerate it, and you would not need to even if it could. The body stores enough carbohydrate (glycogen) in muscles and liver to cover roughly 1,800 to 2,000 calories of running. The fuelling problem is bridging the gap between what you stored and what the race demands.

The Bonk Explained: Why Mile 20 Is the Wall

The “wall” is not a metaphor. It is glycogen depletion. Your muscles run primarily on a blend of carbohydrate and fat. The harder you push, the more carbohydrate you burn as a percentage. At marathon pace, that mix relies heavily on the limited carbohydrate stores in your legs and liver.

Most well-trained runners have about 90 minutes to two hours of stored carbohydrate at race pace. After that, the tank empties. When glycogen runs low, the body has to drop pace dramatically so it can switch to burning more fat. That sudden pace cliff, usually somewhere between mile 18 and 22, is the bonk.

You delay it two ways: store more carbohydrate before the race, and take in carbohydrate during the race so your stores last longer. Both matter.

This article is for informational purposes only and does not constitute medical or nutritional advice. Endurance fuelling carries individual variation and risks (including gastrointestinal distress and electrolyte imbalances). Speak to a registered sports dietitian or qualified healthcare professional before changing your race-day nutrition, particularly if you have a medical condition or take medication.

How Much to Eat the Week Before a Marathon

Carb-loading works, but it is not a single Saturday-night pasta dinner. Research led by Asker Jeukendrup, one of the world’s leading sports nutrition scientists, shows that two to three days of high-carbohydrate eating reliably increases muscle glycogen stores in trained runners (Jeukendrup, 2014, Sports Medicine).

The practical version:

• Three days out: shift to roughly 8 to 10 g of carbohydrate per kg of body weight per day. For a 70 kg runner, that is 560 to 700 g of carbs daily – rice, pasta, bread, potatoes, oats, fruit.
• Cut back on fibre and fat in the final 48 hours to ease digestion on race morning.
• Hydrate steadily with water and a small amount of sodium. Pale-yellow urine is the target. Drinking gallons does not help and can dilute sodium.

Race-day breakfast should land two to three hours before the gun. Aim for 1 to 2 g of carbohydrate per kg of body weight, with very low fibre and fat. Porridge, white toast with honey, a banana and a small coffee is a classic combination that works for most stomachs.

Carbs Per Hour: The Race-Day Numbers That Matter

The American College of Sports Medicine, in its joint position stand on nutrition and athletic performance, recommends 30 to 60 g of carbohydrate per hour for events lasting one to two and a half hours, and up to 90 g per hour for events over two and a half hours when using a mix of carbohydrate types (Thomas et al., 2016, Medicine & Science in Sports & Exercise).

Translate that into a finishing time:

• Sub-3:00 marathon: aim for 60 to 90 g of carbs per hour. Total over the race: 180 to 270 g.
• 3:00 to 4:00 marathon: 45 to 75 g per hour. Total: 150 to 250 g.
• 4:00 to 5:00 marathon: 30 to 60 g per hour. Total: 150 to 240 g.
• 5:00 plus: 30 to 45 g per hour, with attention to electrolytes and small amounts of protein later on.

For context, a typical running energy gel contains 20 to 30 g of carbohydrate. So 60 g per hour means a gel every 20 to 25 minutes from about 30 minutes into the race.

The 90 g protocol

If you want to push toward the upper limit of 90 g per hour, you need a mix of carbohydrate types. Glucose alone saturates one set of gut transporters at around 60 g per hour. Adding fructose, which uses a separate transporter, lifts the ceiling to roughly 90 g per hour. Most modern high-carb sports drinks and gels labelled “dual-source” or “2:1 ratio” are formulated for this. You cannot just take more single-source gels and expect the same effect – the gut will not absorb them and you will get sick.

Sub-elite and elite marathoners now routinely train their gut to tolerate 90 to 100 g per hour. Recreational runners do not need to chase that ceiling, but understanding the principle helps you choose products that match your goals.

When to Take Gels During a Marathon

The best fuelling plan is the one you can actually execute under pressure. A simple structure:

• Mile 5 (or 30 minutes in): first gel. You are still feeling fresh and your stomach is calm.
• Every 20 to 25 minutes after: next gel, washed down with water.
• From halfway: consider alternating gels with energy chews if you are sick of the gel taste. Chews are slower to absorb but easier on the stomach for some runners.
• Last 5 miles: a small caffeine gel can help if you tolerate caffeine. Around 100 to 200 mg of caffeine across the race improves perceived effort and pacing in the closing stages.

The cardinal rule of marathon fuelling: nothing new on race day. Every gel brand, every flavour, every drink mix must be tested on long runs. Practise the exact race-day schedule on two or three of your longest training runs. If a product upsets your stomach in week 14 of training, it will destroy you in week 18.

Hydration and Electrolytes Belong With Fuelling

Carbs and fluid are not separate problems. Most gels need 100 to 200 ml of water to absorb properly. Take a gel without water and you risk pulling fluid into your gut, which causes cramping and nausea.

Aim for 400 to 800 ml of fluid per hour during a marathon, depending on weather, sweat rate and body size. In hot conditions, the upper end. In a cool race, the lower end. Drink to thirst rather than forcing fluid – over-drinking water without sodium can cause hyponatraemia, which is dangerous.

Sodium losses vary massively between runners. Heavy sweaters can lose over 1,000 mg of sodium per litre of sweat. Electrolyte tablets dropped into your bottles, or an isotonic drink mix, replace what aid stations alone often miss. If you finish long training runs with white salt rings on your kit or a pounding salt-craving headache, you are a heavy sodium loser – plan accordingly.

Carrying your own fuel matters if you race a course with sparse aid stations or want full control over what you take in. A running hydration belt with two small flasks lets you mix your own sports drink at the concentration that has worked in training, rather than guessing what the course is handing out.

Generate Running Plan

The Fuelling Difference in the Final 10 km

Look at any large mass-participation marathon and the pattern in the final 10 km is the same. Two runners with similar training, similar weekly mileage and the same goal time start side by side. The one who skipped breakfast, grabbed unfamiliar gels off a friend the day before and trusted adrenaline ends up walking from mile 20 and finishing 15 to 25 minutes outside goal. The one who carb-loaded for three days, ate a low-fibre breakfast 2-3 hours before the gun, took a gel at mile 5 then every 25 minutes, alternated water and electrolyte at every aid station and used a caffeinated gel late on holds marathon pace while everyone around them is not.

Same fitness, very different last 10 km. Marathon-finisher data shows the back half of the field slows by roughly 8 to 12% on average between miles 20 and 26, and a large slice of that comes down to fuelling, not fitness. It is the cheapest, fastest improvement available to most marathoners.

What to Eat After a Marathon

The 30 to 60 minutes after you finish is when your muscles are most receptive to refuelling. You do not have to force food down immediately, but getting carbohydrate and a little protein in within the first hour speeds glycogen replenishment and starts repair.

A simple post-race target: 1 to 1.2 g of carbs per kg of body weight, plus 15 to 25 g of protein. A banana, a recovery drink, and something savoury like a salty pretzel or a sandwich covers it well. Continue eating normally for the next 24 hours – chasing perfect macros while you are exhausted is unnecessary. Use the pace calculator later in the week to recalibrate your training paces based on what you actually ran, then read our guide on what to do in the two weeks after a big race for a full recovery plan.

And if you are now planning your next race already, the RunReps Running Plan Generator builds a training schedule that includes specific long-run weeks where you can practise the exact fuelling protocol you intend to use on race day.

Questions Marathon Runners Ask About Fuelling

How many calories do you burn in a marathon?

A typical runner burns around 2,600 calories during a marathon, but the range is roughly 2,200 to 3,500 calories depending on body weight, pace and course. A quick estimate is your body weight in kg multiplied by 1.05, then multiplied by 42.2 km. Heavier runners and hillier courses sit at the higher end.

How many calories should I eat during a marathon?

You will not replace what you burn while running. The target is 30 to 60 g of carbohydrate per hour for most recreational marathoners, rising to 60 to 90 g per hour for faster finishers using dual-source carb products. That works out at roughly 120 to 360 calories per hour from gels, drinks or chews.

Why do runners hit the wall around mile 20?

Muscle and liver glycogen stores hold around 1,800 to 2,000 calories of fuel at marathon pace, which lasts about 90 minutes to two hours. Without carbohydrate intake during the race, those stores run out, and the body has to slow dramatically to switch to burning more fat. That sudden pace drop is the bonk.

Should I take gels with water?

Yes. Most energy gels are concentrated carbohydrate solutions that need 100 to 200 ml of water to dilute and absorb properly. Taking a gel without water can cause stomach cramps, nausea or diarrhoea. The exception is isotonic gels, which are pre-diluted, but even these go down more comfortably with a sip of water.

How do I train my gut for higher carb intake?

Practise on long runs. Start at 30 g per hour. Each week, push the intake up by 5 to 10 g per hour during one key long run. Within six to eight weeks, most runners can tolerate 60 to 90 g per hour without gastrointestinal issues. Like any system in endurance training, the gut adapts to repeated stress.