Mavr Sports Nutrition Team
September 24, 2026. 10 min read.
Running a marathon burns about 1 kcal for every kilogram of body weight for every kilometer. For a 70 kg (154 lb) runner that is roughly 2,950 kcal over 42.2 km, more than a full day of normal food. Pace changes the number less than most people think. Body weight changes it a lot.
This guide shows the numbers by body weight, explains where the energy comes from and why that leads to the wall, and turns it into what you should eat before, during, and after the race.
The rule of thumb comes from measurements of the energy cost of running, which is close to 1 kcal per kg per km across normal running speeds. Running economy differs between runners, so treat these numbers as good estimates, not exact values.
| Body weight | Half marathon (21.1 km) | Marathon (42.2 km) |
|---|---|---|
| 50 kg (110 lb) | About 1,050 kcal | About 2,110 kcal |
| 60 kg (132 lb) | About 1,270 kcal | About 2,530 kcal |
| 70 kg (154 lb) | About 1,480 kcal | About 2,950 kcal |
| 80 kg (176 lb) | About 1,690 kcal | About 3,380 kcal |
| 90 kg (198 lb) | About 1,900 kcal | About 3,800 kcal |
| 100 kg (220 lb) | About 2,110 kcal | About 4,220 kcal |
Real races match this well. In a study of recreational marathon runners, men burned about 2,800 kcal and women about 2,400 kcal for the full distance. Larger runners burned more, and ran slower, than smaller runners.
These are total numbers. They include the energy you would have used anyway just being alive, about 1 kcal per kg per hour. For a 70 kg runner finishing in four hours, that is roughly 280 kcal, so the extra cost of the race itself is closer to 2,650 kcal.
Less than you would expect. Per kilometer, running costs about the same energy whether you run fast or slow. A faster runner burns more calories per hour but for fewer hours, so the total for the distance comes out about the same.
| 70 kg runner finishing in | Calories per hour | Total for the marathon |
|---|---|---|
| 3:00 | About 985 kcal | About 2,950 kcal |
| 4:00 | About 740 kcal | About 2,950 kcal |
| 5:00 | About 590 kcal | About 2,950 kcal |
| 6:00 | About 490 kcal | About 2,950 kcal |
What pace does change is the fuel mix. The harder you run compared with your fitness, the more of the energy comes from carbohydrate instead of fat. At a hard, sustained effort most of it comes from carbohydrate. Slower runners burn a bit more fat, but they are on the course for longer, so they still use a lot of carbohydrate in total.
Your muscles and liver store carbohydrate as glycogen, but only a limited amount, roughly a few hundred grams depending on your size and how well you have loaded. At marathon pace a large share of the energy comes from carbohydrate, so a runner who does not eat during the race can run low somewhere in the last third. That is the wall: the pace drops, the legs feel heavy, and thinking gets foggy. Modeling studies estimate that more than two in five marathon runners hit it.
Two things push the wall back or remove it: starting with full glycogen stores and eating carbohydrate during the race. You are not trying to replace every calorie. You only need to keep enough carbohydrate coming in to hold your pace.
You cannot replace 3,000 kcal on the run. Your gut absorbs about 60 g of glucose per hour, or up to about 90 g when you mix glucose and fructose. That is 240 to 360 kcal per hour, a fraction of what you burn, and it is enough.
| Finish time | Carbs per hour | Calories from race fuel | Share of what a 70 kg runner burns |
|---|---|---|---|
| 3:00 | 60 to 90 g | 720 to 1,080 kcal | About 25 to 35 percent |
| 4:00 | 45 to 75 g | 720 to 1,200 kcal | About 25 to 40 percent |
| 5:00 | 40 to 60 g | 800 to 1,200 kcal | About 25 to 40 percent |
The rest comes from glycogen and fat. That split is why these amounts are enough to hold pace for most runners who start with full stores. Above 60 g per hour, use glucose-fructose products and only amounts you have practiced. The gel guide linked below turns this into a packet count.
Carb loading does not mean eating 3,000 extra calories. It means making carbohydrate the biggest part of your food for 36 to 48 hours, about 8 to 10 g per kg of body weight per day, while training is light. For a 70 kg runner that is 560 to 700 g of carbohydrate, about 2,250 to 2,800 kcal from carbs alone. Total calories go up a little, carbs go up a lot, and fat and fiber come down.
You finish with a big energy gap and low glycogen stores. You do not need to eat it all back at once. In the first hour, aim for about 1 to 1.2 g of carbohydrate per kg plus 20 to 30 g of protein, and add more carbohydrate every couple of hours if you can. Over the next two days, eat to appetite with plenty of carbohydrate and protein. Hunger often comes back strongly the day after the race.
On race day you may be 1 to 3 kg lighter on the scale, but almost all of that is water and glycogen. It comes back within a day or two as you eat and drink. The whole race is worth about the energy stored in 0.4 kg of body fat, and only part of the energy actually comes from fat.
Training is different. Sixteen to twenty weeks of marathon training burn a lot of energy, and runners who under-eat in that block are the ones who get injured or sick. If you want to lose weight, do it slowly and far from race week. The weight loss guide linked below explains how.
Garmin, Apple Watch, and Strava estimate calories from heart rate, pace, and your profile. Lab studies find their calorie numbers are often 20 to 40 percent off for a single workout, even when heart rate is accurate, and some show only active calories. Use the 1 kcal per kg per km rule as a cross-check. If your watch is far away from it, trust the formula.
About 1 kcal per kg of body weight per km: roughly 2,530 kcal for a 60 kg runner, 2,950 kcal at 70 kg, and 3,380 kcal at 80 kg. Pace changes the total very little; body weight changes it a lot.
About half of a marathon: roughly 1,270 kcal at 60 kg, 1,480 kcal at 70 kg, and 1,690 kcal at 80 kg.
Think in carbs, not calories: 30 to 60 g of carbohydrate per hour for most runners, up to 90 g per hour if you have trained your gut. That is 120 to 360 kcal per hour, much less than you burn, and enough to prevent the wall for most runners.
Per hour, yes. For the whole race, not really. The energy cost per kilometer is about the same across normal running speeds, so a faster runner burns more per hour for fewer hours.
There is no need to replace everything at once. Eat 1 to 1.2 g of carbohydrate per kg plus 20 to 30 g of protein in the first hour, then normal meals with plenty of carbs and protein, eating to appetite for the next two days.
It is an estimate. Lab studies find watch calorie numbers are often 20 to 40 percent off for a single workout. The 1 kcal per kg per km rule is a good cross-check.
General guidance for healthy adults, not medical advice. Talk to a doctor or registered dietitian if you have a health condition, take medicines or have special dietary needs.
Mavr turns your weight and goal time into a marathon fuel plan: carbs per hour, gels to carry, fluids, and sodium, plus how much to eat in the days before and after.
Ask Kai, the coach in the app