The finding. Race fueling guidelines are written per hour, but a gel plan written in mile markers gives a different hourly dose at every pace. With an example gel of 25 g, one gel every 5 miles gives a 3:00 marathoner about 44 g of carbohydrate per hour and a 5:00 marathoner about 26 g, below the 30 g floor of the guideline. To hold 60 g per hour with that gel, the spacing is every 25 minutes: 3.6 miles apart at 3:00 pace, 2.7 at 4:00 and 2.2 at 5:00.
The guideline numbers, as published
Two sets of numbers matter here, and they work on different scales.
Before the race (loading). A 2021 review in Sports Medicine by Holtzman and Ackerman summarises the current recommendation, drawing on the 2016 ACSM joint position statement on nutrition and athletic performance (Thomas, Erdman and Burke) and on Burke and colleagues (2011). For an event lasting more than 90 minutes, the recommendation is 10 to 12 g of carbohydrate for each kilogram of body weight, every day, over the final 36 to 48 hours.
During the race. The 2011 paper by Burke, Hawley, Wong and Jeukendrup, written for the IOC's sports nutrition consensus, puts the target for longer sports at 30 to 60 g an hour and allows up to 90 g an hour once an event passes 2.5 hours. In efforts lasting roughly one hour, a little carbohydrate, or simply rinsing it round the mouth, does the job. Jeukendrup's 2014 review in Sports Medicine gives about 60 g per hour for 2 to 3 hours and about 90 g per hour for ultra-endurance, using multiple transportable carbohydrates (mixes such as glucose and fructose), and states that the advice does not depend on body weight.
So loading scales with your weight and not your race time, while race fueling scales with your race time and not your weight. Everything below is those rules multiplied out.
Carb loading in grams, by body weight
Daily grams = body weight in kg x 10 to 12. Totals for 36 hours multiply the daily figure by 1.5, for 48 hours by 2. If you think in pounds, the range works out at 4.5 to 5.4 g for each pound you weigh, per day.
| Body weight | kg | Per day at 10 g/kg | Per day at 12 g/kg | 36 hours (1.5 days) | 48 hours (2 days) |
|---|---|---|---|---|---|
| 110 lb | 49.9 | 499 g | 599 g | 748 to 898 g | 998 to 1,197 g |
| 130 lb | 59.0 | 590 g | 708 g | 885 to 1,061 g | 1,179 to 1,415 g |
| 150 lb | 68.0 | 680 g | 816 g | 1,021 to 1,225 g | 1,361 to 1,633 g |
| 170 lb | 77.1 | 771 g | 925 g | 1,157 to 1,388 g | 1,542 to 1,851 g |
| 190 lb | 86.2 | 862 g | 1,034 g | 1,293 to 1,551 g | 1,724 to 2,068 g |
| 210 lb | 95.3 | 953 g | 1,143 g | 1,429 to 1,715 g | 1,905 to 2,286 g |
| 230 lb | 104.3 | 1,043 g | 1,252 g | 1,565 to 1,878 g | 2,087 to 2,504 g |
Worked example: a 150 lb runner weighs 68.0 kg (150 x 0.4536). Ten grams per kg gives 680 g a day, twelve gives 816 g; doubled for a 48 hour load, the total lands between 1,361 and 1,633 g. The carb loading calculator does this for any weight in pounds or kilograms.
The spread is wide: at the bottom of the range, a 110 lb runner loads 499 g a day; at the top, a 230 lb runner loads 1,252 g. Those are very different menus, which is why a single gram figure cannot fit every runner.
During the race, in grams by finish time
Total grams = hours x grams per hour. The 90 g column only applies past 2.5 hours, because that is the threshold the 2011 paper uses. The gel column divides by an example gel of 25 g and rounds up, the same way the race fueling calculator does; 25 g is only an example, so check the carbohydrate on your gel's label and use that figure.
| Finish time (any race) | Total at 30 g/h | Total at 60 g/h | Total at 90 g/h (over 2.5 h only) | Example 25 g gels: 30 / 60 / 90 g/h |
|---|---|---|---|---|
| 1:30 | 45 g | 90 g | n/a | 2 / 4 / n/a |
| 2:00 | 60 g | 120 g | n/a | 3 / 5 / n/a |
| 2:30 | 75 g | 150 g | n/a | 3 / 6 / n/a |
| 3:00 | 90 g | 180 g | 270 g | 4 / 8 / 11 |
| 3:30 | 105 g | 210 g | 315 g | 5 / 9 / 13 |
| 4:00 | 120 g | 240 g | 360 g | 5 / 10 / 15 |
| 4:30 | 135 g | 270 g | 405 g | 6 / 11 / 17 |
| 5:00 | 150 g | 300 g | 450 g | 6 / 12 / 18 |
| 5:30 | 165 g | 330 g | 495 g | 7 / 14 / 20 |
| 6:00 | 180 g | 360 g | 540 g | 8 / 15 / 22 |
The table may run against intuition. Because the target is per hour, slower finishers need more carbohydrate in total, not less: a 5:00 marathon at 60 g per hour adds up to 300 g, a 3:00 marathon to 180 g. A 2:30 finish sits exactly on the threshold; the guideline says "over 2.5 h", so the table and the calculator leave 90 g out there.
How often to take a gel, at your gel size
Minutes between gels = gel grams / target grams per hour x 60. A 25 g gel at 60 g per hour: 25 / 60 x 60 = 25 minutes. The gel sizes below are examples to show the arithmetic, not a statement about any brand; read the grams of carbohydrate per gel on your own packet.
| Gel size (example) | At 30 g/h | At 60 g/h | At 90 g/h |
|---|---|---|---|
| 20 g | 40 min | 20 min | 13.3 min |
| 25 g | 50 min | 25 min | 16.7 min |
| 30 g | 60 min | 30 min | 20 min |
Drinks and chews count the same way: add their grams to the hourly total. If half your carbohydrate comes from a sports drink, the gap between gels doubles.
Turning minutes into mile markers
On race day it is often easier to take a gel at a mile marker or an aid station than on a timer. The conversion needs your pace, which the marathon pace chart gives for any finish time. Distance between gels = minutes between gels / minutes per mile.
Worked example for a 4:00 marathon: pace is 9:09 per mile (4 hours / 26.219 miles). A gel every 25 minutes is 25 / 9.15 = 2.7 miles. Turned round, a 25 g gel every 5 miles means one gel per 5 x 9:09 = 45.8 minutes, which is 25 / 45.8 x 60 = 32.8 g per hour: inside the guideline, but only just.
| Marathon finish | Pace per mile | Pace per km | One 25 g gel every 25 min (60 g/h) is every | One 25 g gel every 5 miles gives |
|---|---|---|---|---|
| 3:00 | 6:52 | 4:16 | 3.6 mi (5.9 km) | 43.7 g/h |
| 3:30 | 8:01 | 4:59 | 3.1 mi (5.0 km) | 37.5 g/h |
| 4:00 | 9:09 | 5:41 | 2.7 mi (4.4 km) | 32.8 g/h |
| 4:30 | 10:18 | 6:24 | 2.4 mi (3.9 km) | 29.1 g/h |
| 5:00 | 11:27 | 7:07 | 2.2 mi (3.5 km) | 26.2 g/h |
That is the finding in one table. A plan written as "a gel every so many miles" quietly gives faster runners more carbohydrate per hour and slower runners less. For the 4:30 and 5:00 finishers, a gel every 5 miles falls below 30 g per hour. Writing the plan in minutes first, then converting to the mile markers on your course, keeps the hourly dose where the guideline puts it.
Loading versus race day
The two numbers are not the same size. The 150 lb runner in the loading example eats 1,361 to 1,633 g over the two days before a race. During a 4:00 marathon at 60 g per hour the same runner takes 240 g, so loading is 5.7 to 6.8 times the race-day total. That is why most of the planning work sits in the days before, and why the race-day number is mostly a question of timing and of what your stomach tolerates.
Body weight changes the first number and not the second. A 110 lb and a 230 lb runner finishing in the same time get the same 30 to 60 g per hour during the race, by Jeukendrup's 2014 reading, while their loading targets differ by a factor of more than two.
Limits, and when to ask a professional
These are population guidelines multiplied out, not a personal plan and not medical advice. The higher in-race rates rely on mixes of carbohydrate types, and a stomach can object to amounts it has not practised, so try your plan on long training runs, starting at the low end. Anyone living with diabetes, a gut condition, an eating disorder or another medical condition should build both the loading days and the race plan with a registered dietitian or a doctor.
Fluid is a separate question with its own arithmetic: the sweat rate calculator measures it, and the sweat rate test guide shows how the 2% body-weight line turns into a drinking target. For pacing your goal time, the half marathon pace chart covers the shorter race.
Sources: Burke, Hawley, Wong and Jeukendrup (2011), Carbohydrates for training and competition, J Sports Sci; Jeukendrup (2014), A step towards personalized sports nutrition: carbohydrate intake during exercise, Sports Med; Thomas, Erdman and Burke (2016), ACSM joint position statement: Nutrition and Athletic Performance, Med Sci Sports Exerc; Holtzman and Ackerman (2021), Recommendations and nutritional considerations for female athletes, Sports Med. Rules checked . Found a problem? Tell us.
Questions
How many grams of carbs should I eat when carb loading?
The guideline is 10 to 12 g for each kg you weigh, daily, across the last 36 to 48 hours before a race lasting over 90 minutes. A 150 lb (68 kg) runner lands at 680 to 816 g a day.
How often should I take a gel in a marathon?
Divide the grams in your gel by your hourly target and multiply by 60. With a 25 g gel at 60 g per hour that is every 25 minutes: about every 2.7 miles at 4:00 marathon pace or every 3.6 miles at 3:00 pace.
Do slower marathon runners need more carbs?
In total, yes. The guideline is per hour, so a 5:00 marathon at 60 g per hour adds up to 300 g, against 180 g for a 3:00 marathon.
Do heavier runners need more carbs during a race?
Not by the guideline: Jeukendrup (2014) gives in-race carbohydrate targets per hour that do not change with body weight. Weight only changes the loading target in the days before.