Marathon Fuel Calculator
Carbs and gels needed for long runs from hours and burn rate.
What it does
Bodies store roughly ninety minutes of hard running glycogen, then the tank gauge lies and legs follow. Hourly carbs from gels, chews, or drink extend that range across marathons and ultras, with forty five grams an hour as the standard starting dose.
The tool above multiplies hours by hourly target for total carbs and divides by gel size for the carry count. Water at stations washes each dose down.
Gut training matters as much as leg training. Stomachs learn to absorb more with practice, climbing toward ninety grams an hour with mixed glucose and fructose sources, while untrained guts rebel past sixty.
Early beats late. First fuel near forty five minutes tops the tank before the dip, while waiting for hunger means absorbing uphill against depletion.
How to use
- Enter expected hours on feet, not goal optimistic.
- Set hourly carbs: 30 starting, 45 standard, more only if trained.
- Read the label carbs per gel honestly.
- Carry the rounded count plus one spare.
Formula
- hours
- Moving time expected on course.
- grams per hour
- Absorption target within gut training.
- ceil
- Round gels up. Partial gels do not exist mid race.
Caffeine gels add stimulation on top of carbs. Count their milligrams separately toward daily limits.
Worked example
Example: 4 hour marathon at 45 g per hour, 25 g gels
Four hours running, standard fueling, typical gels.
- Total: 4 x 45 = 180 g carbs.
- Gels: 180 / 25 = 7.2, round up to 8.
- Schedule: one every 30 minutes from 0:45.
- Water: a few gulps per gel at every station.
Reference table
| Race | Carbs | Gels |
|---|---|---|
| Half marathon 2 h | 90 g | 4 |
| Marathon 3.5 h | 158 g | 7 |
| Marathon 4 h | 180 g | 8 |
| Marathon 5 h | 225 g | 9 |
| 50K ultra 6 h | 270 g | 11 |
Limits
- Carbs only sized. Water and salt need separate plans.
- Gut training assumed. Untrained stomachs tolerate less.
- Aid station products may differ from training ones.
- Heat raises needs and slosh risk together.
Common mistakes
- Waiting for hunger before the first gel.
- Debuting new products on race day.
- Skipping water with concentrated gels.
- Underfueling long ultras with marathon doses.
- Forgetting caffeine totals across gels.
Try it yourself
Quick checks. Answers sit tucked under each question.
3 hours at 60 g per hour, 20 g chews?
180 g total. Then 180 / 20 = 9 chews.
5 hours at 30 g, 25 g gels?
150 g total. Then 150 / 25 = 6 gels.
Questions
How many gels for a marathon?
Seven to nine at standard dosing: four hours at 45 g per hour needs 8 gels of 25 g.
How often should I take gels?
Every 25 to 35 minutes from minute 45, with water each time.
Can I overdo carbs?
Yes past gut training. Nausea and sloshing signal backing off.
What about sports drink?
Count its carbs toward the hourly total instead of adding on top.
Do I need salt too?
Long hot races, yes. Electrolytes ride alongside, not instead of, carbs.
Real food instead of gels?
Works in ultras at easy effort. Fast marathons favor gels for speed of uptake.