How many gels do I need for a marathon?

It depends on your target finish time, your carb target per hour, the carb content of each gel, and whether you use sports drink. This calculator estimates both gels per hour and total gels needed.

Should I fuel by kilometres or time?

Time is usually more reliable because carbohydrate intake is time-based. Kilometre checkpoints can be useful as a backup reminder.

When should I take my first gel in a marathon?

Many runners do better starting early, often around 20 to 35 minutes into the race, then repeating on a consistent schedule.

Marathon Fueling Calculator

Updated: 12 Mar 2026 · Free tool

Work out your marathon fueling plan based on finish time, carb target, gel size, and sports drink use. This calculator estimates gels per hour, total gels needed, gel timing, and approximate kilometre checkpoints.

Calculator inputs

Target finish time Example: 3:30

Hours

Minutes

Carb target per hour (g) Common range: 30–90 g/h

Carbs per gel (g) Check your label

First gel at Minutes into the race

Gel interval

Extra backup gels Recommended: 1 extra

Include sports drink in carb total

Tip: fuel by time first. Kilometre markers are helpful as reminders, but your carbohydrate intake is time-based.

Your fueling plan

Target race time 4:00

Approx pace per km 5:41 /km

Carbs from gels per hour 60 g/h

Gels per hour 2.4

Planned gels during race 9

Carry this many gels 10

Start at 25 minutes, then take 1 gel every 25 minutes.

#	Race time	Approx km	Fuel	Notes

Copyable race-day plan

Educational use only. Always test your exact products and timing in training before race day.

Recommended fueling products

Once you know your timing plan, the next step is choosing products you can actually tolerate and repeat under marathon effort.

Product	Best for	Typical use
Maurten Gel 100	Runners who want simple carb delivery and minimal flavour	Used as a standard gel within 20–30 minute fueling schedules
GU Energy Gel	Runners who want lots of flavour options and easy availability	Works well when building a repeatable gel-by-time plan
SiS Beta Fuel Gel	Runners pushing higher carb targets per hour	Useful when trying to reduce how often you need to take gels
Tailwind Endurance Fuel Drink Mix	Runners who prefer drinking part of their carbohydrate intake	Can support gels or replace some gel intake on longer runs

Important: practice exact products in training before race day. A good plan on paper is only useful if your stomach accepts it at marathon effort.

Disclosure: Some links on this page are affiliate links. This means MarathonPaceKM may earn a small commission at no extra cost to you.

How to use this calculator

Choose a realistic marathon finish time.
Set your carb target per hour based on your experience and gut tolerance.
Enter the carb content of your gel.
Decide when you will take your first gel and how often you want reminders.
If you use sports drink, include it so the calculator reduces how much you need from gels alone.

Important: A higher carb target is not automatically better. The best plan is the one you can actually tolerate and execute.