How much should you eat during a race?
Your body stores only enough carbohydrate (glycogen) for roughly 90 minutes of hard running. Beyond that, if you have not been topping up, you hit the wall. Fuelling is simply about drip-feeding carbohydrate so your tank never runs dry.
The guidelines scale with race duration:
- Under 1 hour: usually no fuel needed — you have enough stored glycogen.
- 1 to 2.5 hours: 30–60 g of carbohydrate per hour.
- Over 2.5 hours: 60–90 g per hour, using glucose-plus-fructose products to absorb the higher end.
Turning carbs per hour into gels
Multiply your target carbs per hour by your race time to get total carbohydrate, then divide by the carbs in each gel to find how many to carry. A 3:30 marathoner at 60 g/hr needs about 210 g — roughly 10 gels of 22 g each — taken about every 20 minutes.
The calculator above gives you those three numbers for your own finish time, target, and gel size, plus a suggested interval so the fuelling is evenly spaced.
Marathon fuelling at a glance
Total carbohydrate and gel count for common marathon finish times, at 60 g of carbohydrate per hour with 22 g gels. These are the values this calculator produces.
| Finish time | Carbs / hour | Total carbs | Gels (22 g) |
|---|---|---|---|
| 3:00 | 60 g | 180 g | 9 |
| 3:30 | 60 g | 210 g | 10 |
| 4:00 | 60 g | 240 g | 11 |
| 4:30 | 60 g | 270 g | 13 |
| 5:00 | 60 g | 300 g | 14 |
Train your gut before race day
The single biggest fuelling mistake is trying a new strategy on race day. Your gut is trainable: practising 60–90 g per hour on long runs for several weeks teaches it to absorb carbohydrate under load, so you avoid the nausea and cramps that catch out under-prepared runners.
Rehearse your exact race-day products, timing, and quantities on at least two or three long runs. What the calculator gives you is the plan — the long runs make it work.
Race day is more than fuelling
The Running Genie builds your whole marathon block — long runs, pace work, taper, and race strategy — so your fuelling plan lands on top of real fitness.
Frequently asked questions
How many carbs should I eat during a marathon?
For efforts over about 2.5 hours, aim for 60 to 90 grams of carbohydrate per hour. For races of 1 to 2.5 hours, 30 to 60 grams per hour is usually enough. Under an hour you generally do not need to fuel. A 4-hour marathoner at 60 g/hr needs roughly 240 grams in total.
How many gels do I need for a marathon?
It depends on your finish time and the carbs per gel. Most gels contain 20 to 25 grams. A 4-hour marathoner needing about 240 grams would take roughly 10 to 12 gels. Enter your time and gel size above for your exact number.
When should I take gels during a race?
Start fuelling early — within the first 30 to 45 minutes — rather than waiting until you feel low. Space your gels evenly, typically one every 20 to 40 minutes, and always take them with water. This calculator gives you a suggested interval based on your time and target.
Can I absorb 90 grams of carbs per hour?
Rates above about 60 grams per hour require a mix of glucose and fructose (often labelled 2:1) so you use multiple transporters in the gut, and they need to be trained. Practise your race-day intake on long runs for several weeks so your stomach adapts.
Do I need to fuel for a 10K or half marathon?
For a 10K, generally no — it is short enough to run on stored glycogen. For a half marathon, most runners benefit from one or two gels, especially over about 90 minutes. Anything beyond roughly two hours makes consistent fuelling clearly worthwhile.
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