How it works
carbs/hour by duration: <1 h ≈ 0 · 1–2 h ≈ 30 g · ≥2 h ≈ 60 g (up to 90)
Your body stores only enough carbohydrate for roughly 90 minutes of hard running, so longer efforts need carbs from outside. Guidelines scale intake with duration: under an hour you generally need little beyond what you ate beforehand; between one and about two and a half hours, around 30–60 grams per hour works well; and for races over about two and a half to three hours, trained runners can absorb up to 90 grams per hour using a mix of glucose and fructose. This tool uses 0, 30 and 60 g/h bands as a sensible default, multiplies by your race time for the total, and divides by 25 grams to estimate gels. Sodium loss varies with sweat, but roughly 300–800 mg per hour is typical; we show 500 mg/h as a starting point — measure your sweat rate to personalise it. Always practise your fueling in training: the gut adapts, and race day is no time to try something new.
Sources
- Carbohydrate during exercise Jeukendrup, A. (2014). “A step towards personalized sports nutrition: carbohydrate intake during exercise.” Sports Medicine 44(S1), 25–33 — 30–60 g/h, up to ~90 g/h with multiple transportable carbohydrates for long events.
- ACSM/ISSN guidance American College of Sports Medicine / International Society of Sports Nutrition — carbohydrate intake scales with exercise duration; ~30–90 g/h for events over ~1–2.5 h.
- Sodium replacement Sweat sodium losses commonly fall around 300–800 mg per hour; replacement needs are individual and rise with heat and sweat rate.
FAQ
How many carbs do I need during a race?
It depends on how long you are out there. Under an hour you usually need little; for 1–2.5 hours aim for roughly 30–60 grams of carbohydrate per hour; and for races beyond about 2.5 hours, up to 90 g/h if your gut is trained. This calculator gives a per-hour target and a total from your expected finish time.
How many gels is that?
Most energy gels contain about 25 grams of carbohydrate, so the calculator divides your total carbs by 25. A 3:30 marathon at 60 g/h needs about 210 g, or roughly nine gels — spread evenly, not all at once.
When should I start fueling?
Begin before you feel low — typically within the first 30–45 minutes — and keep a steady intake rather than waiting until you fade. Starting early keeps your blood sugar and muscle fuel topped up so you avoid the late-race bonk.
How much sodium and fluid do I need?
Sodium needs vary, but 300–800 mg per hour is a common range; this tool shows 500 mg/h as a starting point. Fluid depends on your sweat rate — measure it with the sweat rate calculator and aim to limit body-mass loss to under about 2%.
Can I really absorb 90 g of carbs per hour?
Only with training and the right products. Above about 60 g/h you need multiple transportable carbohydrates (glucose plus fructose) and a gut that has practised high intakes. Build up gradually in long runs; pushing 90 g/h untrained often causes stomach trouble.
Do I need to fuel for a 10K or shorter?
Usually not during the race — your stored carbohydrate covers efforts up to about 90 minutes. Focus on a good carbohydrate meal a few hours before and on hydration. Fueling on the run matters most for the half marathon, marathon and ultras.
Fueling targets are general guidelines, not personalised nutrition advice; individual tolerance and needs vary widely. Always rehearse fueling in training and consult a sports dietitian for a tailored plan. General information, not medical advice.