Free marathon & ultra fueling calculator
HOW MUCH SHOULD YOU EATON RACE DAY?
Bonking and stomach trouble end more races than tired legs. Enter your half marathon, marathon or ultra and I'll work out how many carbs per hour to eat, the same number I'd give one of my own athletes.
Made by Jonathan Fors, UESCA-certified ultrarunning coach. Top 5% on ITRA, elite field at MDS Legendary, 744 km across Portugal.
Your race fueling plan
Step 1 of 2
A real coach's method, not a generic formula.
I'm Jonathan, a UESCA-certified ultra coach. I've raced from the Sahara to 744 km across Portugal and I coach runners from their first half to 100 miles. This tool runs the exact rules I use when I set up an athlete's race day.
Carbs scaled to your gut
From 30 g/h for short races to 90 g/h for a trained gut on long ultras. Never more than you've practised.
Fluid and salt for the day
Adjusted for heat and how heavily you sweat, so you don't slosh or cramp.
Real food, not just gels
Long races switch to savoury food from hour three, the way the best ultra finishers fuel.
A plan to train your gut
A ramp for your long runs so race day isn't the first time you eat that much.
The record
THE TRACK RECORD OF YOUR COACH
- Top 5%
- Ranked worldwide on ITRA
- Elite runner
- Ranked as an elite runner in MDS Legendary 2025
- 744 km
- Longest run (Project Portugal)
Project Portugal · 1–10 August 2026
Most recently, I ran the entire coastline of Portugal, in 10 days, for men's mental health, with my partner Niki driving support. It was the second attempt. Two years earlier we set out on the same route and my immune system gave out at 480 km.
The second one is the line on the page. The first one is the reason it's worth anything: I know what it is to do the work, do it properly, and still have to stop — and then to decide whether you go back.
Enter your race and a few honest numbers
See your headline result instantly
Get the full coach plan in your inbox
How many carbs per hour should I eat in a marathon?+
Most runners do well with 30–60 g per hour in a marathon. A trained gut can take 60–90 g. Use the calculator above and it will give you your own number from your finish time and gut training.
How many carbs per hour in an ultramarathon?+
Usually 60–90 g per hour, but only up to what you've practised in training. Past hour three, mix savoury real food in with gels so your stomach doesn't shut down.
Is 90 g of carbs per hour safe?+
Only if you've trained your gut for it. The calculator caps your target lower if you're new to fueling and gives you a ramp to build up.
How much sodium and fluid do I need per hour?+
It depends on the heat and how heavily you sweat. Your full plan by email gives your per-hour fluid and sodium targets for race day.
Do I need to fuel during a half marathon?+
Under about 75 minutes you don't need to eat during the race. Slower than that, 30 g of carbs per hour helps you finish strong.
Who made this fueling calculator?+
Jonathan Fors, a UESCA-certified ultrarunning coach. It uses the same fueling rules he sets up with his coached athletes.
Is this medical advice?+
No. It's a coach's starting point. If you have a medical condition affecting fluid or salt, check with your doctor.
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