Run Nutrition Calculator
Estimate hydration and calorie needs for your trail run to avoid bonking. No GPX needed — just enter your route numbers.
Estimated duration: 15h 0m
How it works
Hydration is estimated at ~500 ml/hour base rate, adjusted +15% per 5°C above 20°C. Calorie burn uses ~8 kcal/kg/hour as a rough trail running baseline. These are estimates — adjust based on your experience.
Why fueling matters on the trail
Bonking — running out of glycogen — is the most common reason trail runners slow down or DNF in ultras. Your body can only store about 2,000 kcal of glycogen, but a 50K mountain run can burn 4,000–6,000 kcal. The gap has to be covered by eating and drinking during the effort.
Dehydration compounds the problem: even a 2% fluid loss reduces endurance performance measurably. In hot conditions or at altitude, sweat rates climb and the margin shrinks. Planning your nutrition and hydration before race day — and knowing how many gels, bars, or liters you need per aid-station interval — prevents the guesswork that leads to stomach trouble or energy crashes.
Privacy
All calculations happen in your browser. Nothing is sent to a server.
Frequently asked questions
How much water do I need on a long trail run?
The page assumes about 500 ml an hour. Above 20°C it adds 15% for each 5°C or part of 5°C, so 21–25°C gives 575 ml an hour. It multiplies that by the duration and shows the total, the amount per 30 minutes and the number of 500 ml flasks.
How does the page estimate calories?
It uses a rough rate of energy used, 8 kcal per kg of body weight per hour, so a 70 kg runner gets 560 kcal an hour, and shows the total for the run as gels of about 100 kcal each, rounded up. That is energy used, not an amount to eat each hour: most runners take in less than they use, so start from what your stomach has handled on long runs.
How long will my trail run take?
With Auto-estimate on, the page allows 1 hour for every 5 km plus 1 hour for every 600 m of climb (Naismith's rule). 50 km with 3,000 m of climb gives 15 hours. Untick it to enter your own time; the water and calorie totals follow it.
Is anything sent anywhere?
No. All the sums run in your browser. Distance, climb and duration are written into the page address, so the Shareable link opens the same plan. Body weight and temperature are not in that link.