Free Ultra Fueling Calculator: Your Hour-by-Hour Plan¶
Fueling is the part of an ultra that gets planned last and decides the most. The training block gets sixteen weeks of attention. The eating gets a scroll through a forum thread the night before, and a number somebody half remembers from a podcast.
Then you are nine hours in, you have lost count of what you have eaten, and the arithmetic you need is the arithmetic you are least able to do.
So I built the free ultra fueling calculator to do that math before you leave the house. Three inputs, no account, no signup. It hands back what to take in each hour and what your running total should look like at every aid station.
This post is about the tool: what it asks, what it gives you, and where it stops. If you want the fueling science underneath it, that lives in how many carbs per hour an ultramarathon needs.
What the free ultra fueling calculator asks for¶
Three things, and nothing else:
- Body weight in pounds. Heavier runners burn more and sweat more, so weight nudges both the carb number and the sodium number.
- Time on feet, in hours. Not distance. Not a goal finish time. How long you expect to be out there, which is the number that decides how much fuel the day needs.
- Conditions: cool, moderate, or hot. This scales fluid and sodium. Heat is a hydration problem before it is a calorie problem.
That is the whole form. No email required, no login, nothing held back behind an account. You type three numbers and press the button.
Asking for hours instead of miles is deliberate. A 50K on runnable dirt and a 50K with 9,000 feet of climbing are the same distance and different days, and your stomach only knows about the hours.
The numbers it hands back¶
Three per-hour targets, up top:
- Carbs per hour, in grams
- Fluid per hour, in ounces
- Sodium per hour, in milligrams
For a 160 lb runner expecting 10 hours in moderate conditions, that comes out at 78 g of carbs, 22 oz of fluid, and 508 mg of sodium per hour.
Under that, the totals for the day. Same runner: 780 g of carbs, about 3,120 calories from those carbs, 220 oz of fluid, 5,080 mg of sodium. Then a line that reads "roughly 35 gels' worth of carbs."
That gel line is not a shopping list. Nobody is eating 35 gels, and a day built out of nothing but gels is a day that ends badly. It is there to give the number a shape you can feel. 780 grams sounds abstract. Thirty-five gels sounds like a lot of chewing, which is the honest reaction, and it is the reason real food and drink mix carry so much of an ultra. Real food fueling goes into what that looks like at the table.
The aid-station plan is the part worth printing¶
Below the totals, the calculator splits your day into one-hour segments. Each row carries that hour's carbs, fluid, and sodium, and then the running cumulative total beside it. There is a chart of cumulative carbs under the table.
The cumulative column is the one that earns its place. Per-hour targets are easy to agree with and easy to drift from. Being 20 g light in an hour feels like nothing. By hour six it is 120 g, which is an hour and a half of fuel you no longer have, and the first place you find out is your legs on the next climb.
For the 160 lb, 10-hour example, the table reads 78 g at hour one, 156 g at hour two, 234 g at hour three, and 468 g by the end of hour six. Those are the numbers to check yourself against when you roll into an aid station and someone asks how you are doing. It is a lot easier to answer that question against a printed line than against a feeling.
If the day does not divide into whole hours, the last segment is a partial one and the math scales to it. A 10.5 hour plan ends with a half-hour row.
When the results come up you can share the result, download it as an image, or have the plan emailed to you so it is on your phone at the start line. All of that is free and none of it needs an account.
Run your numbers in the free ultra fueling calculator
How many carbs per hour for an ultramarathon¶
The calculator starts every runner at 60 g per hour and moves up from there with time on feet and body mass. It never goes below 60 or above 120, because those are the edges of what the endurance-nutrition research supports.
Three runs through it, so you can see how the number moves:
| Runner | Time on feet | Conditions | Carbs per hour |
|---|---|---|---|
| 155 lb | 5 hours | moderate | 66 g |
| 160 lb | 10 hours | moderate | 78 g |
| 150 lb | 30 hours | moderate | 89 g |
Longer days push the number up, which reads backwards until you think about it. A 30-hour finish is slower per mile, so you are burning less per minute, but you are burning for a very long time and there is no way to bank enough of it in advance. The deficit is what gets you, and it accumulates.
The ceiling is a gut ceiling, not a want-to ceiling. Glucose moves across the gut wall on one transporter, and fructose uses a different one, so a mixed glucose and fructose product absorbs faster than glucose alone. That is what lifts the practical range from around 60 g per hour to around 90, and up toward 120 for a runner who has trained for it. The calculator will print a number in that band. Whether your stomach honors it is decided in training, not on race day, which is what gut training is for.
One thing the calculator deliberately does not do: heat does not change your carb number. Your muscles need the same fuel at 85 degrees as at 45. What heat changes is fluid and sodium.
Fluid, sodium, and the part to be careful about¶
Take that same 160 lb runner and switch the conditions from moderate to hot. Carbs stay at 78 g per hour. Fluid goes from 22 oz to 28 oz per hour. Sodium goes from 508 mg to 635 mg.
The calculator stays inside published ranges here too: roughly 16 to 27 oz of fluid per hour and 300 to 800 mg of sodium per hour, scaled for heat and for body mass.
Now the careful part, and it is worth reading slowly. Drinking to a number, without sodium alongside it, is how runners get into trouble in long hot races. Take in a lot more water than you are sweating out and the sodium in your blood gets diluted. That is exercise-associated hyponatremia, it is uncommon, and it is serious when it happens. It is also the reason this tool hands you a sodium number next to the fluid number instead of a fluid number on its own.
What that means in practice is unglamorous. Small, frequent sips beat big gulps. If your stomach is sloshing, you are ahead of what you can absorb, and the answer is to back off the fluid rather than push more through. Take your sodium with your fluid, not separately from it. If you want the full picture on sweat rate testing and electrolytes, the hydration and sodium guide covers it properly.
This is not medical advice, and the tool says so on the page. The numbers are a research-anchored starting point, not a prescription. Your gut and your sweat are yours. If you have any health concerns, talk to a professional before you build a race day around a number a calculator printed.
What the calculator cannot see¶
It is arithmetic on three inputs. That is its strength, because it works for anyone in about ten seconds with no account, and it is also the honest limit.
It does not know your sweat rate. It does not know that your stomach turns on you around hour fourteen, or that you can drink 200 calories of mix an hour without thinking but cannot look at a gel after noon. It does not know that miles 40 through 50 of your race are a 4,000-foot climb where eating is hard and drinking is harder. And it has no idea what your training has been telling you.
If you would rather build the whole thing by hand, the calories-per-hour fueling framework walks the same math through on paper.
So use the plan as the baseline and let your long runs correct it. Take the per-hour numbers into a five-hour training day, run them for real, and write down what your stomach did. Two or three of those and you have your own numbers, which are worth more than any calculator's. That is also where an aid station strategy starts to take shape: what you grab, in what order, and how long you stand there.
The calculator works from what you type in. NavRun works from what you have run. Connect Strava and it reads your training, builds your plan around your real long runs and volume, and updates as your fitness changes. Getting a training plan costs nothing.
Questions¶
Is this medical advice?¶
No. It is a research-anchored starting point, not medical or dietetic advice. Your gut and your sweat rate are personal, so test these numbers on long training runs and adjust. If you have health concerns, talk to a professional.
Do I need an account to use the free ultra fueling calculator?¶
No. There is no signup, no login, and no email required to see your plan. You can have the plan emailed to you if you want it saved somewhere, but that is your choice and not a gate.
Where do the numbers come from?¶
Accepted endurance-nutrition ranges: roughly 60 to 90 g of carbs per hour, up to around 120 with a trained gut, about 16 to 27 oz of fluid per hour, and 300 to 800 mg of sodium per hour, scaled up for heat and for heavier runners. The tool holds hard guard rails so the output can never leave those bands.
Why does it ask for hours instead of miles?¶
Because your stomach keeps time, not distance. Thirty miles of runnable dirt and thirty miles of steep technical trail are the same number on paper and hours apart in reality, and the fueling follows the hours.
Why did my carb number not change when I set conditions to hot?¶
Heat does not change what your muscles need. It changes what you lose. So the conditions setting scales fluid and sodium, and leaves carbs where they are.
What if I cannot hit the carb number?¶
Then you have found the thing to work on, and you have found it in training instead of at mile 60. Gut tolerance moves with practice. Start below the target on your long runs and build toward it over a block rather than trying to jump to it on race day.
Does it work for a 50K, or only for 100 milers?¶
Any duration. Enter four hours or thirty-six. Shorter days sit near the bottom of the carb range, longer days trend up, and the aid-station table is built to match whatever you entered.
Take it into your next long run¶
The plan is worth what your training says it is worth. Print the aid-station table, run it on a long day, and change the numbers that were wrong for you. That is the whole loop.
Open the free ultra fueling calculator. Free, no signup.
Then connect Strava and let NavRun build the training around it.