How Much Water to Carry on Each Ultra Leg¶
Water runs out between stations.
Not at the station. There is a table there, and someone refilling bottles. It runs out on the long stretch after it: the exposed climb in the afternoon, the leg that takes an hour longer than the miles suggest. That is where a runner finds out whether two soft flasks were enough, and out there it is too late to go back for a third.
Most of us guess. We look at the distance between aid stations and pick the bottles we always use. This post swaps the guess for arithmetic you can do yourself, and shows the part of NavRun that does it with you.
Why ultras make this harder¶
An aid station is a stop on the course with water and food. In a road marathon they come every mile or two, so what you carry barely matters. On a trail ultra they can be 5 miles apart or 15, and the distance is the least useful number on the page.
Time is what empties a bottle. A leg is the stretch from one point to the next: the start to the first station, station to station, the last station to the finish. Eight flat miles and eight miles with 3,000 feet of climbing are not the same leg. The second can take twice as long, and you sweat for every minute of it.
A 50K in summer heat or a 100 miler through the night, the first time or the tenth, the question is the same: how many hours am I out between refills, and how much do I lose in that time?
What NavRun does with it¶
Body numbers¶
Three numbers about you: your sweat rate in ounces an hour, your sodium loss in milligrams an hour, and the carbohydrate you know you can take in grams an hour. You type them in once and they follow you to every race.
Each one is marked as your own or as a general starting number. Where you have not measured, NavRun uses a general starting number (22 oz an hour for sweat, 500 mg for sodium, 60 g for carbohydrate) and says, in the same line: "That is a general starting number for a lot of runners and it is not about you."
If you have never measured your sweat rate, there is a calculator beside the boxes. Weight before, weight after, how long you were out, and what you drank. It works out ounces an hour and saves nothing until you choose to.
Sodium and carbohydrate stay as hourly numbers: a guide for each hour you are out, whatever leg you are on. Water is the one NavRun works out leg by leg.
Web (Preferences, Body Numbers) and iPhone (Settings, Body numbers). Free.
What I can carry¶
List what you carry water in: a name, the size in ounces, how many, and where it goes. Two 17 oz flasks in the vest, a 50 oz bladder in the back. NavRun adds them up, and the list stays on your account for every race.
Web and iPhone. Free.
Fuel and water on your race strategy page¶
Every race you add has a strategy page, and under the cutoff times (when each station closes) is a Fuel and water section. It shows one row per leg with the distance, the climb, the time at your goal pace, and the water that leg takes at your own rate.
When NavRun knows both your goal time and what you carry, each leg reads "fits" or "more than you carry." When it does not know one of them, it leaves that blank rather than guessing. With no goal time set, the section says nobody has said a goal for this race yet, with a link to set one.
Web and iPhone. Free, for every runner.
Where the water runs out¶
This part is the Crew, NavRun's paid plan. On a leg that asks for more than you carry, the Crew adds a line saying where you run dry, for example: "Leaving Mount Falcon with 20 oz, that runs out near mile 15.4."
That is a mile marker you can plan around: one more flask for that leg only, or a spare bottle from your crew, the friends or family who meet you at stations.
Web and iPhone. The Crew. Adding your first race starts 30 days of the Crew, no card needed.
What it does not do yet¶
The water on each leg is your hourly rate times the hours at your goal pace. It does not raise the number for a hot afternoon or drop it for a cold night, so on a race where the weather swings, adjust it yourself. The point where water runs out is a straight share of the leg's distance, and it does not yet account for where on the leg the climb falls.
Sodium and carbohydrate show as your hourly numbers, not per leg. And races with no set distance, like backyard ultras and 24 hour races, do not fit the leg table yet.
None of this is medical advice. Test every number on long training runs.
How to measure your own sweat rate¶
You can do this with a bathroom scale and a notebook. The method below follows the field protocol from the Gatorade Sports Science Institute (Baker, 2017).
- Use the toilet, then weigh yourself with as little clothing as possible.
- Run for about an hour at the effort you plan to race at. Note exactly how much you drink.
- Towel off, and weigh yourself again in the same clothing.
- Pounds lost times 16 gives ounces lost. Add the ounces you drank. Divide by the hours you ran.
Say you lost 1.5 pounds (24 oz) and drank 12 oz over an hour. That is 36 oz an hour.
One warm day is not a whole picture. Weigh in on a few runs in different weather and see where your own number settles.
A sweat rate is what you lose, not a drinking target¶
Your sweat rate tells you what a leg takes out of you. It is not an amount you have to put back. A 2017 review of exercise-associated hyponatremia, a dangerous drop in blood sodium, names drinking beyond thirst as its most common cause, and advises drinking to thirst (Hew-Butler et al., Frontiers in Medicine). Gaining weight during a race is a sign you are taking in more than you lose.
So use the leg's number to decide what is worth carrying out of a station, and let thirst decide what you drink from it. Having water on a long leg and choosing not to drink it is a very different day from wanting it and having none.
If you never sign up¶
Take the course map and write down every leg. For each one, estimate the hours it takes: distance at your easy trail pace, plus extra for the climbing, plus more if it falls in the heat of the day. Multiply by your sweat rate. Compare it with what your bottles hold. Circle every leg where the number is bigger.
Those circled legs are the ones to plan for.
For an hourly starting point on carbohydrate, fluid and sodium, the free ultra fueling calculator takes your weight, time on feet and conditions, with no login. For stations themselves, read the aid station guide. For sodium in depth, read ultra hydration and electrolytes, and for people meeting you out there, the crew and pacer guide.
Common Questions¶
How much water should I carry between aid stations in an ultra?¶
Multiply your sweat rate in ounces an hour by the hours the leg takes you, not its miles. Carry enough to have that much with you, and drink to thirst.
How do I measure my sweat rate for running?¶
Weigh yourself before and after a run of about an hour, in the same minimal clothing. Pounds lost times 16, plus ounces drunk, divided by hours, is your sweat rate in ounces an hour.
What is a normal sweat rate for runners?¶
It varies a lot between runners and with the weather. NavRun's general starting number is 22 oz an hour, and it is labelled as not being about you until you measure your own.
Should I drink my whole sweat rate back every hour?¶
No. The hyponatremia research advises drinking to thirst, and gaining weight during a race means you are drinking more than you lose. Your sweat rate is for deciding what to carry.
Is the Fuel and water section free?¶
Yes. Your body numbers, your carry list and the water for every leg, with "fits" or "more than you carry," are free. The line saying where your water runs out on a long leg is part of the Crew.
Does it work for backyard ultras?¶
Not yet. A backyard or a 24 hour race has no set distance, so there are no legs to work out. Your body numbers and carry list still work for them.
The long leg¶
The long, exposed stretch will be there on race day, in whatever heat the afternoon brings. What you can have in hand is a number for it, worked out at home, so you leave the station with the water that leg takes.
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