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Ultra Running Fueling Calculator

Here is your carb, sodium, fluid, and caffeine plan per hour for any ultra, built on the science and not guesswork. The numbers climb as the race gets longer, scale to your body weight, and bump up for the heat. It is the same math the Summit Line app uses to build a race-day fueling plan.

⏵ YOUR RACE

DIAL IT IN

Body weight
lb
Race duration
hr
Forecast high (optional)
°F

Leave blank for a cool-weather baseline. Heat raises fluid and sodium needs above 70°F.

⏵ YOUR PRESCRIPTION

THE LOAD ON THE GUT

Carbs / hour
90g
80–100 g/h range
Total carbs
2160g
Across 24.0 hr
Fluid / hour
0.68L
~23 oz/h · heat-adjusted
Total fluid
16.3L
~551 oz total
Sodium / liter
500mg
8,145 mg total
Caffeine
200mg
Pre-race + top-ups

Strategy · Gut-trained 80–100 g/h

Long ultra: 80–100 g/h only works with a trained gut, ramp in training.

Over 8 hours: ramp toward this carb rate in training. A gut that has never seen 90 g/h will revolt on race day.

⏵ ON THE CLOCK

HOUR BY HOUR

HrCarbsFluidSodium
  1. 0190 g23 oz340 mg
  2. 0290 g23 oz340 mg
  3. 0390 g23 oz340 mg
  4. 0490 g23 oz340 mg
  5. 0590 g23 oz340 mg
  6. 0690 g23 oz340 mg
  7. 0790 g23 oz340 mg
  8. 0890 g23 oz340 mg
  9. 0990 g23 oz340 mg
  10. 1090 g23 oz340 mg
  11. 1190 g23 oz340 mg
  12. 1290 g23 oz340 mg

... same load each hour through hour 24

A steady drip beats big dumps. Spread carbs across the hour (a gel every 20 to 30 minutes), sip fluid continuously, and pair sodium with what you drink.

⏵ This is the generic prescription

Summit Line dials it to YOUR actual course profile, aid stations, heat forecast, and projected splits, then tracks how your gut handles it in training so race day is rehearsed, not a guess.

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How ultra running fueling works

Fueling an ultra is not one number you memorize and call it done. It moves with how long you will be out there, how big you are, and how hot it is. Here is the thinking behind this calculator so the numbers above actually mean something to you.

Carbs ramp with duration: 60 to 90 to 120 g/h

The biggest lever you have in ultra fueling is carbs per hour, and it goes up the longer the race is. Under about 4 hours you can get away with 60 to 70 grams an hour from a single glucose source, because you are mostly running on the glycogen you already stored. Between 4 and 8 hours that target moves up to roughly 70 to 90 g/h. Past 8 hours, well-trained runners push toward 90 and even 120 g/h, simply because a long day burns way more fuel than your glycogen can hold.

You might wonder how you can take in more than the old 60 g/h ceiling people used to talk about. It comes down to transporters. Glucose moves into your blood through one set of gut transporters that max out around 60 g/h. Add fructose, which rides a separate transporter, and the combined ceiling goes up. That is the whole reason long-race fueling leans on a glucose plus fructose blend, usually around a 1 to 0.8 ratio, instead of just glucose.

Past 8 hours, your gut is the bottleneck

The number on the calculator is a target, not a promise your stomach will play along. Taking in 90 grams of carbs an hour while you run is a skill, and an untrained gut will fight you with nausea, bloating, or worse. The fix is gut training. Practice your race-day carb rate on your long runs in the weeks before the race so your gut builds up its sugar transporters and learns to empty while you are working hard.

Treat your longest, most race-like runs as fueling rehearsals. Use the exact products you will race with, hit the same hourly carb number, and pay attention to what your stomach can handle. By race week, 90 g/h should feel routine. Not an experiment.

Sodium tracks your fluid and the heat

Sodium is not a set hourly dose, it is a concentration in the fluid you are already drinking, usually 300 to 700 milligrams per liter. On a cool day with a moderate sweat rate, something around the middle, 500 mg/L, makes sense. As the forecast heats up and you sweat more, both your fluid and your sodium concentration climb toward 700 mg/L, because you are losing more salt in your sweat.

Heavy, salty sweaters, the ones who finish caked in white crust, need the high end and sometimes more than that. And honestly, cramping, a sloshy stomach, and that wrung-out feeling late in a race are usually sodium and fluid problems, not fitness problems.

Hydration scales with body weight and temperature

How much you need to drink goes with how big you are and how hot it is. A bigger runner has more surface area and a higher sweat rate, so this calculator scales the baseline fluid band by your body weight, then adds a heat multiplier of up to 50 percent for forecasts well above 70°F. The output is a starting point, not the final word.

The best number you can get is your own measured sweat rate. Weigh yourself before and after a long run in conditions like your race, and every pound you lost is roughly 16 ounces of fluid you did not put back. Drink to thirst, trust that personal number, and use the calculator to check your plan, not to argue with your own body.

Ultra fueling FAQ

How many carbs per hour for a 100 miler?

For a race over 8 hours, most ultrarunners aim for 80 to 100 grams of carbohydrate per hour, and a good middle to shoot for is about 90 g/h. But you only get there two ways: using more than one kind of carb (a glucose to fructose blend, roughly 1 to 0.8) and, the big one, a gut you have actually trained to take that much. If you have not practiced eating that much while running, start nearer 60 to 70 g/h and build it up over the weeks before race day. Do not find your ceiling on the start line.

How much sodium do I need in an ultra?

Sodium goes with how much you drink, not some fixed hourly number. A range that works is 300 to 700 milligrams of sodium per liter of fluid, lean high in the heat and sit in the middle when it is cool. If you are drinking roughly half a liter to a liter an hour, that puts most runners between 300 and 700 mg of sodium per hour. Heavy, salty sweaters need more. Cool days and light sweaters need less. Simple as that.

How do I train my gut for more carbs?

Your gut is trainable, same as your legs. Practice your race-day carb rate on your long runs: take in 70, then 80, then 90 grams an hour on back-to-back long efforts and your gut starts building up the transporters that move sugar into your blood. Use the exact products you will race with, not whatever is in the cabinet. You want 90 g/h to feel boring and routine well before race week, never something you are trying for the first time on race morning.

How much should I drink per hour in an ultra?

How much you drink goes with your size and the heat, there is no one number that fits everyone. A decent baseline is roughly 0.4 to 0.8 liters an hour (about 14 to 27 ounces) for a mid-size runner in cool weather, going up as much as 50 percent in heat above 70°F. Honestly though, the real answer is drink to thirst and to your own sweat rate. Weigh yourself before and after a long run in heat like your race, that gives you your real number. Use this calculator as the starting point and go from there.

Do I need caffeine in an ultra, and how much?

Caffeine is optional, but it helps, especially late in a long race or overnight when you are dragging. A safe range is about 1 to 3 milligrams per kilogram of body weight spread across the whole race, usually a pre-race dose plus small top-ups every 3 to 4 hours. For anything under 2 hours your pre-race coffee already has you covered. And do not save it all for one big hit. Steady little doses keep you from the jitters and the crash that comes after.

Why does carb intake go up as the race gets longer?

Short, fast efforts mostly run on the glycogen you already have stored, so a single glucose source clears fast enough and 60 g/h is plenty. But as the race stretches past 4 and then 8 hours, you burn more total fuel than your glycogen can hold, so you have to bring more in from outside. The longer the day, the more it leans on multiple carb types and a trained gut to keep the supply ahead of what your body is asking for. Long days are eating contests, basically.

Take the plan from generic to race-specific

Summit Line builds your fueling plan around your real course profile and aid stations, projects your splits, and tracks how your gut handles the load across your whole training block. Pace that comes from your own runs, an AI race brief, and a fueling plan you have actually practiced before you toe the line.

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This calculator gives general guidance based on standard sports nutrition consensus for endurance and ultra-endurance events. It is not medical advice. Everybody is different, so always test your fueling in training and talk to a qualified professional for any medical or dietary concerns.