Race Fueling Calculator

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Walk-through

How to Use This Calculator

5 steps

1

Pick your distance and finish time

Choose your race distance and expected finish time (hours and minutes) — everything else scales off how long you'll be out there.

2

Enter your bodyweight and carb strategy

Add your bodyweight and pick a carb target from 60 to 120 g/hr. Higher targets need practiced gut tolerance from training runs.

3

Run a sweat test or let it estimate

Weigh yourself before and after a training run for a personal sweat rate, or skip it and the calculator estimates fluid from your weight and temperature.

4

Set your race-day temperature

Enter the expected race temperature — heat raises both your fluid and sodium targets, so this shifts your plan noticeably.

5

Pick your gel and read the plan

Choose your gel brand (or a custom carb count) to see your exact gel count, plus a full timeline of fuel and fluid stops by mile.

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Reference

Formula & Methodology

3 formulas

Total carbs & gel count

totalCarbs = gramsPerHour × hours; gels = ceil(totalCarbs ÷ gelCarbsPerServing)

Your target grams of carbohydrate per hour, multiplied by your finish time, gives total carbs needed. Divide by your gel's carb content and round up for the exact gel count.

Fluid target

fluidRate = sweatTestRate (if measured), else estimate(bodyweight, temperature); totalFluid = fluidRate × hours

A sweat test — pre-run weight minus post-run weight, about 1 kg per liter — overrides a bodyweight-and-temperature estimate whenever you provide one. Either way, the hourly rate is multiplied by your finish time.

Sodium target

sodiumRate = 300–700 mg/hr, scaled up with heat; totalSodium = sodiumRate × hours

Sodium needs scale with sweat losses, so hotter race conditions push the rate toward the higher end of the 300–700 mg/hr range used in endurance-nutrition guidance.

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YMYL

Trust, Methodology & Sources

Written by Morgan Reyes · Updated 2026-07-12 · 3 sources

Editorial accountability

Author: Morgan Reyes - Sports & endurance content author, Calculover

Owner: Morgan Reyes - Sports & fitness content owner, Calculover

Last reviewed: 2026-07-12

Last verified: 2026-07-12

Methodology

Carb targets (60-90 g/hr, up to 120 g/hr if gut-trained) follow the joint ACSM/ISSN endurance-nutrition position stand and Jeukendrup's carbohydrate-oxidation research on multiple transportable carbohydrates. Fluid defaults to a bodyweight- and temperature-scaled estimate unless a sweat test (pre-run weight minus post-run weight over duration, about 1 kg per liter) is provided. Sodium targets of 300-700 mg/hr scale up with race temperature.

Assumption: Your finish time, bodyweight, and race temperature are entered accurately; the plan scales directly from these inputs.

Assumption: Without a sweat test, fluid needs are estimated from bodyweight and temperature and may not match your true sweat rate.

Assumption: Carb, fluid, and sodium ranges reflect population-level sports-nutrition guidance, not a personalized clinical assessment.

Limitations & guidance

Individual carbohydrate and fluid tolerance varies — some runners need to stay below 60 g/hr even with training.

This tool does not account for medical conditions such as diabetes, kidney disease, or a history of hyponatremia.

Extreme heat, altitude, or unusually long ultra distances may need fueling adjustments beyond this general model.

Professional guidance: This calculator provides general endurance-nutrition guidance and is not medical or dietary advice. Individual tolerance varies — test any new fueling plan in training before race day, and consult a doctor or sports dietitian if you have a medical condition.

Primary sources

Nutrition and Athletic Performance (Joint Position Stand) - American College of Sports Medicine, Academy of Nutrition and Dietetics, Dietitians of Canada

Carbohydrate feeding during exercise (Jeukendrup AE) - European Journal of Sport Science

Exercise and Fluid Replacement (Position Stand) - American College of Sports Medicine

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Glossary

Key Terms Explained

8 terms

Carbohydrate oxidation The rate at which your body burns ingested carbohydrate for fuel during exercise — the basis for the 60-90 g/hr guidance.

Gut training Practicing high carb intake during training runs so your digestive system adapts to absorb 90 or more grams per hour without distress.

Sweat rate How much fluid you lose per hour of exercise, measured by weighing yourself before and after a run of known duration.

Sodium (electrolytes) A mineral lost in sweat that helps the body retain fluid and prevent cramping; needs rise with heat and sweat rate.

Glycogen Stored carbohydrate in muscles and the liver that fuels exercise; race-day fueling helps top it off as it depletes.

Hyponatremia A dangerous low blood-sodium condition caused by drinking too much fluid without enough sodium — the real risk of overdrinking.

Bonking Running out of stored glycogen mid-race, causing a sudden and severe drop in energy, focus, and pace.

Gel / chew / drink mix Portable carbohydrate sources used during a race — energy gels, chewable candies, and carbohydrate-electrolyte drink powders.

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Scenarios

Real-World Examples

3 worked examples

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Dana, 4:00 marathon

75 g/hr, generic gels

Distance Marathon Finish time 4:00 Carb strategy 75 g/hr

At 75 g/hr for 4 hours, Dana needs 300 g of carbs — 12 generic 25g gels — plus about 2.4 L of fluid using the built-in estimate since she skipped a sweat test.

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Sam, sweat-test runner

1.2 kg lost in a 1-hour test run

Weight before 160 lb Weight after 157.4 lb Duration 60 min

Losing 1.2 kg over a 1-hour test run means Sam's real sweat rate is about 1.2 L/hr — a personalized number that replaces the calculator's generic bodyweight-based estimate.

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Jordan, 100K ultra

90 g/hr over 10 hours

Distance 100K Finish time 10:00 Carb strategy 90 g/hr

At 90 g/hr for 10 hours, Jordan needs 900 g of carbs — well into gut-training territory — so the calculator flags practicing that intake on long runs before race day.

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Deep Dive

How many gels do I need? Start with grams of carbs per hour instead

4 chapters · 3 min read

"How many gels do I need?" is the wrong first question. Start with grams of carbohydrate per hour and your personal sweat rate, and the gel count — along with your fluid and sodium targets — falls out automatically.

Why carbs-per-hour beats counting gels

Every gel brand packs a different amount of carbohydrate, so "how many gels" has no fixed answer until you know your target grams per hour. Once you fix that number — 60 to 90 g/hr for most events, up to 120 g/hr if you've trained for it — the gel count is just division. This calculator starts there instead of assuming a brand.

Your sweat rate matters more than a generic fluid number

Fluid needs vary enormously between runners and conditions — a generic 400-800 mL/hr range can be off by double for a heavy sweater in the heat. A simple sweat test (weigh before and after a training run) gives you a personal number that this calculator uses in place of the estimate whenever you provide it.

Practice your carb target before race day

The gut can be trained to absorb more carbohydrate, but only with practice. If your target is 90 g/hr or higher, rehearse it on long training runs first — race day is the wrong place to discover you can't tolerate that much sugar at once.

Heat changes fluid and sodium, not carbs

Hotter race-day temperatures push sweat and sodium losses up, so this calculator scales your fluid and sodium targets with the temperature you enter. Your carbohydrate target, by contrast, stays about the same regardless of weather — it's driven by how hard and how long you're working, not how hot it is.

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Questions

Frequently Asked Questions

8 questions

How many gels do I need for a marathon?+

Divide your total race-time carbs (grams per hour times hours) by the carbs in your gel. At 75 g/hr for a 4-hour marathon, that's 300 g total — about 12 gels at 25 g each, often supplemented with real food on longer races.

How many carbs per hour should I eat while running?+

60-90 g/hr is the well-supported range for events over 2.5 hours, and up to 120 g/hr if you've specifically trained your gut to absorb that much using multiple transportable carbohydrate sources like glucose and fructose together.

How much should I drink during a race?+

400-800 mL/hr is the general range, but your own sweat rate — measured by weighing yourself before and after a training run of known duration — is far more accurate than any generic number.

How much sodium do I need during a race?+

300-700 mg/hr covers most runners, with heavier or saltier sweaters and hot race-day conditions needing the higher end of that range to help with cramping and fluid retention.

What is a sweat test?+

Weigh yourself nude immediately before and after a run of known duration, without a bathroom break in between. Each kilogram lost is roughly a liter of fluid you need to replace over that same period.

Can I train my gut to handle more carbs?+

Yes. Practicing high carb intake of 90 or more grams per hour during long training runs over several weeks improves gut tolerance and meaningfully reduces GI distress on race day.

Does hot weather change my fueling plan?+

Yes — heat raises sweat and sodium losses, so both your fluid and sodium targets should increase on hot race days, while your carbohydrate needs stay about the same.

Is real food okay to use instead of gels?+

Yes, especially for ultras — bananas, dates, chews, boiled potatoes, and even sandwiches all work fine as long as you hit your total carbohydrate target for the hour.

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Your race

Your fuel plan

60 vs. 90 g/hr

Cool vs. hot conditions

Sweat rate test

Your sweat rate

Cumulative carbs & fluid

Aid-station checkpoints

How to Use This Calculator

Pick your distance and finish time

Enter your bodyweight and carb strategy

Run a sweat test or let it estimate

Set your race-day temperature

Pick your gel and read the plan

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Dana, 4:00 marathon

Sam, sweat-test runner

Jordan, 100K ultra

How many gels do I need? Start with grams of carbs per hour instead

Why carbs-per-hour beats counting gels

Your sweat rate matters more than a generic fluid number

Practice your carb target before race day

Heat changes fluid and sodium, not carbs

Frequently Asked Questions

Get a branded PDF of your results

Race Fueling Calculator

Your race

Your fuel plan

60 vs. 90 g/hr

Cool vs. hot conditions

Sweat rate test

Your sweat rate

Cumulative carbs & fluid

Aid-station checkpoints

How to Use This Calculator

Pick your distance and finish time

Enter your bodyweight and carb strategy

Run a sweat test or let it estimate

Set your race-day temperature

Pick your gel and read the plan

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

How many gels do I need? Start with grams of carbs per hour instead

Why carbs-per-hour beats counting gels

Your sweat rate matters more than a generic fluid number

Practice your carb target before race day

Heat changes fluid and sodium, not carbs

Frequently Asked Questions

Get a branded PDF of your results

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