How many calories do I burn in a marathon?

A runner weighing 70 kg burns approximately 3,100–3,500 kcal during a marathon at a moderate race effort, equivalent to roughly 1.05 kcal per kilogram per kilometre. Heavier runners burn more; lighter runners burn less. Calorie burn increases at higher intensities as a greater proportion of energy comes from carbohydrate oxidation. This calculator uses your body weight and selected intensity to give a personalised estimate.

How many gels do I need for a marathon?

Most marathon runners need 5–8 gels for a standard race at 4–5 hours pace, depending on body weight and how aggressively they fuel. Current sports science recommends consuming 60–90 g of carbohydrate per hour from race nutrition. A standard gel provides approximately 22–27 g of carbs, so you would need one gel every 30–40 minutes from about 45 minutes into the race.

How much water should I drink during a race?

Water requirements during racing depend on sweat rate, temperature and humidity. As a general guideline, aim to drink 400–800 ml per hour depending on conditions. At a moderate effort in typical UK conditions, 600–700 ml per hour is a reasonable target. Avoid drinking purely to a fixed schedule — drink to thirst and adjust for conditions. Electrolyte replacement is especially important in events exceeding 60–90 minutes.

Do I need to fuel for a 5K or 10K race?

For most runners, fuelling during a 5K or 10K race is not necessary as glycogen stores are sufficient for efforts under 60–75 minutes. A light pre-race meal 2–3 hours before is more important. Focus hydration on the hour before the race with 400–600 ml of water. This calculator adjusts fuel recommendations accordingly for shorter distances.

RunCalcs

Race Fuelling Calculator — Calories, Gels, Water & Electrolytes

Calculate your race-day nutrition needs including total calories, carbohydrate grams, gel count, water volume and electrolyte tablets based on distance, target time, body weight and intensity.

Race Distance

Select your race distance

Body Weight

Your body weight in kilograms

Target Time (hours)

Target race finish time — hours

Target Time (minutes)

Target race finish time — minutes

Race Intensity

Your planned effort level for the race

How We Calculate This

How the calculation works

Total calories = Body weight (kg) × Distance (km) × Calorie rate (kcal/kg/km)

Calorie burn rate varies by intensity: 0.95 (easy), 1.05 (moderate), 1.15 (hard) kcal/kg/km.

Carbohydrate and fat split

Carbohydrate fraction of total calories by intensity:

Easy: 50% carbs / 50% fat
Moderate: 65% carbs / 35% fat
Hard: 80% carbs / 20% fat

Carbs (g) = (Total calories × Carb fraction) ÷ 4

Gel count

Gel count is based on exogenous carbohydrates needed (capped at 90 g/hr), divided by carbs per gel (default 25 g).

Water and electrolytes

Water (L) = Sweat rate (ml/hr) × Race hours

Electrolyte tabs: 1 tab per 500 ml of water.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always consult a coach or medical professional for personalised training advice.