When should I start carb loading?

Most sports nutritionists recommend starting 2-3 days before a marathon or ultra. For a half marathon, 1-2 days of increased carbs is usually sufficient.

How many grams of carbs per kg for carb loading?

For marathon and ultra distances, aim for 8-12 g/kg/day. For a half marathon, 6-8 g/kg/day is typical. This calculator uses middle-range values.

What are good carb loading foods?

Pasta, rice, bread, potatoes, bananas, porridge oats, energy bars and fruit juice are all good options. Stick to foods your stomach tolerates well.

Can carb loading cause weight gain?

You may gain 1-2 kg temporarily because your body stores water alongside glycogen (about 3 g water per 1 g glycogen). This is normal and beneficial for race day.

RunCalcs

Carb Loading Calculator — Race Week Fuelling

Calculate your daily carbohydrate targets for race-week carb loading.

Body Weight

Race Distance

Loading Days

How We Calculate This

Carb Loading Science

Carb loading maximises muscle glycogen stores before a race. The body can store approximately 400-500 g of glycogen in muscles and 80-100 g in the liver.

Daily Carb Target = Body Weight (kg) x Carb Rate (g/kg)

Marathon/ultra runners should aim for 10 g/kg/day, half marathon runners 7 g/kg/day. Spread intake across 3-4 eating occasions per day.

Frequently Asked Questions

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Last updated: February 2026

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