Fuelling · Free tool
Marathon Carb Loading Calculator
Enter your body weight, your expected race time, how hard you want to load and for how many days. We turn that into grams of carbohydrate per day and per meal, the calories that represents, the total for the load, and a race-morning breakfast.
What carb loading does
Your muscles and liver hold roughly 400–600 g of glycogen — enough for about 90 minutes of hard running. Carb loading packs those stores as full as they will go so a marathon or long race starts with the tank brimming, which research shows improves endurance performance by 2–3% and delays the wall.
The modern protocol is simple: 8–12 g of carbohydrate per kilogram of body weight per day for the final 36–48 hours, while training tapers to almost nothing. The old depletion phase — a week of low carbs and hard running before the load — is unnecessary and just makes you miserable.
How to eat that much
For a 70 kg runner 10 g/kg is 700 g of carbohydrate a day, which is far more than a normal diet. It only fits if carbohydrate replaces fat, protein and fibre rather than being added on top: white rice, pasta, bread, potatoes, bagels, fruit juice, sports drinks and low-fibre cereals. Spread it across four meals and a couple of snacks, and use liquids when solid food gets tedious.
Expect the scales to rise by 1–2 kg. Every gram of glycogen is stored with about three grams of water, so the gain is the load working, not fat — and it doubles as a hydration reserve for the race.
Race morning and who can skip it
Top up the liver overnight with a breakfast of 1–4 g/kg of carbohydrate 1–4 hours before the start — the calculator uses 2 g/kg, about 140 g for a 70 kg runner. Familiar, low-fibre food only; race morning is not the time to experiment.
If your race is under about 90 minutes — a 5K, 10K, or a fast half marathon — your normal stores are enough. A regular high-carb day of 5–7 g/kg and a decent breakfast will do; a full load just adds weight you have to carry.
Frequently asked questions
How many carbs should I eat before a marathon?
About 8–12 g per kilogram of body weight per day for the final two days — 560–840 g a day for a 70 kg runner — then 1–4 g/kg at breakfast on race morning. The calculator gives your exact figures and splits them across meals.
How many days should I carb load?
Two days is the sweet spot for most runners: 36–48 hours of high carbohydrate with minimal training fills the stores. A single day works if you can eat enough; three days adds little beyond extra weight and is mainly useful if you struggle to eat large amounts at once.
Do I need to carb load for a half marathon?
Only if you expect to be out for more than about 90 minutes. Runners finishing a half in under that have enough glycogen from a normal high-carb diet; anyone slower than about 1:45 benefits from at least a one-day load.