Carb Loading Calculator for Marathon Runners
Calculate your pre-race carb loading targets based on your race, body weight, and experience level.
The science behind carb loading
Your body stores roughly 500g of glycogen across muscles and liver. That's enough fuel for about 75-90 minutes of hard running. If your race is longer than that, you're going to dip into those stores, and when they run low, your body shifts to burning fat. Fat burns too slowly to hold race pace. That's the wall.
Carb loading delays the wall by supercompensating your glycogen stores before the gun goes off. Research shows it can improve marathon performance by roughly 3% (Burke et al. 2011). For a 3-hour marathoner, that's 5+ minutes.
The old protocol is dead
The 7-day depletion/loading protocol is outdated. Bussau et al. (2002) found that a single day of 10 g/kg with minimal exercise matched the old multi-day approach in trained athletes. No starving yourself. No depletion runs. Areta & Hopkins (2018) reviewed 181 muscle biopsy studies and confirmed that fitness level and carb intake are the two biggest predictors of glycogen storage.
The activation session
A short run or set of strides the morning before your race primes your muscles to absorb and store more fuel. Tim Podlogar, a sports nutrition researcher who advises Tour de France teams, uses this protocol with his athletes: a brief activation session in the morning, then aggressive carb intake the rest of the day.
Weight gain is normal
Water follows glycogen into your muscles. Expect 1-2 kg on the scale during loading. It's fuel, not fat. It disappears during the race. A single pasta dinner provides roughly 200g of carbs. A 70 kg runner targeting 10 g/kg needs 700g. That's an entire day of focused eating, not one meal.
For the full science on how your body absorbs and uses carbs during racing, read our guide to the science of race fueling.
Frequently Asked Questions
How many carbs should I eat before a marathon?
Most marathoners should target 8-10 grams of carbs per kilogram of body weight per day during the loading window. For a 70 kg (154 lb) runner, that is 560-700g of carbs per day. First-timers should aim for the lower end (6-8 g/kg) and experienced runners can push toward 10-12 g/kg. The loading window is typically 24-48 hours before race day.
How long before a marathon should I carb load?
Research shows that a single day of aggressive carb loading (10 g/kg) is as effective as the old multi-day protocol (Bussau et al. 2002). First-timers benefit from a gentler 36-48 hour approach. Experienced runners can use a 24-hour protocol. The key is a brief activation session (strides or easy run) the morning before loading begins to prime muscles for glycogen storage.
Do you need to carb load for a half marathon?
It depends on your finish time. If your half marathon takes under 90 minutes, a full carb load is unnecessary—just emphasize carbs the day before (5-8 g/kg). If your half takes 90 minutes or longer, carb loading is beneficial because you will begin to deplete glycogen stores. A 24-hour protocol at 6-8 g/kg is sufficient for most half marathoners.
Is it normal to gain weight while carb loading?
Yes. Expect to gain 1-2 kg (2-4 lbs) during carb loading. Water follows glycogen into your muscles at a ratio of about 3g of water per 1g of glycogen stored. This is fuel, not fat. The extra weight disappears during the race as you burn through your glycogen stores. Do not cut carbs to avoid the scale increase—it means the loading is working.
What should I eat when carb loading before a race?
Focus on simple, low-fiber carbs: white rice, white bread, bagels, pancakes with syrup, pretzels, honey, jam, maple syrup, sports drinks, fruit juice, gummy bears, bananas, and rice cakes. Liquid carbs like apple juice (55-60g per glass) are an easy way to top up. Avoid whole grains, high-fiber foods, raw vegetables, beans, heavy sauces, and high-fat foods that slow digestion.