Carb Loading Calculator
Top off glycogen, not your gut. Most endurance athletes need roughly 8 to 12 grams of carbohydrate per kilogram of body weight per day for the final 24 to 72 hours, built from low-residue foods like white rice, bananas, and sports drinks. This tool turns that into an hour-by-hour countdown to the gun.
Enter your weight, loading target, window, gut sensitivity, and dietary preference to get a front-loaded, no-bloat countdown with gram-accurate food picks. We call the 3-day output your 3-Day Pre-Race Glycogen Matrix.
EB says
10-12 g
carbs / kg over 36h
Per Carbohydrates for training and competition (Journal of Sports Sciences (Burke et al., 2011))
Enter your details above to see your carb loading protocol.
Gear up
Low-residue carb sources athletes actually buy
Keep it simple and low-fiber the day before. Test any new food in training first, never on race weekend.
- Instant white rice (microwave cups)The easiest low-residue staple. ~45 g carbs per cup with zero prep on race weekend.View on Amazon →
- Pure maple syrupFast, low-fiber liquid carbs. ~13 g per tablespoon to stir into oats or sip late.View on Amazon →
- Low-fiber sports drink mixLiquid carbs for the final hours when solids feel heavy. Easy on a nervous gut.View on Amazon →
- Digital food scaleHit your gram targets exactly instead of guessing portions during loading.View on Amazon →
As an Amazon Associate we earn from qualifying purchases.
Fueling on course?
DIY Sports Drink Calculator
Carry your loading carbs into the race with a matched maltodextrin and fructose bottle recipe.
Salt it too?
Sodium Calculator
Pair your carb load with a per-hour sodium and fluid target and a pre-race loading card.
Know your goal time?
Marathon Time Predictor
Predict your finish from a recent race and your weekly mileage, then load and pace to that number.
Why do you gain weight when carb loading?
Gaining 1 to 2 kg (2 to 4 lb) during a carb load is normal and is the sign it worked, not a problem. Your body stores roughly 3 grams of water with every gram of glycogen, so topping off your glycogen tank pulls water in with it. That weight is stored fuel and hydration, not fat, and most of it is burned and sweated off during the race.
| Athlete weight | Glycogen stored (loaded) | Expected scale gain |
|---|---|---|
| 60 kg (132 lb) | ~500 g | 1.0 - 1.5 kg |
| 75 kg (165 lb) | ~600 g | 1.5 - 2.0 kg |
| 90 kg (198 lb) | ~750 g | 2.0 - 2.5 kg |
If you feel slightly heavy and well-hydrated on race morning, the load worked. Athletes who panic and cut carbs to avoid the scale change arrive under-fueled.
Carb loading calculator FAQ
How many carbs should I eat before a marathon?
For a marathon, aim for roughly 8 to 12 grams of carbohydrate per kilogram of body weight per day in the final day or two before the race. A 70 kg (154 lb) runner targeting the standard 10 g/kg lands near 700 grams of carbohydrate per day. Spread that across the waking hours in regular meals and snacks rather than a few huge plates, and lean on low-fiber, low-fat sources so your gut is light on race morning.
What is low-residue carb loading?
Low-residue carb loading means hitting your high carbohydrate target using foods that leave little undigested material in the gut, so you top off glycogen without bloating, gas, or an urgent bathroom stop at the start. It favors white rice, white bread and bagels, ripe bananas, applesauce, peeled potatoes, sports drinks, honey, and maple syrup, and it avoids high-fiber and high-fat foods like whole grains, beans, raw vegetables, and nuts in the final 24 hours.
When should I start carb loading?
For most road races up to a marathon, a 24 to 36 hour loading window is plenty; the muscle can refill its glycogen stores in about a day when carbohydrate intake is high and training volume is tapered. For ultra-distance events of 100 miles or more, a 48 to 72 hour window using the 3-Day Pre-Race Glycogen Matrix approach gives the body more time to saturate. Starting much earlier than 72 hours adds little and often just adds weight and bloat.
What do vegan athletes eat during carb loading?
Vegan athletes can hit high carbohydrate targets easily because the best low-residue carb sources are already plant-based: white rice, peeled potatoes, ripe bananas, applesauce, rice cakes, medjool dates, fruit puree pouches, maple syrup, and sports drinks all qualify. The only common carb-loading food that is not vegan is honey. The vegan filter in this calculator removes honey and surfaces the best plant-based alternatives so your plan stays high-carb, low-residue, and animal-product free.
Can you carb load gluten-free before a marathon?
Yes. Gluten-free carb loading is straightforward because many of the best low-residue carb sources contain no gluten at all: white rice, peeled potatoes, ripe bananas, applesauce, certified GF quick oats, rice cakes, medjool dates, fruit pouches, and sports drinks. The gluten-free filter in this calculator removes wheat-based foods like bagels, white bread, and graham crackers and replaces them with GF equivalents so your gram targets stay the same.
Why do my legs feel heavy after carb loading?
Every gram of stored glycogen binds roughly 3 grams of water, so a successful load genuinely adds a few pounds of glycogen-plus-water weight. That heavy, slightly puffy feeling the day before is normal and expected; it is fuel and fluid you will burn through during the race, not fat. If you feel bloated and gassy rather than just heavy, that points to too much fiber or fat, which is exactly what the low-residue food picks here are designed to avoid.
Do I need to carb load for races under 90 minutes?
Generally no. Normal glycogen stores cover efforts up to about 90 minutes, so a 5K, 10K, or short sprint triathlon does not require a dedicated load; a normal carbohydrate-rich dinner and breakfast is enough. Carb loading pays off for marathons, long-course triathlons, gran fondos, and ultras where you would otherwise run low on stored fuel. Use a shorter window in the calculator if your event sits near that 90-minute line.
See also: the 36-hour carb-loading food list and what to eat on race morning.
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