Carb Loading for a Marathon: How Many Grams, and When
Carb loading for a marathon in grams: why 8–12 g/kg and the 85–95% rule differ by 245 g a day, what 700 g looks like on a plate, plus a 36-hour schedule.
Kristian Hoffmann
SaaS founder and operator
Carb loading for a marathon comes out at roughly 8–12 g of carbohydrate per kilogram of body mass per day across the final 36–48 hours; Baylor College of Medicine puts the range at 10 to 12 g/kg per day. For a 70 kg runner that is 560–840 g a day, eaten as low-fibre food across five or six sittings rather than one enormous dinner. Most carb loading marathon advice stops at that sentence — which is where it goes wrong, because the other rule in wide circulation produces a different answer for the same runner.
The two published rules disagree by 245 g a day
Carbohydrate loading means deliberately raising carbohydrate intake above your normal training diet in the last day or two before a long race, while training volume is already low. Two ways of expressing the target dominate the search results: grams per kilogram of body mass, and a percentage of total calories. Run both against one runner and they part company.
| Rule as published | Applied to a 70 kg runner | Carbohydrate per day |
|---|---|---|
| 8 g/kg/day (lower end) | 70 × 8 | 560 g |
| 10 g/kg/day | 70 × 10 | 700 g |
| 12 g/kg/day | 70 × 12 | 840 g |
| 85% of calories on a 2,800 kcal taper day | 2,380 kcal ÷ 4 kcal/g | 595 g |
| 95% of calories on a 2,800 kcal taper day | 2,660 kcal ÷ 4 kcal/g | 665 g |
Top to bottom, that is a 245 g spread — around 875 g of cooked white rice, a full extra dinner, for the same person on the same day. Carbohydrate is counted at 4 kcal per gram, the standard Atwater factor used on nutrition labels, so the conversion itself is not in dispute. The disagreement sits in what the percentage rule assumes about your total intake.
What the percentage rule assumes but never states
Work it backwards. To reach 700 g of carbohydrate at 90% of calories, the 70 kg runner needs 700 × 4 ÷ 0.90 = 3,111 kcal on the day. At 85% it is 3,294 kcal; at 95%, 2,947 kcal. So the percentage rule only lands on 10 g/kg if total intake climbs to roughly 2,950–3,300 kcal — in the week when the long runs have gone and appetite usually follows them down.
That is the trap. A runner who cuts back at the table because they are running less, then makes 90% of what remains carbohydrate, has obediently followed the advice and eaten about 500 g. Carb loading fails on arithmetic far more often than on willpower.
Which number to plan with
Plan in grams per kilogram and treat percentages as a description of the result, not the instruction. Grams scale to the body doing the running: a 55 kg runner at 10 g/kg needs 550 g, a 90 kg runner needs 900 g, and the gap between those two people is more carbohydrate than many runners eat in an ordinary day.
Whether the effort pays is a separate question from how much. Carbohydrate loading can allow a competitor to hold their optimal pace for longer before fatiguing, which is the mechanism described in Louise Burke's review of nutrition strategies for the marathon. A recurring counter-position on r/Marathon_Training is that loading is obsolete because a normal training diet already leaves you well stocked on race morning. Both camps are arguing about the same thing: what you actually ate, not what you meant to eat. Count one day and you will know which camp you are in.
What 700 g looks like on a plate
Grams per kilogram are useless until they become portions. These are typical label figures — the packet in your kitchen is the authority, and supermarket own-brands vary — but they are close enough to build a day from.
| Food | Portion | Approx. carbohydrate |
|---|---|---|
| Cooked white rice | 250 g | 70 g |
| Cooked white pasta | 250 g | 75 g |
| Bagel, plain | 1 medium | 50 g |
| White bread | 2 slices | 30 g |
| Banana | 1 medium | 25 g |
| Orange juice | 250 ml | 24 g |
| Jam or honey | 1 tbsp | 16 g |
| Sports drink, 6% | 500 ml | 30 g |
| Dried dates | 100 g | 68 g |
Seven hundred grams is not one pasta party. It is roughly ten of those portions spread across a waking day, which is why the schedule below breaks the target into six sittings.
The fibre number nobody publishes
Fibre is the part of the carbohydrate that passes through largely undigested — it counts on the packet but does little for you at kilometre 32. Build a 700 g day out of wholemeal staples and, by label values, you carry roughly 40–50 g of fibre. Build the same 700 g out of the refined versions of those foods and you land near 12–15 g, with no loss of carbohydrate.
Swapping brown rice for white, wholemeal pasta for white, and skins off the fruit is the single change that keeps the gram target while dropping most of the residue. How much fibre you personally tolerate before a race is not something an article can tell you; rehearse it before a long run six weeks out and you will have your own number instead of a stranger's.
Loading in a hotel room, which is where October and November happen
The autumn city-marathon block means a lot of runners load in a hotel in a country whose labels they cannot read, two days after their last supermarket shop at home. Restaurant portions are unknown quantities, the expo sells things you have never eaten, and the pasta party arrives with a cream sauce and a glass of wine.
A list that works in almost any European supermarket, bought the moment you land: white rice or instant rice pots, plain white rolls, jam or honey, bananas, fruit juice, plain salted crackers or rice cakes, and a sports drink powder you already use. Nothing here needs a kitchen or a translation. Two practical rules for the trip: nothing new in the last 48 hours, and no alcohol standing in for a portion you still have to eat.
Cool weather adds a quieter failure. At 6–14 °C you drink less than you would in July, and juice and sports drinks are usually 100–150 g of the daily target. Drop them because you are not thirsty and the day ends short without a single skipped meal.
The final 36 hours, counted back from your start time
Fix your start time, call it T, and work backwards. Slots for a 70 kg runner on a 700 g target, none of them above 150 g:
| Slot | Timing | Target |
|---|---|---|
| Breakfast | T−30 h | 150 g |
| Mid-morning | T−27 h | 80 g |
| Lunch | T−24 h | 150 g |
| Afternoon | T−21 h | 80 g |
| Dinner | T−18 h | 150 g |
| Evening | T−15 h | 90 g |
If you prefer the two-day version, repeat the same six slots one day earlier and keep the day-of-race-eve numbers identical. Adding a third day mostly adds discomfort; the last 36 hours are where the plan earns its keep.
Race morning and the clock you forgot
Most plans put the last real meal about three hours before the gun, at whatever offset you rehearsed in training. Do the subtraction with the corral time, not the start time. A 09:00 start with corrals closing at 08:15 and a 30-minute walk from the hotel means eating at 06:00 and then standing around for two hours — a longer gap than anything you practised on a Sunday morning, and the reason a small top-up in the corral belongs in the plan rather than in the improvisation.
Write the top-up trigger on the same page as your splits. A printable pace band with a fuel column turns "I'll take a gel when I feel like it" into a checkpoint you either hit or notice missing.
What typically goes wrong
Four failure modes account for most of it, and three are detectable the day before:
- Silent calorie cut. Training volume drops, appetite drops with it, the percentage rule reports success at 500 g. Count one day rather than trusting the feeling.
- One-meal loading. A single vast dinner is uncomfortable and still leaves the day short; six sittings of 80–150 g is the same total without the misery.
- Liquid carbohydrate dropped in the cold. See above — 100–150 g can disappear without a single missed meal.
- New food in the last 48 hours. The expo, the sponsor's gel, the restaurant's sauce. Race week is the wrong laboratory.
One thing not to read into: the scale on race morning. Body mass moves for several reasons during a load, and none of them tell you anything useful about your fitness at that point.
When loading is not worth the trouble
Featherstone Nutrition puts the threshold at any race lasting over 90–120 minutes, which is the cleanest decision rule available: check your predicted finish time first, then decide. Below that duration, the protocol tends to cost more in bloating and admin than it returns.
That makes the goal-time question upstream of the nutrition question. If you are unsure whether you will be out there for 3:10 or 4:30, settle it with a marathon goal pace calculator before you buy the rice — the two ends of that range are different fuelling problems, not the same one at different speeds.
FAQ
How many days of carb loading for a marathon?
Two days at most for the great majority of runners, and the final 36 hours carry the bulk of it. Longer protocols existed historically with a depletion phase attached; current mainstream advice in the ranking guidance clusters at 36 hours to three days. If you can only do one thing properly, make it the day before.
How many carbs should I eat per hour during a marathon?
Race fuelling is a separate number from loading and is usually prescribed in the region of 30–60 g per hour, with higher intakes depending on trained tolerance and multiple carbohydrate sources; Burke's review covers in-race fuelling alongside loading. The practical answer is whatever you rehearsed and absorbed in long runs at race pace. Race day is not the place to discover your ceiling.
What is a good carb loading meal plan for marathons?
Take your body mass in kilograms, multiply by 10, and split the result across six sittings of roughly 80–150 g using the portion table above. White rice, white pasta, bagels, bananas, jam, juice and a sports drink will get almost anyone there without a recipe. The plan is good when you can add the six numbers up and reach your target before you start eating.
Should you carb load for a 10K?
No — a 10K sits well under the 90–120 minute mark where loading is generally considered worthwhile, so a normal high-carbohydrate day and a familiar breakfast cover it. The exception is a runner finishing a 10K in well over 90 minutes, for whom the duration argument starts to apply. For a half marathon, treat the same duration test as the deciding factor rather than the distance on the entry form.