Carb Loading Calculator — Marathon Fueling Plan

How the Carb Loading Calculator Works

The calculator picks the protocol for your race distance and applies the 90-minute rule: events under 90 minutes do not need carb loading. Top-Up (6-7 g/kg) covers 5K/10K and sub-90-minute halves; Modified Loading (7-8 g/kg) covers halves over 90 minutes; Classic Carb Loading (10-12 g/kg) covers marathons and ultras. Daily targets ramp progressively across the loading window and split across your chosen meal count, with a food reference table adjusted for your diet. Enter an optional expected finish time and a sub-90-minute half is automatically downgraded to a top-up.

The Science of Glycogen Supercompensation

1966 → 67 — the founding observations. Bergström and Hultman (1966) first showed that muscle glycogen can be roughly doubled when high-carbohydrate intake follows exercise-induced depletion. The classical race-prep protocol — a deliberate "bonk-out" workout 7 days before the race, then high-carb loading — was refined in Bergström, Hermansen, Hultman & Saltin (1967). Painful, risky, and now obsolete.

1981 → 2002 — the depletion phase falls away. Sherman et al. (1981) showed a modified protocol (3-day taper + progressive carb increase to ~75% high-CHO) achieves the same glycogen peak without the depletion workout. Bussau et al. (2002) compressed it further: a single day at 10 g/kg combined with complete physical rest reached supercompensation within 24 hours in trained endurance athletes (about 95 → 180 mmol/kg wet weight).

Modern consensus (2011 → 2016). The authoritative reference today is the ACSM / AND / DC Joint Position Statement (Thomas, Erdman & Burke, 2016), building on Burke et al. (2011): target 10–12 g/kg/day for 36–48 hours before races lasting more than 90 minutes. The 7-day depletion protocol is no longer recommended for general use; this calculator does not offer it.

Individual variance is real. Glycogen response to a given protocol varies substantially between individuals — often by tens of percent (Burke et al. 2011). The calculator outputs the protocol mean — expect to fine-tune across 2–3 races to find your personal sweet spot. Pair carb loading with in-race fueling (60–90 g/h carbs) and gut training (Costa et al. 2017) for the full glycogen pipeline.

Practical Meal Planning During Loading

Spreading intake across 5-6 meals is key. Main meals get 25-35% of daily carbs; snacks get 10-15%. Choose low-fiber, low-fat foods. Liquid carbs (juice, sports drinks) add carbs without fullness. A sample 600g day: 2 bagels with honey + OJ (120g), banana + bar (70g), pasta + bread (130g), pretzels + drink (75g), rice bowl + bread (140g), applesauce + dates (65g). Don't restrict salt — sodium helps glycogen storage. For longer or hot-weather races, pair loading with a race-day sodium plan from the electrolyte & sodium loss calculator.

Common Carb Loading Mistakes

Not eating enough: Adding 'some extra pasta' isn't loading — you need 7-8+ g/kg. Too much fiber/fat: Whole grains and salads defeat the purpose. Wrong timing: Starting too early (1 week) or too late (night before). New foods: Race week isn't for experimenting. Ignoring taper: Without 50-75% reduced training, muscles burn glycogen faster than you store it.

Race-Week Carb Loading Timeline

A reliable day-by-day schedule for marathon week, which the calculator auto-generates to match your weight and meal count:

Monday-Wednesday (5-7 days out): Normal balanced diet, ~5 g/kg carbs. Focus on tapering mileage, sleep, and hydration. No special loading yet.

Thursday (3 days out): Start increasing to ~7 g/kg. Swap whole grains for white rice and pasta. Cut heavy salads.

Friday (2 days out): 8-10 g/kg. Add a mid-afternoon snack (pretzels + juice). Largest meal at lunch, lighter carb dinner.

Saturday (race eve): Peak at 10-12 g/kg. Lunch is your biggest meal of the week — pasta plate, bread, dessert. Dinner small and familiar (rice bowl, noodles). Bed early.

Sunday (race morning): 1-2 g/kg, 2-4 hours before the gun. White toast + banana + honey + sports drink. Sip water, stop 60 minutes before start. You're fully loaded.

Carb Loading for a Half Marathon (1-2 Day Protocol)

A half marathon needs less loading than a full marathon — but more than you'd think if you're aiming for a sub-2:00 or running it in heat. The dividing line is the 90-minute mark: under 90 minutes you usually finish on stored glycogen alone and a normal pre-race meal is enough, so skip the multi-day load and just top up. Over 90 minutes (roughly a 2:00-plus finish), a compressed 2-3 day modified load at 7-8 g/kg raises muscle glycogen 25-40% above baseline — enough to blunt the late fade at km 18-21 without the bloat of a full marathon load.

Where the half protocol diverges from the marathon protocol:

For a 70 kg half-marathoner that peak day is 490-560 g of carbs — about two-thirds of the marathon load, comfortably hit with bagels, pasta, bananas, and juice across 4-5 meals (use the food reference table below). Because the window is shorter, you can start Friday for a Sunday race: ramp from a normal ~5 g/kg into the 7-8 g/kg peak the day before, keep the biggest carb meal at lunch, and run a lighter dinner. Select "Half Marathon" in the calculator and it generates this compressed 2-3 day schedule automatically; pick "Marathon" only for the full 3-4 day, 10-12 g/kg plan.