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The carb-up is a tool. This is its instruction manual.
The cyclical ketogenic diet — five to six days keto, one to two days of structured carbs — lives or dies on the carb-up math, and Lyle McDonald wrote the book on it (The Ketogenic Diet, Ultimate Diet 2.0). This calculator does his arithmetic: protein anchored to lean body mass, carb-up volume scaled to lean mass for 24-hour and 48-hour refeeds, and the number that stops the Monday-morning panic — how much of the scale jump is glycogen and water, not fat.
Keto lifters use this every Friday: the carb-up is the most-mismanaged part of cyclical keto, and both overeating and under-filling glycogen cost performance. It is also the panic-stopper — the math that explains why Monday's scale jump is water, not failure.
Glycogen and water, almost entirely. Stored glycogen binds roughly 3 g of water per gram, so a 400 g carb-up that stores ~240 g of glycogen puts about 2 lb of glycogen-plus-water on the scale. It is fuel in the tank, not fat — it drains back out over the keto week.
Trainees doing intense glycolytic work (lifting, sprinting) several times a week, who need glycogen for performance but want keto's appetite control the rest of the week. Sedentary keto dieters do not need carb-ups; endurance athletes need different fueling entirely.
Yes — but only from calories above maintenance, same as always. The carb-up targets here are sized to refill glycogen, not to be a cheat day. Stay near maintenance calories and the refeed overwhelmingly becomes glycogen and water.