Carbohydrate is a fuel budget, not a moral category
Most carb advice online is a verdict: carbs are bad, or carbs are essential. Sports nutrition treats the question completely differently, and more usefully — carbohydrate is the fuel for high-intensity work, so the amount you need is a function of how much high-intensity work you actually do. A person training twice a day and a person walking the dog do not have the same carb requirement, and no percentage-based rule can tell them apart.
The mechanism is glycogen. Carbohydrate is stored in muscle and liver as glycogen — roughly 400–500 g in a trained adult, about 1,600–2,000 kcal of it. Above about 65% of VO₂max your body cannot supply energy fast enough from fat alone, so it draws on glycogen. Run that store down and the wall arrives: pace collapses, perceived effort spikes, and the next day's session is worse. Fat oxidation continues quietly the whole time, but it cannot cover the top gear.
Grams per kilogram, by training load
These are the bands used in the joint position stand from the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine — the same numbers the calculator applies to your body weight:
| Training load | g / kg / day | 75 kg athlete |
|---|---|---|
| Rest, skill or light activity | 3–5 | 225–375 g |
| Moderate — about 1 h/day | 5–7 | 375–525 g |
| Endurance — 1–3 h/day | 6–10 | 450–750 g |
| Extreme — more than 4 h/day | 8–12 | 600–900 g |
Two honest caveats. First, the upper end of these bands is written for competitive athletes in heavy blocks, not for a keen amateur who did a long ride on Sunday — match the band to your typical week, then flex up on your biggest days. Second, if you are in a calorie deficit you cannot have both the top of this range and your deficit; the deficit wins, and carbs get squeezed. That is a normal trade-off, not a mistake.
The percentage view
The other way to set carbs is as a share of energy. The Acceptable Macronutrient Distribution Range for carbohydrate is 45–65% of total calories, and the WHO caps free sugars at under 10% of calories, ideally under 5%:
Percentages are convenient but blunt: 50% of 1,800 kcal and 50% of 3,400 kcal are wildly different absolute amounts of fuel for the same body. Use the g/kg number as your primary target and the percentage as a sanity check — if your g/kg target lands you at 75% of calories, your calorie total is probably too low, not your carbs too high.
Timing: where carbs earn their keep
Daily total does most of the work, but placement matters for hard or long sessions.
- 1–4 hours before a long or intense session: 1–4 g/kg, lower in fibre and fat if your stomach is sensitive.
- During sessions over ~60–90 minutes: 30–60 g per hour, and up to 90 g/h with a glucose-plus-fructose mix for events over 2.5 hours.
- After, only if you train again within about eight hours: 1–1.2 g/kg per hour for the first four hours. Otherwise your normal meals refill glycogen in plenty of time — the “anabolic window” is far wider than supplement marketing suggests.
On rest days, drop towards the bottom of your band. Carb cycling is not a metabolic trick, it is just matching fuel to work.
Low-carb, discussed honestly
Low-carb and ketogenic diets work for fat loss — and so do higher-carb diets. When trials match calories and protein, the difference in fat lost is small enough to be noise. What low-carb genuinely does well is appetite: cutting a whole food category removes decisions, and protein and fat are filling. Some people also find blood-glucose control and cravings improve markedly.
What it costs is top-end performance. Fat oxidation adapts within a few weeks, so steady-state aerobic work is largely fine, but repeated sprints, heavy lifting and race-pace efforts rely on glycogen and get measurably worse. There is also an energetic penalty: fat is a less efficient fuel per litre of oxygen at high intensity. If you compete in anything with a finish line, going low-carb is a performance decision, not just a diet one.
The reasonable middle ground for most people: set protein with the protein calculator, set fat to at least 20% of calories with the fat intake calculator, and let carbs take the rest — then check the whole plate in the macro calculator. Carbs are the macro you adjust when training changes, which makes them the flexible one, not the disposable one.
Quality inside the number
The calculator gives grams; grams say nothing about what those carbs arrive in. Fibre is the marker worth watching — 14 g per 1,000 kcal is the reference intake, and most people manage about half. Carbs from oats, potatoes, beans, fruit and whole grains bring fibre, micronutrients and satiety with them. Sugar-sweetened drinks bring none of that, which is exactly why the free-sugar ceiling exists as a separate number from your carb target.