Carbs Explained — What They Are and How Much You Need
Carbohydrates are the body's fastest fuel. Gram for gram they provide 4 kcal, the same as protein, but they are the only macronutrient the body stores specifically for high-intensity work: as glycogen in muscle and liver. A typical adult stores roughly 300–500 g of glycogen, enough for about 90 minutes of hard exercise before stores run low and performance drops off a cliff.
The headline answer is that most active adults do well on 3–7 g of carbohydrate per kilogram of body weight a day, with the exact figure depending on training volume: around 3–5 g/kg for light training, 5–7 g/kg for moderate training, and 8–12 g/kg for very heavy endurance work (Thomas et al., Journal of the Academy of Nutrition and Dietetics, 2016). At 80 kg and moderate training that is roughly 400–560 g a day — far more than most diet articles suggest, and the reason low-carb diets feel so flat in the gym.
Use the calculator
Start with your own numbers: try our macro calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Fats Explained, Fibre and Weight Loss, What Are Macros?. You can also browse the full guides section for the rest of the series.
The key points
Carbs are not one thing. Sugars, starches and fibre are all carbohydrates, but they behave very differently. Starch in oats, rice and potatoes digests steadily and refills glycogen; fibre in vegetables, beans and whole grains slows digestion and feeds gut bacteria; added sugars in drinks and sweets spike blood glucose and add calories without fullness. Judging a food by its total carbohydrate number alone misses all of this — 50 g of carbs from lentils and 50 g from cola are not the same meal.
Glycogen decides how hard you can train. Muscle glycogen is the fuel for efforts above roughly 70% of maximum heart rate: heavy lifting sets, intervals, and the second half of a long run. When glycogen is low, the body cannot simply switch to fat at the same intensity — perceived effort rises, power falls, and sessions get cut short. This is why athletes eat carbohydrate before and during long sessions, and why a very-low-carb diet usually costs performance for the first few weeks at least.
Fibre is the part that matters for weight loss. Fibre adds bulk and slows stomach emptying without adding usable calories, which is why high-fibre meals keep you full for longer on the same calories. Most adults in the UK and US eat only 15–18 g of fibre a day against a recommended 25 g (women) to 38 g (men) — closing that gap is often the single easiest change in a fat-loss diet, and it does not require cutting any food group out.
Timing matters more than the total for athletes. For a single daily session, total daily carbohydrate matters most. But around training, timing helps: 1–4 g/kg in the 1–4 hours before a hard session, and 1.0–1.2 g/kg per hour for the first few hours after a glycogen-depleting session, with protein alongside. For two sessions a day, that post-session window is when refuelling actually matters; for everyone else, spreading carbs across meals is fine.
At a glance
| Food | Carbs per 100 g | Fibre per 100 g | Notes |
|---|---|---|---|
| Cooked white rice | 28 g | 0.4 g | Fast-digesting, good post-training |
| Cooked brown rice | 23 g | 1.8 g | Slower, more fibre and magnesium |
| Rolled oats (dry) | 66 g | 10 g | Very filling per calorie |
| Wholemeal bread | 41 g | 7 g | Practical everyday source |
| Banana | 23 g | 2.6 g | Portable pre-training snack |
| Cooked lentils | 20 g | 8 g | Carbs plus protein plus fibre |
| Potato (boiled) | 17 g | 2 g | High satiety per calorie |
| Cola | 10.6 g | 0 g | All added sugar, no fullness |
Values are typical USDA FoodData Central figures and vary by brand and cooking method. The pattern is what matters: whole foods carry fibre and micronutrients with their carbohydrate, while drinks and sweets carry carbohydrate alone.
The formula behind it
Carbohydrate provides 4 kcal per gram (the Atwater factor, used in food labelling worldwide). Daily need is usually set by body weight and training load rather than by a percentage: carbohydrate (g) = body weight (kg) × g/kg target. The Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine joint position stand (Thomas et al., 2016) gives 3–5 g/kg for light training, 5–7 g/kg for moderate training, 6–10 g/kg for heavy training and 8–12 g/kg for very heavy training.
Worked example — an 80 kg lifter training four times a week at moderate volume, maintenance about 2,600 kcal a day. At 5 g/kg, carbohydrate is 80 × 5 = 400 g, which is 400 × 4 = 1,600 kcal, or about 62% of maintenance. With protein set at 1.6 g/kg (128 g, 512 kcal), that leaves roughly 488 kcal for fat, about 54 g. If the same person cuts to 2,100 kcal, carbohydrate can drop to 3.5 g/kg (280 g, 1,120 kcal) while protein stays high and fat takes about 55 g — the cut comes mostly from carbohydrate, because protein protects muscle. Note the floor: 2,100 kcal is comfortably above the 1,500 kcal minimum for men and the 1,200 kcal minimum for women, so this is a normal deficit, not a very-low-calorie diet.
Your plan, step by step
- Work out your training load honestly: sessions per week, their length and their intensity, not the week you wish you had.
- Pick a g/kg target from the ranges above — 3–5 g/kg for light training, 5–7 g/kg for moderate, more only for heavy endurance work.
- Set protein first (1.6–2.2 g/kg), then fill the remaining calories with carbohydrate and fat; do not set carbohydrate as a percentage and hope protein fits.
- Put most carbohydrate around training on hard days and spread it evenly on rest days, when total need is lower.
- Re-check after two weeks: if sessions feel flat and heavy, add 50–100 g of carbohydrate a day before changing anything else.
How to use this guide
Use this guide to set a starting carbohydrate target, then judge it by training quality and body weight over two to four weeks, not by a single day. If weight is falling at 0.5–1% of body weight a week and sessions feel strong, the target is right. If weight is stalling, trim 100–200 kcal — usually from carbohydrate — and reassess. If sessions feel flat, add carbohydrate before you add caffeine or cut training volume.
Common mistakes
Cutting carbs to cut calories, then wondering why training died. A low-carb diet is a low-energy diet for training. If strength or sprint performance matters to you, carbohydrate is not the enemy — total calories are the lever for fat loss, and carbohydrate is the fuel that lets you keep training while you cut.
Counting net carbs and ignoring total intake. Fibre is subtracted from total carbohydrate on some labels, but the calories still have to be counted somewhere. For fat loss, total calories and total protein matter far more than whether a food's carbs are 'net' or 'total'.
Eating all your carbs at dinner and none before training. A morning session fuelled by nothing but coffee is a session run on an empty glycogen tank. Move some carbohydrate to the meal before training and the session usually feels noticeably easier within a week.
What this guide does NOT tell you
Carbohydrate needs vary with training, body size and health conditions, and the ranges on this page are population estimates from sports-nutrition position stands, not personal advice. People with diabetes, insulin resistance or other metabolic conditions should set carbohydrate targets with a clinician or registered dietitian, because blood-glucose response changes the answer. This guide does not diagnose or treat any condition, and it does not predict your individual response — it gives you a defensible starting point and a way to adjust it.
Frequently asked questions
How many carbs should I eat a day?
For most active adults, 3–7 g per kilogram of body weight a day covers the range: 3–5 g/kg for light training, 5–7 g/kg for moderate training, and 8–12 g/kg only for very heavy endurance training (Thomas et al., 2016). At 80 kg that is 240–560 g a day depending on training load. Sedentary adults need less, and the figure should fall when calories are cut for fat loss — but protein should stay high while carbohydrate is the part that flexes.
Are carbs bad for weight loss?
No. Fat loss is driven by the calorie deficit, and controlled trials find similar fat loss from low-carb and low-fat diets when calories and protein are matched. Carbohydrate is often blamed because the foods people overeat — biscuits, crisps, sugary drinks — happen to be high in carbohydrate and fat together. Cutting carbohydrate works for some people because it removes those foods; it is not required, and it can cost training performance.
Should I eat carbs before a workout?
For sessions longer than about 60–90 minutes or at high intensity, yes: 1–4 g/kg in the 1–4 hours before training tops up glycogen and usually improves performance. A banana and some toast an hour before is enough for most sessions. For a short, easy session, it matters far less — and some people train fasted for convenience without any problem, as long as total daily carbohydrate is adequate.
What is the difference between simple and complex carbs?
Simple carbohydrates (sugars) are short chains that digest quickly; complex carbohydrates (starches) are long chains that digest more slowly. In practice the distinction matters less than the food itself: fruit is 'simple' but comes with fibre and water, while white bread is 'complex' but digests fast. Judge foods by fibre content, satiety and how they fit your total intake, not by the simple/complex label alone.
Do I need carbs after a workout?
After a hard, glycogen-depleting session, 1.0–1.2 g/kg of carbohydrate per hour for the first few hours speeds refuelling, especially if you train again the same day. For one session a day, total daily carbohydrate matters more than the post-workout window — your next meal covers it. Add 20–40 g of protein to that meal as well, since protein, not carbohydrate, is what drives muscle repair.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.