Fuel the ride before you bonk
Running out of carbohydrate — bonking — is the classic way a long ride falls apart, and under-drinking quietly drains performance long before thirst hits. This calculator turns ride duration, intensity, and bodyweight into concrete targets: carbohydrate per hour, total carbs to carry, fluid per hour, and estimated sodium to replace.
How it works
Carbohydrate need is driven mostly by intensity and duration, not weight, so the tool picks a grams-per-hour rate by intensity:
easy / endurance -> 40 g/h
moderate / tempo -> 60 g/h
hard / race -> 85 g/h
total_carbs = rate * hoursFluid and sodium scale with sweat, which rises with both intensity and body mass:
fluid_ml_per_hour = base_rate(intensity) * (weight_kg / 70)
sodium_mg_per_hour = fluid_litres_per_hour * 900The base sweat rate ranges from about 500 ml/h easy to 850 ml/h at race pace, normalised to a 70 kg reference rider, and 900 mg/L reflects a typical sweat sodium concentration.
Example and tips
A 75 kg rider planning a 4-hour tempo ride needs about 60 g/h × 4 = 240g of carbs, roughly 650 × (75/70) ≈ 700 ml of fluid per hour, and around 630 mg of sodium per hour. Carry that as a mix of bars, gels, and drink mix, start fuelling within the first 30 minutes rather than waiting until you feel empty, and practise your race-day intake on training rides first.
Turning the numbers into a carry plan
Knowing you need 240 g of carbs is only half the job — you need to carry it. Here is how to translate grams into products. These are illustrative figures; check the label on whatever you are actually carrying:
- A typical energy gel contains roughly 20–25 g of carbohydrate.
- A standard energy bar contains roughly 30–45 g.
- A two-scoop serving of most drink mixes provides roughly 40–60 g per 500 ml bottle.
A 4-hour ride at 60 g/h could be covered with two bottles of drink mix (about 80–120 g) plus a bar and a gel each hour (roughly 50–70 g/h combined). The exact products matter less than hitting the total — but variety helps if your gut is sensitive to fructose or a particular brand.
The 90 g/h ceiling and why a glucose-fructose mix matters
The human gut can absorb approximately 60 g of glucose per hour via one transporter (SGLT1). Adding fructose, which uses a separate transporter (GLUT5), opens a second absorption pathway and raises the ceiling to around 90 g/h for trained athletes who have practised high-carbohydrate fuelling.
Most gels use maltodextrin (a glucose chain) plus fructose in roughly a 2:1 ratio for this reason. If you are targeting above 60 g/h, check that your products use a mixed carbohydrate source. Pure glucose-only products hit a gut bottleneck at around 60 g/h even if your muscles could use more.
Hydration: weigh-in method for calibration
The fluid estimate here is a starting point. The most accurate way to know your personal sweat rate is to weigh yourself (naked, dried off) immediately before and after a one-hour ride of known intensity in typical conditions. Each kilogram of weight lost is approximately one litre of sweat. Do this on a few rides in different weather, and you will know your real rate rather than relying on a population average.