You're about to run a marathon. Or maybe you're halfway through a four-hour gravel ride. Either way, your body is burning through two fuel tanks simultaneously: tank number one filled with carbohydrates (glycogen, stored in your muscles and liver) and tank number 2 is filled with fat.
There's many factors that will influence how quickly these tanks are draining as you exercise. That's the whole game with our calculators! Every calculator we've built is trying, more or less, to answer the same question: how fast are you draining tank number one (the carb tank), and how much should you pour back in?
Let's pop the hood on each one.
The Race Calculator
This one's built for runners lining up for a half or full marathon, and it leans on a beautifully simple equation from the physiologist Luc Léger.
Here's what it does. You give it your weight and your target finish time, and it works backwards to figure out how fast you'll be running. That speed gets plugged into the Léger equation, which estimates how much oxygen your body will consume per minute (your VO2). Oxygen consumption is basically a proxy for total energy burn — roughly 5 kilocalories for every litre of O2 you use.
But not all of that energy comes from carbs. Slower runners burn proportionally more fat. Faster runners lean harder on glycogen. The calculator models this shift: at 8 km/h, about 50% of your fuel comes from carbs. Push past 12 km/h and that climbs to 70%.
Now for the clever part. It estimates how much glycogen you're already carrying — about 12.5 grams per kilogram of muscle mass, plus around 100 grams in your liver. You can't use all of it (bonking happens well before the tank is truly empty), so it assumes about 70% is accessible. The target? Replace roughly 50–60% of the carbs you're burning through, on top of what your body already has on board.
Round to the nearest five, cap it based on your gut tolerance, and you've got a decent number in grams per hour as well as what precise kilometre to take on your fuel that is an excellent starting point for a race day plan.
The Carb Calculator: Intensity Times Duration, Simplified
Not everyone has a power meter or a race bib. Sometimes you just need to know: I'm going mountain biking for three hours at a pretty hard effort — how much should I eat?
This calculator takes a different approach. Instead of modelling your physiology from oxygen consumption, it uses a lookup table anchored to two inputs: how hard (rated 1–10) and how long.
Duration sets the base rate. Under 45 minutes? You probably don't need anything. Between one and two-and-a-half hours? Around 45 grams per hour. Beyond three hours, the base climbs to 75 g/hr. Intensity then scales that number — an easy spin (RPE 3) cuts the rate to about 60% of baseline, while an all-out effort (RPE 9–10) pushes it up to 130%.
Sport type nudges things too, but mostly in a practical direction. Running shakes your gut more than cycling, so the rate gets a small 10% haircut. Hiking, where intensity rarely spikes, drops further. And if you tick "hot and humid" or "high altitude" in the optional section, the calculator bumps your target up, as heat and altitude both increase carbohydrate reliance.
It's less granular than the race calculator, but that's the point. It's designed to give a credible recommendation to anyone, for any sport, with minimal inputs.
The Cycling Calculator
Cyclists live in a world of data. This calculator speaks their language.
The core idea is elegant: if you know your average power output in watts and how long you rode, you know exactly how much mechanical work you did (power × time = energy in kilojoules). But your body isn't 100% efficient — most of the energy you produce is lost as heat. The fraction that actually turns the pedals is called gross efficiency, and for most trained cyclists it sits around 20–22%.
Divide the mechanical work by your efficiency, and you get total metabolic energy expenditure. That's the whole pie.
Now you need to slice it. How much of that pie was carbs versus fat? This is where the calculator uses your intensity factor — your average power divided by your FTP (the highest power you could sustain for an hour). The higher the intensity factor, the more your body relies on carbohydrates. Physiologically, this maps to something called the respiratory exchange ratio (RER): at low intensities the RER hovers around 0.80 (more fat), and it climbs toward 1.00 (almost pure carbs) as you approach threshold.
From there, the logic is the same as the race calculator: estimate your glycogen stores, figure out how many grams of carbohydrate your body oxidized, and recommend replacing 50–60% of that with food you carry in your pockets.
So… Why Not Just Say "Eat 60 Grams Per Hour"?
Because a 55 kg runner finishing a half marathon in 1:45 and a 90 kg cyclist grinding through five hours at tempo are living in completely different metabolic realities. A flat recommendation would overfeed one and underfeed the other.
The math isn't perfect (no model is FYI). Your actual glycogen stores depend on what you ate yesterday. Your efficiency varies with fatigue. Your gut absorption has a ceiling that training can shift. But a personalised estimate grounded in exercise physiology will always beat a guess. And that's really all these calculators are doing: turning well-established science into a number you can act on before you clip in or lace up.
Now go eat your snacks at the right time.