How to calculate your carb needs for any workout

Every runner knows carbs matter. But most have no idea how many they actually need — so they guess. They take one gel on a 2-hour run and wonder why they bonk. Or they carb-load for a 5K and wonder why they feel heavy.

Carb needs are not a mystery. They follow a clear formula based on how long and how hard you're working. Here is how to calculate them for any workout.

The quick formula

Your carb needs come from two sources: what you eat before the workout and what you consume during it.

• Pre-workout: 1–2g carbs per kg of body weight, 2–3 hours before
• During workout: based on duration (see table below)
• Post-workout: 1–1.2g carbs per kg within 2 hours

Carb targets by workout duration

Use this table to figure out how many carbs you need during the workout itself:

Carbs per hour for running: 30 vs 60 vs 90g

The duration bands give you the range. Where you land inside it depends mostly on what your gut can absorb and repeat without stomach trouble.

• 30g/hour: the easiest starting point, especially if you have a sensitive stomach.
• 60g/hour: the sweet spot for many long runs and marathons.
• 90g/hour: works for the longest efforts, but only after gut training with dual-source fuel.

In gels, one gel is roughly 25–30g of carbs. So 30g/hour is about one gel, and 60g/hour is about two gels, or one gel plus a 500 ml bottle of sports drink (a high-carb drink mix can supply 60g on its own; check the label). 90g/hour usually needs a mix, such as gels plus a carb drink. Carbs from sports drink count toward the hourly total, which often makes higher targets easier.

The schedule matters more than the number

Most runners who bonk picked a reasonable number and then started too late. By the time you feel the crash, you are already behind.

• Take the first dose about 15–20 minutes in, once the run has settled, not when you start to fade.
• Repeat on a fixed interval that matches your target, such as a gel every 30 minutes or a smaller dose every 15–20 minutes.
• Smaller, more frequent doses usually sit better than one big dose.
• Stop fueling close to the finish so your stomach is not sloshing.

How intensity changes things

Duration is the biggest factor, but intensity matters too. Higher intensity burns more carbs per minute.

At race intensity, a 70kg runner burns through roughly 70–90g of carbs per hour. Your body can only absorb 30–90g per hour (depending on the carb source and your gut training), which is why fueling is always a race against depletion.

Worked example: 2-hour long run for a 70kg runner

• Pre-run meal (2.5 hours before): 1.5g/kg = 105g carbs (oatmeal + banana + honey + juice)
• Pre-run top-up (15 min before): 20g carbs (1 gel)
• During run: 45g/hour x 2 hours = 90g total (1 gel every 30 min)
• Post-run recovery: 1g/kg = 70g carbs + 25g protein
• Total for the session: roughly 285g carbs across the day

Compare that to a rest day where the same runner might only need 200–250g total. The difference is significant — and most runners never adjust.

Carb needs by sport

Why most generic calculators get it wrong

Most carb calculators give you a single daily number: "eat 300g of carbs per day." But your needs change dramatically depending on whether it's a long-run day, an easy day, or a rest day. A flat daily target means you're overfueling on easy days and underfueling on hard days.

The right approach is per-session carb targeting — calculating needs for each workout individually, then building your daily total from there.

How to do the per-session math

Per-session targeting sounds technical but is mostly two quick calculations. For daily carbs, multiply your bodyweight by a factor set by the day's load: roughly 3-5g/kg on easy or rest days, 6-8 on hard days, and 8-10 around long sessions and race week. For during-workout carbs, multiply your planned rate (30-60g per hour for most efforts over 75-90 minutes) by the duration to get a session total, then divide into timed feeds. A 70kg runner on a 2-hour long-run day might land near 560g of carbs across the day and 90-120g taken during the run itself.

Running those numbers for every session automatically — from a Runna, Garmin, or TrainingPeaks calendar — is what Mavr does so you never reach for a calculator. But the two multiplications above are genuinely all the math is, and doing them yourself once makes the logic stick.

How the Mavr workout fuel calculator applies these numbers

Mavr runs the same math from your training calendar, then turns the result into a plan you can follow mid-workout. These are the inputs it uses:

• It plans the key session first. When a long run is coming up, that is where fueling mistakes cost the most.
• Targets are rounded to real servings (gels, chews, drink mix), because a precise number you cannot follow when tired does not help.
• The first dose and the interval are set for you, so you follow "take this at minute X" instead of doing math mid-run.
• On long sessions, the fuel shows up as a "Workout Fueling" entry in your day plan, next to your meals, so it is not forgotten.
• An estimated duration is fine. The plan still works when the workout length is a guess.

Hydration works the same way. Two runners at the same pace can lose very different amounts of sweat and sodium. If you have done a sweat test, Mavr uses it to set your fluid and sodium ranges. Without one, you still get sensible default ranges.

Frequently asked questions