Marathon & Running
How to Calculate Your Carb Needs for Any Workout
Stop guessing how many carbs you need. A simple formula based on workout type, duration, and intensity — no nutrition degree required.
Quick answer
To calculate carb needs for any workout, multiply your body weight in kg by the carb rate for your workout duration: under 60 minutes needs minimal carbs, 60-90 minutes needs 30-45g/hour, 90-150 minutes needs 45-60g/hour, and 150+ minutes needs 60-90g/hour. Add 1-2g/kg for pre-workout fueling 2-3 hours before. MAVR automates this calculation for every session in your training plan.
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:
| Workout Duration | Carb Target | Why | Practical Example (70kg runner) |
|---|---|---|---|
| Under 45 min | Not needed during | Glycogen stores sufficient | No fueling required |
| 45–60 min | Optional: 15–20g | Marginal benefit for very hard efforts | Mouth rinse or small gel |
| 60–90 min | 20–30g/hour | Glycogen starting to deplete | 1 gel + sips of sports drink |
| 90–150 min | 30–60g/hour | Glycogen depleting fast | 1 gel every 30–40 min + drink mix |
| 150+ min | 60–90g/hour | Glycogen nearly gone; dual-source needed | Dual-source fuel every 20 min |
How Intensity Changes Things
Duration is the biggest factor, but intensity matters too. Higher intensity burns more carbs per minute.
| Intensity | Carb Burn Rate | Fueling Implication |
|---|---|---|
| Easy / Zone 2 | 0.5–0.8g/min | Lower end of carb targets is fine |
| Moderate / Tempo | 0.8–1.2g/min | Use mid-range targets |
| Hard / Intervals / Race | 1.0–1.5g/min | Upper end of targets; start fueling early |
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
| Sport | Typical Session | During-Session Carbs |
|---|---|---|
| Running | Long run (90+ min) | 30–60g/hour |
| Cycling | Long ride (2+ hours) | 60–90g/hour (easier on the gut) |
| Swimming | Continuous (60+ min) | 30–45g/hour (hard to fuel while swimming) |
| Gym / Strength | 60–90 min session | Not usually needed during; pre-workout meal sufficient |
| CrossFit / Hyrox | 45–75 min competition | 20–30g/hour (high intensity, short duration) |
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.
Get personalized carb targets for every workout in your training plan.
Frequently Asked Questions
How many carbs do I need per hour of running?
For runs under 60 minutes, you generally do not need carbs during the run. For 60–90 minutes, aim for 20–30g/hour. For 90–150 minutes, target 30–60g/hour. For runs over 2.5 hours, aim for 60–90g/hour using dual-source carbs (glucose + fructose).
Do I need to eat carbs before every workout?
For workouts under 45 minutes at moderate intensity, a pre-workout meal is helpful but not critical. For anything over 60 minutes, or for high-intensity sessions, eating 1–2g carbs/kg 2–3 hours before makes a significant difference in performance.
How do I know if I'm eating enough carbs?
The clearest signs you're underfueling: you bonk on long runs, you feel heavy-legged in hard sessions, you're not recovering between sessions, or you're losing weight unintentionally. If any of these sound familiar, increase your carb intake by 20–30% and see how you feel.
Should I eat the same amount of carbs every day?
No. This is one of the biggest mistakes runners make. Your carb needs vary significantly based on training load. Eat more on hard and long days, less on rest and easy days. This is called periodized nutrition, and it is standard practice among elite endurance athletes.
Written by
MAVR Sports Nutrition Team
The MAVR sports-nutrition team translates peer-reviewed endurance research (Jeukendrup, Burke, the ISSN) into daily fueling for runners and triathletes.