Running Fueling Calculator
Work through the questions below. You'll get a target for carbs, drink, sodium and caffeine, plus a schedule you can follow on the run.
Free to use.
What are you fueling for?
A race asks more of your stomach than a training run.
How hard will you run?
Easy means you can talk. Steady is a strong long run. All out is race effort.
How long will you be running?
At least 1 hour.
What will the weather be like?
Cool is under 10°C (50°F). Hot is over 25°C (77°F). Heat means you need more to drink.
How well does your stomach handle gels and drinks?
Pick what you have already managed on a run without feeling sick. Choose 90g+ only if you regularly take more than 90 grams of carbs an hour.
Optional. Used only to suggest a caffeine amount. Skip it if you prefer.
Will you use caffeine?
Coffee, a caffeine pill, or a caffeinated gel all count.
How much caffeine do you usually have?
Rarely means almost never. 1–2 coffees is a normal daily habit. 3+ includes caffeine gels or pills you already use.
Fueling basics
Carbs per hour, gels, and fluids.
How many carbs per hour running?
The amount of carbohydrates you need per hour while running depends on your intensity, duration, and gut tolerance. Current sports science recommends 30–60g per hour for moderate efforts lasting 1–2.5 hours, and 60–90g per hour for high-intensity efforts longer than 2.5 hours. In this calculator, Low tolerance is capped at 70g/hr, Moderate at 80g/hr, High at 90g/hr; only Elite (extensively gut-trained, dual-carb protocols) allows plans above that, up to about 120g/hr when other inputs support it.
The hourly number went up once research showed that glucose and fructose use separate intestinal transporters — SGLT1 for glucose and GLUT5 for fructose. Combining both sugars, typically in a 2:1 ratio, lets you absorb more carbohydrate per hour without overwhelming either pathway. For most recreational runners, 40–70g/hr is a realistic target. The calculator above personalises this number from your inputs.
What to eat during a marathon
Once you're moving, the menu is short. These are the options that hold up when you're figuring out what to eat during a marathon:
Energy Gels
The most popular choice; compact, pre-measured, and fast-absorbing. Look for glucose-fructose blends (2:1 ratio) for maximum absorption. Always take with water, not sports drink.
Energy Chews
Clif Bloks, Skratch Chews, and similar products offer a chewable alternative. Good for runners who dislike gel texture. Easier to control portion size.
Sports Drinks
Serve double duty, providing carbohydrates and fluids simultaneously. Products like Maurten Drink Mix or Gatorade Endurance can supply 30–45g carbs per 500ml.
Real Food
For ultra-distances and longer marathons: banana pieces, rice balls, boiled potatoes with salt, or PB&J sandwich quarters. Slower to digest but easier to face late in a long race.
The rule that matters: never try new products on race day. Test everything on training runs first. Whatever you choose, combine glucose and fructose sources so absorption keeps up.
How often to take gels during a marathon
Take your first gel at 30–40 minutes into the race, not at the start. You still have glycogen from the pre-race meal. After that, keep a steady rhythm:
- Standard gels (22g carbs): Every 20–25 minutes for a target of 60g/hr
- Medium gels (25g carbs): Every 22–28 minutes
- High-carb gels (40g carbs): Every 30–45 minutes
Consistency matters more than a perfect interval. Set a recurring timer on your watch if you tend to forget. Take gels with water, not sports drink — stacking the two raises the sugar concentration and is a common cause of stomach trouble. Spread intake across the race instead of front-loading or saving gels for later.
What changes the numbers
Intensity
Higher intensity redirects blood away from the digestive system to working muscles, reducing gut absorption capacity. All-out race efforts demand more carbs but are harder to digest, making gut training even more important.
Duration
Efforts under 90 minutes can rely mostly on stored glycogen. Beyond 2 hours, carbohydrate you take in becomes critical. For marathon-distance races and longer, a steady fueling plan is what keeps you off the wall.
Weather & Heat
Hot conditions increase sweat rate by 50–100%, dramatically increasing fluid and sodium needs. Heat also reduces gut blood flow, meaning you may need to slightly reduce carb intake while significantly increasing fluids.
Sweat Rate
Individual sweat rates range from 0.5 to 2.5 litres per hour. Heavy sweaters need proportionally more fluid and electrolytes. A simple pre/post-run weigh-in reveals your personal sweat rate.
Gut Tolerance
The gut is the limiting factor in endurance fueling, not fitness. Research shows 30–90% of endurance athletes experience GI issues during races, usually from inadequate gut training or trying unfamiliar products. This tool caps hourly carbs by tier: Low ~70g/hr, Moderate ~80g/hr, High ~90g/hr. Choose Elite only if you have trained specifically to absorb over 90g/hr using glucose-fructose blends.
Caffeine
Caffeine improves endurance performance by 2–4% through reduced perceived effort and delayed fatigue. A useful dose is 3–6mg/kg taken 45–60 minutes before you start. More is not better — side effects pick up above 6mg/kg.
Marathon fueling
How much you'll actually need across 26.2 miles.
How many carbs do you need for a marathon?
A marathon (42.195 km / 26.2 miles) typically takes 3–5 hours for most runners, so total carbohydrate needs range from 120g to 450g+ depending on pace and intensity. Your body stores only about 300–400g of glycogen in muscle and liver, enough for roughly 90–120 minutes of hard running.
A 4-hour marathon at 60g/hr is about 240g of carbohydrate during the race, before you count the pre-race meal or carb-loading days. Carb loading at roughly 7–10g per kg per day for 2–3 days beforehand (often ~8–9g/kg when appetite and gut allow) tops up those stores and pushes "the wall" later, usually somewhere around mile 18–22 (km 29–35). In-race fueling and the days before the race both matter.
How many gels for a marathon?
It depends on the gel size and your hourly carb target. For a 4-hour marathon at 60g per hour (240g total), how many gels for a marathon looks like this:
| Gel Type | Carbs Per Gel | Total Gels Needed | Take Every |
|---|---|---|---|
| Standard (GU, Clif) | ~22g | ~11 gels | ~22 min |
| Maurten Gel 100 | ~25g | ~10 gels | ~24 min |
| High-Carb (Maurten 160, SiS Beta Fuel) | ~40g | ~6 gels | ~40 min |
Most runners use a combination of gels and sports drinks to hit their carb targets. For example, sipping a carb-rich sports drink throughout and supplementing with gels every 30–45 minutes reduces the total number of gels needed while maintaining intake, and is often easier on the stomach.
Marathon fueling plan example
Here's one worked marathon fueling plan example for a 4-hour marathon (70 kg, moderate intensity, moderate gut tolerance, warm weather, caffeine):
| When | What to Do | Details |
|---|---|---|
| 3-4 hrs before | Pre-race meal | 2-3g/kg carbs: bagel with jam, banana, sports drink (~150-200g carbs) |
| 45 min before | Caffeine | 200mg caffeine (coffee or caffeine gel) |
| 15 min before | Final prep | Sip 150-200ml water or sports drink |
| 0:30 | First gel | 1 gel (22-25g carbs) + 150ml water |
| 0:50 | Gel #2 | 1 gel + 150ml water |
| 1:10 | Gel #3 | 1 gel + sip sports drink |
| 1:30 | Gel #4 | 1 gel + 150ml water |
| 1:50 | Gel #5 + caffeine | 1 caffeinated gel (mid-race boost) + water |
| 2:10 | Gel #6 | 1 gel + 150ml water + electrolyte tab |
| 2:30 | Gel #7 | 1 gel + sip sports drink |
| 2:50 | Gel #8 | 1 gel + 150ml water |
| 3:10 | Gel #9 | 1 gel + 150ml water |
| 3:30 | Final gel | Last gel for the final push + water |
This plan delivers approximately 55–60g carbs/hr, 500–600ml fluid/hr, and 400–500mg sodium/hr, well within evidence-based guidelines. Use the calculator above to generate a plan customised to your exact situation.
Practising your strategy
Long runs and gut training.
Fueling during long runs
Fueling during long runs is practice, not just calorie replacement. Your gut adapts. Runners who regularly take carbs on the run can raise how much carbohydrate they absorb by something like 20–40% over 6–8 weeks.
Start around 20–30g/hr on the weekly long run, then creep toward your race-day target. That practice upregulates the SGLT1 and GLUT5 transporters in the gut, so more carbohydrate gets through without the bloating, cramping, or nausea.
Skip that practice and then try 60–90g/hr on race day, and the stomach usually objects. The usual cause of race-day stomach trouble is a lack of rehearsal, not the gel itself. Treat long runs as dress rehearsals for the plan you just calculated.
Common questions
How many carbs per hour do I need for running?
For easy runs under 90 minutes, 0–30g/hr is typically sufficient. For moderate to hard efforts lasting 1–2.5 hours, aim for 30–60g/hr. For marathon-distance races and beyond (2.5+ hours), the current recommendation is 60–90g/hr for most runners. In this calculator, Low tolerance caps at 70g/hr, Moderate at 80g/hr, High at 90g/hr. Athletes with extensively trained guts may exceed 90g/hr (often up to ~120g/hr) using glucose-fructose blends — select Elite for that tier.
How many gels should I take during a marathon?
For a typical marathon with a target of 60g carbs/hr over 4 hours, you'd need approximately 10–11 standard gels (22g each), 9–10 medium gels (25g each), or 6 high-carb gels (40g each). Most runners combine gels with sports drinks to reduce the total number of gels needed and ease digestion.
How often should I take gels during a marathon?
Take your first gel at 30–40 minutes into the race, then every 20–30 minutes for standard gels (22g carbs) or every 30–45 minutes for high-carb gels (40g carbs). The goal is evenly distributed carbohydrate intake throughout the race. Always take gels with water, not sports drink ,to optimise absorption and reduce stomach issues.
What should I eat during a marathon?
The most popular options are energy gels (GU, Maurten, SiS), energy chews (Clif Bloks, Skratch), and sports drinks (Maurten Drink Mix, Gatorade Endurance). For ultra-distance, real food like banana pieces or rice balls also works. Look for products containing a glucose-fructose blend (typically 2:1 ratio) for maximum absorption. The most important rule: never try anything new on race day.
Do I need to fuel during a half marathon?
For half marathons taking longer than 75–90 minutes, yes, fueling can meaningfully improve your performance. Even for faster runners (sub-90 minutes), a gel at 30–40 minutes can help maintain intensity in the final miles. The shorter the race, the less critical in-race fueling becomes, but taking in some carbohydrate rarely hurts performance.
How much water should I drink while running?
Aim for 400–800ml per hour as a baseline, adjusted for weather, sweat rate, and intensity. In hot weather, you may need up to 1,000ml/hr. In cool conditions, 300–500ml/hr may suffice. Sodium / electrolytes are optional but recommended for efforts over about 60 minutes, especially if you sweat a lot or race in heat, to support performance and reduce hyponatraemia risk; adjust to your needs and any medical advice.
Should I use caffeine during a race?
Caffeine can improve endurance performance by 2–4% through reduced perceived effort and enhanced fat oxidation. The recommended dose is 3–6mg per kg of body weight, taken 45–60 minutes before the race. If your event lasts longer than 2 hours, a smaller supplemental dose mid-race can extend the effect. Always test caffeine tolerance in training first, some individuals experience GI issues, anxiety, or elevated heart rate.
What is gut training for runners?
Gut training is the practice of consuming carbohydrates during training runs to condition your digestive system for race-day fueling. By regularly eating and drinking during long runs, you upregulate the intestinal transporters (SGLT1 and GLUT5) that absorb glucose and fructose, increasing your tolerance and absorption capacity over 6–8 weeks. Runners who skip gut training commonly experience nausea, bloating, and cramping when trying to fuel during races.
Fueling is only one part of race day
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