The Ultimate Guide and Calculator: How Many Gels for a 4 Hour Marathon?

November 14, 2025 •

4 min read

Research indicates that up to 40% of runners experience a debilitating energy crash, or 'hitting the wall,' during a marathon due to improper fueling. Knowing exactly how many gels for a 4 hour marathon is crucial for sustaining energy, and this guide provides the calculation and strategy you need to finish strong.

Quick Summary

This guide outlines a calculation for determining the optimal number of energy gels needed for a 4-hour marathon. It covers the necessary carbohydrate intake per hour, proper fueling timing, different gel types, and the vital role of hydration and training practice to maximize performance and prevent energy depletion during the race.

Key Points

Calculate Your Needs: Aim for 60-90 grams of carbohydrates per hour of running, tailoring the number of gels to their specific carb content (e.g., 2-3 standard gels or 1-2 high-carb gels per hour).
Start Early: Begin fueling 15-30 minutes into your race, not when you feel fatigued, to avoid depleting your body's glycogen stores.
Time Consistently: Take a gel every 20-45 minutes throughout the marathon to maintain a steady energy supply.
Hydrate With Gels: Consume most gels with water to aid digestion and absorption, especially classic, non-isotonic varieties.
Practice In Training: Never use new gels or a new fueling strategy on race day. Practice your entire plan during long training runs.
Use the Right Gel: Experiment with different gel types, such as isotonic, caffeinated, or high-carb versions, to find what works best for your body and race strategy.
Consider Other Factors: Adjust your plan based on race day conditions (e.g., temperature) and personal factors like body size and gut tolerance.

The Science Behind Marathon Fueling

For endurance events like a marathon, your body relies on glycogen—a form of stored carbohydrates in your muscles and liver—for primary fuel. A full tank of glycogen typically lasts for about 60 to 90 minutes of running at a moderate to high intensity. A 4-hour marathon pushes well beyond these reserves, necessitating a strategic and consistent intake of carbohydrates throughout the race to prevent fatigue. Your goal is to replenish your glycogen stores as you deplete them, delaying or preventing the infamous 'wall.'

Modern sports nutrition research recommends that endurance athletes, such as marathon runners, consume 60 to 90 grams of carbohydrates per hour. The exact amount depends on factors like body size, training level, and individual gut tolerance. Energy gels are a highly effective delivery system for these carbohydrates, offering a concentrated, fast-acting energy source that is easy to consume on the run.

The 4 Hour Marathon Gel Calculator

To determine how many gels you'll need for a 4-hour marathon, you first need to identify the carbohydrate content of your preferred gel. While most standard gels provide 20 to 25 grams of carbs, some high-carb gels contain 30 to 50 grams per serving.

Step 1: Set Your Carbohydrate Intake Goal

First, choose your target carbohydrate intake per hour. For most 4-hour marathon runners, aiming for 60 grams per hour is a solid starting point. More experienced runners might push this closer to 90 grams per hour. Let's use 60g/hr for our base calculation.

Step 2: Calculate Gels Needed per Hour

Next, divide your hourly carb goal by the carb content of your gel.

Example (Standard 25g gel): 60g (goal) / 25g (per gel) = 2.4 gels per hour.
Example (High-Carb 40g gel): 60g (goal) / 40g (per gel) = 1.5 gels per hour.

Step 3: Calculate Total Gels for the Race

Finally, multiply the gels needed per hour by the 4-hour race time. It's also wise to take an extra gel about 15 minutes before the start to top off your stores.

Using Standard Gels: 2.4 gels/hr x 4 hours = 9.6 gels. Adding the pre-race gel, you'll need approximately 11 gels.
Using High-Carb Gels: 1.5 gels/hr x 4 hours = 6 gels. Adding the pre-race gel, you'll need approximately 7 gels.

Gel Calculation Summary

For 60g Carbs/Hour: 9-11 gels (Standard) or 6-7 gels (High-Carb).
For 75g Carbs/Hour: 12-14 gels (Standard) or 7-8 gels (High-Carb).
For 90g Carbs/Hour: 14-16 gels (Standard) or 8-10 gels (High-Carb).

Comparison of Energy Gel Types

Your choice of energy gel is critical. Different formulas have different characteristics that can impact your fueling strategy and stomach comfort. Always test different brands and types during training to see what works best for you.


Feature	Classic Gels	Isotonic Gels	Caffeinated Gels
Carb Content	20-30g	20-25g	20-30g
Consistency	Thick, syrupy	Thinner, more liquid	Varies (check label)
Water Needed?	Yes, absolutely	No, pre-mixed with water	Yes, usually
Electrolytes	Varies by brand	Often included	Varies, usually higher
Performance Benefit	Standard energy boost	Easier to digest, less mess	Added mental alertness
Potential Downside	Can cause GI distress without water	Bulkier packets due to water content	Can cause jitters; test in training

Practical Fueling Strategy for Race Day

Once you have your number of gels, the next step is planning your intake schedule. The most effective approach is to fuel proactively rather than reactively—don't wait until you feel drained.

Pre-Race: Take your first gel about 15 minutes before the starting gun, especially if you had an early breakfast.
Early Stages (0-60 mins): Take your next gel about 20-30 minutes into the race. This prevents your glycogen stores from dipping too early. Take with water.
Mid-Race (60-180 mins): Continue taking a gel every 25-45 minutes. Mix up flavors if you get flavor fatigue. Use race aid stations for water.
Late Stages (180 mins+): Consider switching to a caffeinated gel for a mental boost in the final hour. Keep hydrating consistently, even if you don't feel thirsty.

The Importance of Hydration and Electrolytes

Consuming gels alone is not enough. Proper hydration is critical for your body to process and absorb the carbohydrates effectively. Many GI issues during a marathon are caused by inadequate hydration rather than the gels themselves. Most gels (except isotonic) require water to prevent them from sitting too heavily in your stomach. Drink small amounts frequently, not large quantities at once. If it's hot, you may need more electrolytes, found in sports drinks or salt tablets, to replace minerals lost through sweat.

Train As You Race

This entire strategy is useless without practice. The golden rule of marathon nutrition is to never try something new on race day. Use your long training runs to experiment with your chosen gels and timing schedule. Test different flavors and see how your stomach tolerates them. This practice helps train your gut to absorb carbohydrates efficiently under race conditions, building confidence in your fueling plan. Remember, the numbers from any calculator are a guide, and your body's specific response is the ultimate determining factor.

Conclusion

A successful 4-hour marathon fueling plan involves more than just a quick calculation. By understanding your hourly carbohydrate needs (60-90g/hr), choosing the right gel type, and practicing your timing and hydration strategy diligently during training, you can ensure a steady energy supply to power you through all 26.2 miles. Listen to your body and adjust as needed, but start with a solid, calculated plan to avoid the energy pitfalls that sideline so many runners. For further reading on performance, see this NIH study on carbohydrates and endurance performance.

Frequently Asked Questions

For a 4-hour marathon, runners typically need between 8 and 12 gels, depending on the carbohydrate content of each gel and their individual fueling strategy. You should also take one extra gel 15 minutes before the start.

You should take your first gel early, about 15-30 minutes into the race, to begin replenishing your glycogen stores before they are significantly depleted.

Yes, unless you are using an isotonic gel. Most gels are highly concentrated and require water to help your body absorb the carbohydrates efficiently and prevent stomach discomfort.

Classic gels have a thicker, more syrupy consistency and must be taken with water. Isotonic gels are pre-mixed with water and can be consumed without extra fluid, which can be easier on the stomach.

Not taking enough gels can lead to depleting your glycogen stores, resulting in extreme fatigue known as 'hitting the wall'.

Taking too many gels, or taking them without enough water, can cause gastrointestinal distress, such as cramping, bloating, and nausea. Listen to your body and stick to a practiced intake schedule.

Caffeinated gels can provide a mental boost during the later stages of a race. However, you should test them in training first to ensure your body tolerates the caffeine well.

Practice your race day fueling strategy, including your chosen gels and hydration plan, during your long training runs to train your gut and avoid race day surprises.