Running Carbs Calculator
Carbohydrate Calculator
How it works: The Science of Fueling
Input the total amount of food and drink you plan to carry or consume during your activity. The calculator will automatically divide it by your estimated running time to give you your exact hourly intake!
During endurance sports, your gut uses two main transporters to absorb carbohydrates:
1. SGLT1 (Glucose): Maxes out at roughly 60 g/h.
2. GLUT5 (Fructose): Maxes out at roughly 45 g/h.
If you only consume glucose (e.g., white bread, standard pasta), you can only absorb 60g per hour. By mixing glucose and fructose, you open both doors simultaneously. The ratio of your sports drink determines your maximum absorption:
- 1:0.8 Ratio: The modern optimal. Maximizes both transporters, allowing up to 105 g/h safely.
- 2:1 Ratio: The classic standard. Maxes out the glucose door first, limiting total intake to roughly 90 g/h.
- 1:1 Ratio: Good for gut comfort at lower hourly intakes, but the fructose door will max out well before the glucose door.
Don't forget the key: Sodium!
SGLT1 stands for Sodium-Glucose Linked Transporter. This door acts like a dual-key lock: it requires one molecule of sodium to absorb one molecule of glucose. Read more below!
Pro tip: Remember to calculate and add the sodium from your solid foods and aid station snacks to the "Extra Sodium" field below!
Want to map this nutrition plan to your race route?
Now you know your optimal hourly carb and sodium needs. Take the next step and plan your exact fueling strategy from aid station to aid station with the ApexPace App.
Get ApexPace NOW! Read more and CLICK HERE!Fuel is an Ultrarunner's Most Important Gear – How to Nail Your Nutrition
Trail and ultra running are brutally honest sports. When distances stretch into dozens or hundreds of kilometers and hours turn into days, physical fitness or sheer willpower is no longer enough. Sooner or later, you will hit the wall if you don't feed your engine the right fuel.
Endurance events are a balancing act: your body burns a massive amount of energy, but your gut's ability to absorb it is limited. Many beginners (and veterans alike) have experienced that dreaded moment when their stomach bloats, energy stops absorbing, and progress halts due to nausea. This is often called "runner's gut."
Usually, the problem isn't eating too little—it's eating the wrong way.
The Two Doors of Your Gut
When you eat or drink carbohydrates on a run, they need to get from your intestines into your bloodstream, and finally to your muscles. You can think of your gut as a nightclub with two entrances:
The Glucose Door (SGLT1): This is the main entrance. It's large and efficient, but it can only let in a maximum of about 60 grams of carbohydrates per hour.
The Fructose Door (GLUT5): This is the side entrance. It can handle about 45 grams per hour.
If you only drink traditional sports drinks or eat white bread (which convert almost 100% into glucose in your body), all the energy tries to squeeze through the main door. If you exceed that 60-gram limit, a bottleneck forms. The energy stays in your gut, drawing in water, and the end result is nausea and GI distress.
The solution is brilliant but simple: open both doors.
By combining glucose and fructose (optimally in a 1:0.8 ratio), you split the traffic between the two doors. This allows you to safely absorb up to 90–105 grams of carbohydrates per hour without stomach issues. That means more power in your legs and a clearer head for those final miles.
The Missing Key: Sodium
Did you know that the glucose door (SGLT1) actually stands for Sodium-Glucose Linked Transporter? This door functions like a dual-key lock. To pull one molecule of glucose into your bloodstream, it requires one molecule of sodium. If you are drinking plain water with your carbohydrates and sweating out your body's salt reserves, your glucose door won't open, leading to an immediate gut disaster. Always make sure your fueling plan includes adequate sodium to unlock the doors!
The Rule of Thumb for Sodium Sodium needs vary wildly from person to person based on sweat rate, genetics, and weather. However, a safe starting point for ultra runners is:
Baseline: Aim for 300–600 mg of sodium per hour.
Hot conditions or "salty sweaters": You may need 800–1000+ mg of sodium per hour.
Important Math: Do not confuse sodium with table salt (NaCl). Table salt is only about 40% sodium. To get 400 mg of sodium, you need to consume exactly 1 gram of table salt. Always check the labels on your sports drink or salt capsules for the sodium content, not just "salt."
Liquids vs. Solid Foods
On paper, it's easy: just mix the right sports drink in your flask and sip it like clockwork. But the reality out in the woods is different. After hours of slogging through rain or scorching heat, you might start craving solid food. You want to chew something. You might crave salty bread or sour candies.
This is completely fine and, in long ultras, even recommended to keep your spirits high! The challenge is that solid food immediately alters your carefully planned glucose and fructose balance. For example, white bread is pure glucose, whereas the carbohydrates in candies are a mix of mostly glucose and some fructose.
So, how do you know how many slices of bread or how many candies you can eat alongside your sports drink without clogging your gut's doors?
Plan Your Fueling Smartly
To solve this puzzle, I built the carbohydrate calculator below.
It allows you to experiment with different combinations of sports drinks and solid foods. The calculator shows you in real-time how much load you are placing on each of your gut's doors. The goal is simple: keep both bars in the green zone.
Feel free to test different options, find the combo that suits your taste, and always remember to test your fueling plan in practice during long training runs before race day!
Official Partnership: Co-Developed with Pogosta Ultra Trail
We are thrilled to announce that this calculator is continuously optimized and co-developed in official partnership with Pogosta Ultra Trail. Featuring a spectacular lineup of races through the remote wilderness of Eastern Finland—culminating in the ultimate, mind-bending 328 km extreme endurance distance—Pogosta is the perfect real-world testing ground for complex pacing, rest, and sleep strategies. Together, we are refining this tool to help runners successfully conquer the toughest terrains and smartest cut-off plans.
