Half marathon fueling calculator. Enter your goal time to calculate carbs per hour, total carbs, and exactly how many gels you need on race day.
Enter your half marathon goal time and get your exact fueling numbers — carbs per hour, total carbs, and gel count. No guesswork, just the math.
Use your goal finish time to get personalized carb and gel numbers for race day.
It gives you a solid starting point based on proven 60-90g/hr guidelines. Fine-tune by practicing in training — every gut is different.
Faster efforts burn through glycogen quicker, but the window is shorter. A 75-minute half needs fewer total carbs than a 2.5-hour half, even if the rate is similar.
| Race | Finish time | Carbs/hour | Total carbs | Calories | Gels |
|---|---|---|---|---|---|
| Half marathon | 1:20 | 45 g/hr | 60 g | 240 kcal | 2 |
| Half marathon | 1:35 | 45 g/hr | 71 g | 284 kcal | 3 |
| Half marathon | 1:50 | 45 g/hr | 83 g | 332 kcal | 3 |
| Half marathon | 2:05 | 45 g/hr | 94 g | 376 kcal | 4 |
| Half marathon | 2:20 | 45 g/hr | 105 g | 420 kcal | 4 |
| Marathon | 2:50 | 75 g/hr | 213 g | 852 kcal | 5 |
| Marathon | 3:15 | 75 g/hr | 244 g | 976 kcal | 5 |
| Marathon | 3:45 | 75 g/hr | 281 g | 1124 kcal | 6 |
| Marathon | 4:15 | 75 g/hr | 319 g | 1276 kcal | 7 |
| Marathon | 4:45 | 75 g/hr | 356 g | 1424 kcal | 7 |
| Marathon | 5:15 | 75 g/hr | 394 g | 1576 kcal | 7 |
Every page on this site has a Markdown mirror at the same path with a .md suffix (for example /calculator.md), and honors an Accept: text/markdown request header on the HTML URL. The calculations here are also available as callable tools through a free public MCP server — no account or API key.