Let's discuss a simple example to make things clear. Mark went on a hike last weekend, which lasted for 12 miles (19.3 km) in total (so 6 miles one-way) and featured a 2500 ft (762 m) elevation gain. Mark is pretty slim himself, weighing 150 pounds (68 kg) and carrying 18 pounds (8.15 kg) of backpack on his back. As you can see from the calculator, the trail averaged around 7.9% grade (remember to use the one-way distance — 6 miles).
Now that we have those numbers, we can calculate the number of calories burned on the way up and down. First, if Mark were to leave his backpack before the trail start, he would burn roughly 924 calories on the way up and 533 on the way down. In total — 1457 calories. Add the backpack, and the numbers go up — 998.5 kcal for going up and 575.5 kcal down. In total, Mark should expect to burn around 1574 calories during his hike.
Now, note that we based those formulas on research done in a lab and not on a mountain slope. This means many outside factors were not taken into consideration, such as Mark's stamina and individual predispositions, weather conditions, trail structure, and speed of hike. Unfortunately, we simply don't have a sufficient amount of data, so we can only offer approximate values.
Regardless, it should work as an interesting way to check the calories needed for each trail and could help hikers better prepare for their trips. Use the tool responsibly, and don't forget to take some water and food with you, even if you're headed just for a short walk. Best of luck!
Not sure how much water to take on your hike? Our water intake calculator will help you with that.