Data journalism website FiveThirtyEight's marathon calculator is a helpful tool for runners looking to complete their first 42 kilometer race. The marathon calculator, built into an article on the site entitled 'How Fast Would You Run A Marathon?,' attempts to predict users' end times based purely on their training and race history.
Marathons are especially difficult for runners to prepare for, not only because they're grueling in and of themselves, but also because the distance means that runners can't simply run a full one in training. This means that, no matter how much training one puts in, they still can't make a confident prediction about their race time in the same way they could with a 10K, for example.
By taking as input a user's average weekly mileage in training and the times in their last two races, the marathon calculator can make a time prediction, helping runners to pace themselves correctly.