The calculator, from Christie Aschwanden and Ella Koeze at FiveThirtyEight, only asks you a few questions about your training, longest recent race, and your time. Once you answer each question, it will instantly give you an estimated run time using a specific equation that automatically incorporates fatigue (you can read about the formula in-depth here).
If you have a marathon coming up, knowing your estimated time can help you plan out your pacing so you don’t hit a wall too early. It also gives you an opportunity to up your training regimen if your current runs aren’t lining up with your goals. You can try it out yourself at the link below.
Go check out the marathon calculator.