Pretty late to the party on this one but thought it was worth adding that the JavaScript behind the runners world calculator is pretty simple. Looking at the function toolSubmit (which is called when the "Calculate" button is clicked) it's clear that it calculates calories burned as:
calories burned = kilometersdistance run (kilometres) x weight of runner (kilograms) x 1.036
This is rounded to a whole number and then formatted as the message displayed in the browser.