Wikipedia's Kalpa(aeon) in Buddhism article says,
In another simple explanation, there are four different lengths of kalpas. A regular kalpa is approximately 16 million years long (16,798,000 years
), and a small kalpa is 1000 regular kalpas, or about 16 billion years. Further, a medium kalpa is roughly 320 billion years, the equivalent of 20 small kalpas. A great kalpa is 4 medium kalpas, or around 1.28 trillion years.
I just went through the Visuddhimagga, and could not find where or how that time in years is calculated.
I was wondering how to calculate it. Please explain how it's calculated, with reference to scripture where Lord Buddha mentioned it, and/or where the calculation or the result of the calculation is described, perhaps in the Thripitaka or Visuddhimagga?
 Wikipedia's reference is to a book:
Epstein, Ronald B.(2002). Buddhist Text Translation Society's Buddhism A to Z p. 204. Buddhist Text Translation Society. ISBN 0-88139-353-3, ISBN 978-0-88139-353-8.
Perhaps that book may have a reference to where they found it, but I still haven't find that book.