 This enabled U.S. President Jimmy Carter to note in a speech in 1977 that in each of the previous two decades the world had used more oil than in all of previous history (The roughly exponential growth in world oil consumption between 1950 and 1970 had a doubling period of under a decade).Given two measurements of a growing quantity, "q"1 at time "t"1 and "q"2 at time "t"2, and assuming a constant growth rate, you can calculate the doubling time as

A constant relative growth rate means simply that the increase per unit time is proportional to the current quantity, i.e.the addition rate per unit amount is constant.