The Rule of 72 is a quick if coarse way of measuring the time it takes a population to double.
First, you’ll need to figure out the growth rate of your population. This is pretty simple: if you’re calculating for a year, then it’s the newer population divided by the older population (e.g. pop2010/pop2009). If you’re calculating for a longer period of time (say a decade), then it’s: (new/old)^(1/time_period). So, for example, for a decade, it would be (new/old)^(1/10). (The ^ symbol is “to the power of”; any decent spreadsheet will do it.)
The Rule of 72 is: divide 72 by the population growth rate to get the number of years to double. If the growth rate is 4%, 72/4=18, so 18 years for the group to double in size.
If the doubling rate of the group is significantly faster than the doubling rate of the population, you’ll have a chance to double or quadruple in size in the same time the population does so once. For example, if the time-to-double is one-quarter that of the national population, it’s possible 4 “doubling generations” could be done in the same time the national population adds one. If the national population growth is 2%, you’d need growth of about 8 to 10% on a regular basis to reach those kinds of levels (72/2 = 36 years to double, 72/8 = 9 years to double.)
That sounds difficult, but 10% growth just means each small group of 10 adds one new member per year. That’s really not that remarkable a growth rate. Let’s say you start with a church of 100. At an 8% growth rate, by the end of a decade you would have a church of 200 (72/8=9 years). Keep that sustained, and by the end of 16 years you would have a church of 400 (in generation 2, the 200 doubled to 400). Do another decade and you will reach 800. Another decade to 1,600. So, at an 8% growth rate, in 40 years you have 1,600 people.
Now, let’s reverse-engineer. Let’s say you have 100,000 people, and you want to have them all be in a church in 40 years. What kind of growth rate would you need? It’s a fairly simple equation to run in a spreadsheet: you’d need a sustained 20% growth rate with very few dips over a 40 year period. How can you do that? Have everyone in a church in a small group, and each small group of 10 adds 2 new members every year, without fail. In 40 years, the church will have grown to 100,000 people.
Sounds easy, but in practice it’s not. The larger the organism gets, the harder it is for it to grow. This is why it’s very important, early on, to train, coach, and encourage all of your people to be making disciples and multiplying.