TL;DR: Divide new members this month by total old members at start of month. Multiply by 100 to growth rate as a percentage. If >6%, you are probably at movement speed, will double in 18 months. If <6%, consider reviewing your systems and processes. If <4% over several months, maybe fizzled or plateaued, definitely review. If <2% over several months, danger: losing ground to population.
Part of the challenge with anything involving growth--whether it is subscriptions, purchases, new Bible Study attendance, baptisms, or church membership--is that not everyone will "act" in response to every opportunity. If you have a church of 1,000, or a Bible Study of 10, or a mailing list of 100 people, how rapid a growth rate do you need to have in order to achieve your goals? How do you know when you are growing fast enough, or when you have fizzled, or when you have plateaued? The old saw is, "If you started with 1 Christian, and each period (week? month? year?) every Christian made another Christian, the world would be completely Christian in (how many?) periods." The answer to how many at this rate of doubling is easy to calculate: 34 doubles are required. For a list of the doubles, see my post "Doubles." But, of course, not every Christian will see another (convert/disciple/whatever) every period. Only a certain percentage of "opportunities to become a disciple" will result in people who act on the opportunity. (Both the Crowds and the Rich Young Ruler left Jesus.) And movements will slow their doubling times over time. For a detailed discussion of this trend, see "and movements make movements." If we want a movement to transform a population, it must become a significant portion of the population. To do this quickly, it must be passing through the doubling stages faster than the general population. What growth rate is required for the movement to grow from 0% to, say, 60% of a given population? This is both a question of the target size (60% of whatpopulation?) and the speed of growth (how many new members are being added each month/year?). To find the speed of growth rate as a percentage, simply divide the new members by the old population and multiply by 100. Given this rate of speed, there is a complex formula for calculating doubling times (T=ln(2)/ln(1+r)), but the Rule of 72 is far simpler (but you actually use 69, not 72, for the most accurate results). Simply divide 69 by the % growth rate to get the number of years required to double. (You can easily calculate how many doubles you need for your movement to get from zero to the required population--usually about 18 to 20.) Let's make this practical. Say you have a church of 1,000 people. You are regularly seeing 2 or 3 baptisms per month. 2/1000=0.2%, and 72/0.2=360 months until you double. In 30 years, your church will have grown from 1,000 to 2,000 people. (We're assuming the 2 is "net"; we're not discussing gains vs. losses here.) We at Beyond typically measure Movements as those which get to 4 generations consistently and rapidly - and most judge this as "every 18 months." As a very rough estimate, consistently doing this means the movement will be doubling once a year. The Rule of 72, charted out, says you must be growing faster than 6% per month to double inside a year (<12 months). Since most normal populations are growing at rates of 1 to 2%, and perhaps 3%, I suggest anything slower than 6 per 100 needs to be seriously examined, and anything slower than 4 per 100 (which will double every 18 months) should be considered plateaued. Growth rates of less than 2 per 100 indicate a movement that has stalled out (or, if it never reached movement stage, a fizzle). All growth is in one sense good--the angels rejoice over any single sinner who repents. But if the existing systems in a movement are yielding growth rates of less than 6 per 100, the systems should be reconsidered. And if systems are yielding growth rates of less than 2 per 100, those systems are actually bad--not because they aren't yielding growth, but because they almost certainly losing ground to general population growth. When more people are entering hell than heaven because our systems aren't reaching them in time, we should certainly reconsider what we are doing.