With three simple numbers you can forecast your worship attendance one year from today. It's quite easy, and surprisingly reliable.

But before we talk about how … let's consider why. Is there value in looking into the future? Or, as Marty McFly discovered, is it just plain trouble to mess with Father Time?

Actually, I am a firm believer in forecasting for the simple reason that if our forecast indicates a potential problem, we can do something to keep it from happening. For example, suppose your forecast indicated that your worship attendance would be down by 10 percent in one year. If you could do something to prevent that situation, wouldn't you? I hope so.

So, let's look at how we can cheat the calendar and peek into the future. It requires three numbers: your visitor volume, your visitor retention, and your back door.

Visitor volume—the number of visitors/newcomers at your church services, as a percentage of your total attendance. To calculate this, add all the first-time visitors who attended a service at your church in the past year, then divide by the total number of persons in attendance (including visitors). The result will be the average percentage of visitors at your services—this is your visitor volume. (Growing churches, by the way, average 4-5 percent.)

Visitor retention—the percentage of your visitors who become involved following their first visit. To calculate, list each person who has visited your church in the past 6-18 months. Then determine how many of them are now regular attenders. Divide the number of involved visitors into your total number of visitors. The result will be your visitor retention rate. (Growing churches average 18-21 percent.)

Back door—the percentage of your total constituency who leave. Simply identify the number of people who stopped attending last year for any reason (transfer, death, inactivity, etc.). Divide this by your present constituency. The result will be your back door rate. (Growing churches average 5-8 percent.)

Once you have these three numbers, take your present attendance and project it for next year. Multiply your present average attendance by your visitor volume to get the average number of visitors you can anticipate. Multiply that number times your visitor retention rate for the number of newcomers who are likely to stay. Subtract the number of people expected to leave. The result is your projected attendance one year from today.

For a stimulating exercise: gather your church leaders together to "play with the numbers." First show the projections of where present trends will take you. Then ask, "Is this where we believe God would have us be in one year?" If the consensus is "no," ask, "Then what do we need to change?" The controllable variables are visitor volume, visitor retention, and the back door. What happens on your spreadsheet if you add one person per week to visitor volume? What if you doubled your visitor retention? Or halved the back door rate?

Such a conversation will lead to a thoughtful exploration of the past, a critical evaluation of the present, and a stimulating vision of what future changes might lead to more effective stewardship of the people God has put under your spiritual care.