Begin with your current metrics (usually your website or mailing list metrics), don’t guess or make these up.
If you have a mailing list of 20,000 and only 2,000 open your emails, you would be foolish to make a membership projection greater than 2,000.
It’s also unlikely all 2,000 people who open their emails will take the action you want (10% to 20% is more realistic). And only a small percentage of these will continue taking that action (5% to 10%).
This is where a big audience of 20,000 whittles down to a small audience of 40 to 50 members. You can influence the percentages a little, but they’re relatively similar across most communities.
But then you have casual web traffic too.
If web pages in similar locations get 5,000 unique clicks per day, you might assume 1% of them will register and 10% of them might be regulars (yet, that’s 5 new regular members per day).
Thus with a mailing list of 20,000 people and 1,000 visitors to similar pages per day, you might get 400 to 600 members after 3 months (and this allows for some churn).
Getting these projections right is important for three reasons:
1) Enterprise platforms lock clients into 3-year contracts based upon membership projections. Getting these projections right can save you a lot of revenue.
2) Your colleagues or boss might want to see big numbers comparable to existing communities. Push back with existing metrics and conversion rates. Making big projections to get internal support today will always cost you tomorrow.
3) If you want to hit higher targets, you have to change the fundamentals behind these metrics. This usually means investing more time in promotion, hiring a copywriter, improving the search rankings, positioning the community more prominently, or offering a really great reason for members to stay active.
p.s. Useful free resource on getting your best members to perform behaviors that matter.