It's actually quite complicated. I think I've worked it out for the latest generation but someone who knows more about population statistics can probably work it out better than I can. I'll have a think about how to work out how to extend the principal beyond the newest generation.
You'd need the attendance figures to work from but as a rough guide I'd go with in any given non PL season we have about 60% of the average attendance as season ticket holders, and say 3-5% opposition fans on average?
The remaining 35% will be mainly people that it's not worth buying a season ticket for, I don't know if it's always been the case but that's about 2/3 of the games. There'll be some that got over that figure but far more will be below it because of people who only get to a game or two a season. If we say the average extra fan attends 1/2 of the games then the total number of unique fans through the gates for the season will be 2/3 of the average attendance plus the season ticket holders.
So last season with over 21k average and 14k season tickets would be a total of 28,000 people attending our home games in the home end.
Extending it over several non PL years you can be pretty certain that almost every fan who's attended a game at the KC as one of our fans attended at least one game last season. There'll have been a few who've died or emigrated (not people like me who've moved away but can come back regularly), but they'll be minimal. Equally when we got promoted from D3 with 17k average I think it's fair to say that almost everybody who'd been to a game there or at Boothferry Park in the previous years of sub 6k attendances would have been to a game at the KC.
At that point you're looking at about 30-35,000 in 15-20 years. If we call it 35,000 in 20 years that's the latest measurable generation. The vast majority of fans attending with their fathers to start with and then with friends as they get older means it should be a fairly stedy number in each generation, kids that go off the rails and glory hunt being replaced by people like me who support the club with no family link due to friends going in around even numbers.
NB: Because we had 21,000 season ticket holders and most of the additional tickets available in the PL were reserved for membership holders (with 2,500 for away fans) and regular home attendance by away fans it's unlikely we had over 28,000 attending as home fans during the PL seasons, though I've no doubt that some of these people haven't returned since it's going to be a negligible error compared to all the others that come in when trying to apply it to the entire history of the club.
