You have reached a number for your market size. How do you check that it is reasonable?

I run two tests. I shrink the number to one household, and I rebuild it by a different route. Say I've sized US haircuts at $66 billion a year. I got there from 330 million people getting about 8 cuts a year, which is 2.64 billion cuts, at an average of $25. First, per household. $66 billion over 130 million households is about $510 a year, or about $42 a month. That's the size of a phone bill, not a rent payment, which is easy to believe for a home of two or three people. Second, another route. A town of 10,000 has maybe 25 stylists and barbers. That's 825,000 across the country. A stylist doing 12 cuts a day for 250 days does 3,000 a year. That makes about 2.5 billion cuts and, at $25, about $62 billion, which is close to my $66 billion. If something looked off, I'd go back and find the assumption that caused the gap.

I run three tests. I shrink the number to one household, I rebuild it by a different route, and I compare it with a ceiling it can't pass. Say I've sized US haircuts at $66 billion a year. I got there from 330 million people getting about 8 cuts a year, which is 2.64 billion cuts, at an average of $25. First, per household. $66 billion over 130 million households is about $510 a year, or about $42 a month. That's the size of a phone bill, not a rent payment, which is easy to believe for a home of two or three people. At $400 a month, I'd know I'd slipped a zero. Second, another route. I built it from customers, so now from the people holding the scissors. A town of 10,000 has maybe 25 stylists and barbers, a handful of shops with a few chairs. That's 825,000 across the country. A stylist doing 12 cuts a day for 250 days does 3,000 a year. That makes about 2.5 billion cuts and, at $25, about $62 billion, which is close to my $66 billion. Both routes use the $25, so I test it alone. A quick chain trim costs less and a salon cut costs more, so $25 across men, women and children is fair. And 3,000 cuts at $25 is $75,000 a stylist before rent and supplies, a modest living that fits the job. Third, an upper bound. Suppose all 330 million people paid $100 a month for cuts. That's $396 billion. My $66 billion is about a sixth of that ceiling. Above it, an input must be wrong. If something looked off, I'd go back and find the assumption that caused the gap. Had the routes disagreed, I wouldn't have averaged them. Here the input I trust least is the $25, since salon cuts could lift the average. So I'd say $60 billion to $85 billion.