Suppose Nike wants to start a subscription shoe service. How would you size the market opportunity and validate it?

Clarification -

Subscription shoe service - 2 possible options - either after sales subscription shoe service or subscription based shoe buying - assumption is after sales subscription based shoe service where customers pay a fixed fee to get after sales support

Any specific country - United Kingdom

Any specific gender - Assuming Gender Agnostic

For calculating the market size I will look at the total number of shoes sold per year in UK and then estimate the % of instances where customers might need after sales support.

Total UK Population - 70 Million

Total Population Wearing Shoes - 90% (excluding children under 1 and adults who maybe bid ridden and don't need shoes) - 63 million

Average number of Pairs owned by customers - Most people have multiple pairs of shoes for different occasions. I will try to estimate the number of shoes owned by each customer based on the occasion for which they need to wear the shoe:

1 formal + 1 casual + 1 sports wear = 3

Total Number of Shoes in UK = 63*3 = 189

Average Life of Shoes = 2 years

Total Number of Shoes Replaced with new shoes every year = 189/2 = 94.5 Million

We can assume 10% shoe defect rate in shoes due to size mismatch, or manufacturing defect.

Total Market Size Opportunity for Subscription Based Shoe = 94.5. Million * 10% = 9.45 Million

We can validate these numbers by checking industry reports on shoe related statistics. We can also look at actual shoe defect rates and average shoe life of Nike shoes.