Estimate the market size for Google Fi.

To estimate the market size for Google Fi in the US, we can break it down into the total addressable market and Google Fi's potential share.

1. Total Addressable Market (TAM): The US population is approximately 330 million. We can exclude very young children and the elderly, leaving roughly 250 million individuals who are likely smartphone users and require telecommunication services.

2. Google Fi's Market Share: While specific subscriber numbers for Google Fi are not public, we can infer its position relative to major carriers. If we assume Google Fi captures a small but growing percentage of the market, perhaps in the low single digits, its customer base would be in the millions. For example, if it aims for 1-2% of the 250 million potential users, that's 2.5 to 5 million customers.

3. Revenue Estimation: To get to a revenue estimate, we'd multiply the number of Google Fi customers by the average revenue per user (ARPU). ARPU for mobile services can range from $50-$100 per month. Therefore, a market size estimate could be (2.5M to 5M customers) * ($50-$100 ARPU/month) * (12 months/year), resulting in an annual market size in the range of $1.5 billion to $6 billion.