Choose the approach that uses the most reliable assumptions and connects most directly to the market you are sizing.

Demand-side works well when customer count, penetration and usage are intuitive. Supply-side can be cleaner when locations, capacity or provider volume are easier to estimate.

You can mention the alternative briefly and explain why you prefer one route.

If time permits, the second approach can be a useful sanity check, but you do not need to build both in full. For worked approaches and practice examples, see the market sizing guide.

I started choosing based on which assumptions I could defend instead of which formula I memorized.