Calculated by estimating potential sales to determine total sales. The bottom-up analysis evaluates where products can be sold, and to whom they can be sold.
For example, you decide to provide a dog walking service. You can estimate based on the number of people in the city.
For example, in New York, there are 600k dogs, and only 8% of them will walk every day.
Then if your average bill is $20 then the maximum possible earnings per day will be equal to 600,000*8%*$20 = $960,000 per day.
Then the whole dog walking market is $350,400,000 per year.
However, you should consider that a large sum of money will be given to dog walkers.
Therefore, your market may be estimated at 600,000*8%*$10*365=$175,200,000.
Where $10 is what you get for walking one dog.
Although estimation with bottom-up analysis requires much more effort, the result is usually much more accurate.