Bottom-up sizing starts from one unit you can picture, such as one store, one chair or one customer, and scales up. It suits questions like "what’s the revenue of one car wash?" where there’s no population to start from.
- Capacity: how much could one unit do at full stretch? Bays × hours open × cars per bay per hour.
- Utilization: nothing runs full all day. Multiply by the share of time it’s busy. Leaving this out is the most common mistake.
- Price: multiply volume by the average price.
- Segments: when customer groups behave very differently, size each group and add them up.
Bottom-up and top-down answers to the same question should land within a factor of two or three of each other. If they don't, one assumption is off.