In project management, the term buttom-up, which means "from the bottom up," is used in planning, preliminary costing, and proposal management.
Buttom-up estimating involves time and cost calculations. In this method, the project duration is estimated and the costs are pre-calculated using reference values of the subtasks in the lower levels of the work breakdown structure. Buttom-up estimating offers the possibility of estimating smaller tasks more precisely in terms of their effort.
In buttom-up estimating, each task is broken down into smaller components. Then, estimates are made of which subtasks are needed to meet the requirements of the project. The estimates for the subtasks are then aggregated to develop a larger estimate for the overall project. Using this method, the estimate for the overall project is much more accurate.