Bottom-up estimating involves the estimation of work at the lowest possible level of detail. These estimates are then aggregated in order to arrive at summary totals. By building detailed cost and time estimates for a work package, the probability of being able to meet the estimated amounts improves substantially. The people who derive these estimates are usually those involved in a project team; they have hands-on knowledge of the proposed work, and so are in the best position to understand the associated work requirements. the only downside to bottom-up estimating is that it can take a substantial amount of time to complete.
Bottom-up estimating is preferred over top-down estimating, where management imposes cost and time figures on a project without having gone through any detailed analysis to substantiate their results.