Bottom Up Estimating

Schedule technique · See it on the map

Runnable here

Estimating each smallest piece of work separately, then adding them up for the most accurate, most labor-intensive total.

When to use it

Use it when the work is decomposable to a level of detail people can estimate confidently, and the decision riding on the estimate is important enough to justify the time it takes — a firm bid, a budget commitment, a task close enough in the schedule that its estimate needs to be trustworthy rather than roughly right.

When to avoid it

Don't use it on work that isn't decomposable yet (see rolling wave planning) — forcing a WBS onto poorly understood work just produces confident-looking nonsense at the leaf level. And don't use it for a rough early estimate where the decomposition cost outweighs the value of the extra precision; analogous or parametric estimating gets you a usable number far faster.

Steps

1. Decompose the task into the smallest components that can each be estimated with reasonable confidence — typically informed by a WBS.
2. Estimate each component independently, using whichever technique fits that component (analogous, parametric or three-point are all fair game at the leaf level).
3. Sum the component estimates to get the task or project total.
4. Add explicit contingency for integration and coordination overhead between components — summing leaf estimates alone systematically undercounts the cost of stitching the pieces together.

What it produces

• A total estimate built from a documented decomposition, where every leaf component's own estimate and basis is visible and traceable back into the sum.

Common pitfalls

• The decomposition stops too early — components are still too coarse to estimate confidently, so the leaf-level estimates are really just analogous guesses wearing a bottom-up label.
• Integration overhead between components is never added, so the sum understates the real total even though every individual leaf estimate was accurate.
• The technique's cost — the time spent decomposing and estimating dozens of small components — gets spent on work where a rougher technique would have answered the question just as well.

Worked example

A kitchen renovation is decomposed into demolition, rough plumbing, rough electrical, drywall, cabinetry, countertop installation, flooring, painting and final fixtures — nine components, each estimated by the trade who'll do it. The components sum to 34 crew-days; the contractor adds three more days of contingency for the coordination and inspection gaps between trades that no single component's estimate accounted for.

Source

• PMBOK-6 §6.4.2.5

Where it comes from: this technique is named by the PMBOK Guide, 6th edition.