Contingency buffer
Extra time or money deliberately held back for project unknowns, so one surprise does not immediately break the plan.
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What it is
A contingency buffer is time or money reserved for uncertainty that belongs inside the agreed job: an integration takes longer than the docs implied, a review uncovers rework, or a dependency arrives late. It is a named reserve, not a secret handful of hours buried inside every estimate. Formal project management splits it in two: a contingency reserve covers the risks you listed and the project manager can spend it, while a management reserve covers the risks nobody saw coming and needs sign-off from above. A two-person studio keeps one line, but the split is why a client's PMO may ask which one you quoted.
Add one while planning work with real unknowns. Estimate the base work first, list the risks that could consume the reserve, then size the buffer from their likelihood and impact: a common starting point is 10 to 15 percent on a build your team has shipped five times, and 30 percent or more when the job hangs on someone else's undocumented API. Keep the reserve visible in the internal budget and record each drawdown so the team knows what remains.
Gotcha: a visible buffer can look like free scope, while a hidden one can become permission to drift. The hidden version has a patron saint: in Star Trek III, Kirk asks Scotty whether he has always multiplied his repair estimates by four, and Scotty asks how else he is supposed to keep his reputation as a miracle worker. It works until someone lines your estimates up against your actuals. State who may release the reserve and what kinds of events qualify. Do not pad every task and add a full project buffer on top, or you count the same uncertainty twice.
Ask AI for it
Build a contingency plan for the project below using three-point PERT estimation. For each uncertain task, list optimistic, most likely, and pessimistic durations, then calculate expected duration as (O + 4M + P) / 6. Separate the base estimate from the contingency buffer, name the risks allowed to consume it, assign an owner who can approve each drawdown, and define the threshold that triggers a change order. Return the calculation table, total reserve, remaining-reserve tracker, and a client-facing explanation that does not expose it as free scope. Do not invent probabilities when the inputs are missing.