This contingency is normally calculated as a percentage. If the phase is 100 days of effort, contingency at 20% would be another 20 days. As the project progresses, the level of risk reduces as the requirements and issues become known, so the percentage will be reduced.
Home builders and remodelers usually allocate between 5% and 10% of a project budget for a construction contingency. This amount creates enough breathing room for unexpected costs. Anyone tracking estimates and costs manually will calculate a contingency percentage on top of all costs before profit margins are applied.
The most basic way to calculate a contingency reserve is to add a fixed percentage to the total project budget, known as the Flat Rate method. Alternatively, if different percentages are applied to unique budget line items, this would be called a Mixed Rate method to establish the reserve.
The contract is characterized as "contingent" because the terms are not final and are based on certain events or conditions occurring. A contingent contract can also be viewed as protection against a future change of plans.