What Is an Assumption?
An assumption is a factor in the planning process that is considered to be true, real or certain without proof or demonstration. Projects rely on many assumptions, and each one carries risk if it turns out to be wrong.
In Practice
Teams record assumptions in an assumptions log, test the most critical ones early, and revisit them as the project progresses. An unexamined assumption — about availability of a resource, a dependency, or a stakeholder’s support — can quietly undermine a plan. Making assumptions explicit converts hidden risk into something the team can manage.
PMBOK 8 Context
Managing assumptions supports the Planning and Uncertainty performance domains of the PMBOK Guide, 8th Edition. Assumptions are closely tied to constraints and to ambiguity.
FAQ
Assumption vs. constraint? An assumption is something believed true without proof; a constraint is a known limitation the project must work within.
Where are assumptions recorded? In an assumptions log, so they can be tracked, validated and converted to risks if needed.
Author: Tom, PMP-certified since 2004. Last updated: July 2026. Aligned with the PMBOK Guide, 8th Edition.