What Is an Assumption Log?

An Assumption Log is a project document that records all assumptions and constraints identified during planning and execution. Assumptions are factors believed to be true for planning purposes but not confirmed as facts; constraints are limiting conditions that affect scope, schedule, or resources. PMBOK 8 treats the Assumption Log as a living artifact — it is first populated during project initiation and updated continuously as new assumptions surface or existing ones are validated or invalidated.

Without a formal Assumption Log, assumptions live in people's heads and in email threads. When an assumption turns out to be wrong, the team scrambles because no one tracked it, analyzed its risk, or assigned an owner. The Assumption Log solves that by making every assumption visible, categorized, and actionable.

What's Inside This Assumption Log Example

This Assumption Log example covers the full lifecycle of Project Phoenix — a $72,250 website launch project for MCG (Marketing Consulting Group) that ran from March 17 to June 13, 2025. The spreadsheet includes the following columns:

• ID — sequential reference (ASM-001 through ASM-014)
• Category — Technical, Vendor, Stakeholder, Regulatory, Resource
• Description — the stated assumption in plain language
• Basis — what information or evidence supports the assumption
• Impact if Wrong — scope, schedule, cost, or quality impact
• Probability — Low / Medium / High likelihood of being wrong
• Owner — team member responsible for monitoring
• Status — Open / Confirmed / Invalidated / Converted to Risk
• Date Resolved — when the assumption was closed or escalated

The log also contains a Constraints tab listing fixed boundaries: the hard launch deadline of June 13, the $72,250 budget cap, the requirement to use MCG's existing hosting provider, and the constraint that all brand assets must be approved by Riley Park before development begins.

How Alex Morgan Used This Assumption Log

Alex Morgan, PMP, created the initial Assumption Log during the project charter workshop in early March. Fourteen assumptions were logged at kickoff. Three of the most consequential were:

• ASM-003: BrightFrame (the design vendor) would deliver approved wireframes by April 4. This assumption was confirmed when BrightFrame hit the deadline, keeping the development phase on track.
• ASM-007: Content migration from the legacy CMS would require no more than 40 hours of effort. This assumption was invalidated in Week 6 when the content audit revealed 180 legacy pages instead of the expected 90. Alex converted ASM-007 to Risk RIS-004 and initiated Change Request CR-002 to adjust the content migration scope.
• ASM-011: Riley Park (COO and project sponsor) would be available for weekly 30-minute check-ins. This assumption held for the first eight weeks but was challenged in Week 9 when Riley Park traveled for three consecutive weeks. Alex adapted by switching to async written briefings, preserving the decision cadence.

Alex reviewed the Assumption Log at the start of every weekly status meeting. Each team member had a column filtered to their name, making it easy to surface assumptions within their domain. By project close, seven assumptions had been confirmed, four invalidated (three converted to risks, one resolved via change request), and three remained open as low-probability items that never materialized.

The Assumption Log was cross-referenced with the Risk Register: every invalidated assumption with a Medium or High impact automatically triggered a risk entry. This tight linkage between the two documents is one of the hallmarks of PMBOK 8's integrated artifact approach.

Download and Customize

This Assumption Log example is available as a free download. Use it as a reference to build your own assumption log, or start with the blank template and fill it in for your project.

• Download the Assumption Log Template — PMBOK 8 (blank, ready to use)
• Read the article: Assumption Log in PMBOK 8 — Guide and Best Practices

Assumption Log Example: Key Takeaways

The Project Phoenix Assumption Log demonstrates that the real value of this document is the discipline of articulating assumptions explicitly at project start. ASM-007 (content migration effort) is the clearest example: the assumption was wrong by a factor of two, but because it was logged, owned, and monitored, Alex detected the problem in Week 6 rather than Week 10. That early detection meant the team had time to negotiate a scope adjustment and reallocate hours without blowing the budget. Projects that skip the Assumption Log tend to discover invalidated assumptions at the worst possible moment — when the deadline is near and options are few.

Want to go deeper? The PMBOK Guide 8th Edition is the definitive reference for modern project management. Get your copy and use it alongside these examples to build a solid, practical understanding of every performance domain.