I would keep this article short, practical, and clearly label the calculation as an estimate, not a scientifically proven formula.
How Many Orphaned Users Do You Have in SharePoint? A Simple Estimation Method
Overview
Many organizations do not know how many orphaned users exist in their SharePoint environment until they start investigating User ID Mismatch incidents.
Unfortunately, Microsoft 365 does not provide a built-in report that can quickly answer this question across an entire tenant. As a result, administrators often need to estimate the scale of the problem before investing time in tenant-wide scanning and cleanup efforts.
This article provides a simple estimation method based on observations from several real-world Microsoft 365 tenants. The goal is not to produce an exact number, but rather to determine whether your organization is likely dealing with dozens, thousands, or millions of orphaned user records.
What Is an Orphaned User?
In SharePoint, an orphaned user is a user record that still exists in a site’s User Information List even though the corresponding account no longer exists in Microsoft Entra ID.
Orphaned users are not necessarily a problem by themselves. However, they can contribute to User ID Mismatch issues when a deleted account is later recreated with the same User Principal Name (UPN).
A Simple Estimation Formula
Based on observations from several production tenants, a rough estimate for the number of orphaned user records can be calculated as:
Estimated Orphan User Records =
Enabled Entra ID Users × 5 × Tenant Age ÷ 7
Where:
- Enabled Entra ID Users = current enabled user accounts
- Tenant Age = age of the Microsoft 365 tenant in years
For tenants that are approximately seven years old, the formula becomes:
Estimated Orphan User Records =
Enabled Entra ID Users × 5
Example Calculations
Example 1
Enabled Users = 10,000
Tenant Age = 7 years
Result:
10,000 × 5 = 50,000
Estimated orphan user records:
50,000
Example 2
Enabled Users = 50,000
Tenant Age = 5 years
Result:
50,000 × 5 × 5 ÷ 7
Estimated orphan user records:
178,571
Example 3
Enabled Users = 250,000
Tenant Age = 8 years
Result:
250,000 × 5 × 8 ÷ 7
Estimated orphan user records:
1,428,571
Understanding the Result
The calculated value represents an estimate of orphaned user records, not necessarily unique users.
For example, if a deleted user exists in 20 SharePoint sites, that user may generate 20 orphaned records.
Therefore:
Number of orphaned records
≠
Number of unique orphaned people
The actual number of unique orphaned identities is typically much lower.
Factors That Increase Orphaned User Counts
The following factors generally increase the number of orphaned users in a tenant:
- Older Microsoft 365 tenants
- Higher employee turnover
- Frequent contractor onboarding and offboarding
- Reuse of UPNs
- Heavy SharePoint adoption
- Large numbers of SharePoint sites
- Lack of cleanup processes during offboarding
Organizations with stable staffing and low turnover tend to accumulate orphaned users more slowly.
Estimating User ID Mismatch Risk
While orphaned users do not automatically cause User ID Mismatch issues, they create the conditions that make those issues possible.
A User ID Mismatch event usually requires all three of the following:
- A user account is deleted.
- The same UPN is reused.
- The legacy SharePoint user record still exists.
Because of this, risk increases as the orphaned user population grows.
Low Risk
- Fewer than 10,000 estimated orphan user records
- New tenant
- Low employee turnover
- UPNs are rarely reused
Medium Risk
- 10,000 to 250,000 estimated orphan user records
- Moderate employee turnover
- Occasional employee rehires
- Large SharePoint footprint
High Risk
- More than 250,000 estimated orphan user records
- Frequent onboarding and offboarding
- Reuse of UPNs
- Multiple User ID Mismatch tickets already reported
A Better Question
Many administrators ask:
“How many orphaned users do I have?”
A more useful question is:
“How likely am I to experience User ID Mismatch issues?”
A tenant may contain thousands of orphaned users and never experience significant problems if UPNs are never reused.
On the other hand, a tenant with fewer orphaned users but aggressive account reuse practices may encounter User ID Mismatch incidents regularly.
For that reason, orphaned user counts should be viewed as a risk indicator rather than an exact measure of operational impact.
Final Thoughts
The formula presented in this article is intentionally simple and should be treated as a rough planning estimate, not a precise calculation.
Estimated Orphan User Records =
Enabled Entra ID Users × 5 × Tenant Age ÷ 7
Its purpose is to help administrators quickly assess the possible scale of orphaned users in their environment and determine whether a deeper investigation is justified. If the estimate suggests hundreds of thousands or millions of orphaned records, it may be worth investing in tenant-wide analysis, cleanup activities, and User ID Mismatch prevention strategies.
===========
Based on the initial data, a very rough estimate for a seven-year-old tenant is approximately five orphaned user-site records per enabled Entra ID user. For tenants of a different age, the result can be adjusted proportionally:
Estimated orphan records = Enabled users × 5 × Tenant age / 7
This is not a precise formula. Actual results can be much lower or much higher depending on SharePoint usage, employee turnover, and the average number of sites associated with each user.