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How to Calculate Task Duration and Work Hours in Microsoft Project

Phi Hung Vo Oct 19, 2026 868 views

Question details

The user needs to understand how Microsoft Project calculates task duration and work hours when using a custom project calendar with non-standard working schedules.

Product: Microsoft Project
Device & OS: not provided
Scenario: Configuring a custom project schedule with 10 hours from Monday to Friday and 5 hours on Saturday, totaling a 55-hour work week.
Observed behavior: Entering a 1-week duration creates 55 hours based on the custom calendar, and manually setting 60 hours of work distributes the time unevenly or causes overallocation based on task settings.

Before you start

Before modifying your task schedules, verify your base project calendar settings and confirm the default hours per day and per week configured in your Microsoft Project options.

Solution 1Recommended

Configure Schedule Options to Match Custom Calendars

Ensure that Microsoft Project correctly converts your entered duration (like days or weeks) into accurate working hours based on your custom calendar.

Microsoft Project converts duration units such as days and weeks into hours using the global schedule settings. If you use a custom calendar (e.g., 55 hours a week), you must update these settings so that 1 week entered in the duration field equals exactly 55 working hours.

Open Project Options

In Microsoft Project, click on 'File' in the top menu ribbon, then select 'Options' at the bottom of the left-hand navigation pane.

Navigate to Schedule Settings

Click on the 'Schedule' tab in the Project Options dialog box to view how the software handles calendars and time.

Adjust Hours per Week

Locate the 'Calendar options for this project' section. Update the 'Hours per day' and 'Hours per week' fields to match your custom calendar (e.g., set Hours per week to 55).

Apply and Enter Duration

Click 'OK' to save your settings. Now, when you enter '1w' or '55h' in a task's duration column, Project will correctly schedule it from Monday morning through your specified Saturday finish time.

Configure Schedule Options to Match Custom Calendars

Best Practice: It is always recommended to modify a copy of the standard calendar and make that copy your specific project or task calendar, rather than overwriting the default Standard calendar.

Solution 2

Manage Work Hours Using Resource Assignments

Avoid entering work directly on unassigned tasks to prevent odd fractional hour distributions and resource overallocation.

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Frequently Asked Questions

Why does my task duration increase when I add work hours in Microsoft Project?

By default, Microsoft Project tasks are set to a 'Fixed Units' task type. If you manually increase the work hours beyond the scheduled duration while keeping the resource units fixed (e.g., at 100%), Project automatically extends the task duration to accommodate the extra work.

How do I fix resource overallocation in Microsoft Project?

Overallocation occurs when you assign more work than the scheduled working hours allow, often on a 'Fixed Duration' task. To fix this, you can increase the task duration, reduce the assigned work, run resource leveling, or adjust the resource's maximum availability units in the Resource Sheet.

Should I manually enter work hours on a task without resources?

It is generally not recommended. Entering work hours without assigning a resource forces Microsoft Project to distribute the work automatically across the duration, which often results in awkward fractional hours (e.g., 10.92 hours per day). Always assign resources first and let the software calculate the work.

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