Hours per day and duration display

Last reviewed 9 September 20261,387 words6 min read

Why an 8-day activity shows as 10

🟩 In one line: P6 stores every duration in hours and converts to days for display using a hours-per-day figure that can come from three different places — if the display figure doesn't match the activity's calendar, the day count you see is wrong even though the dates are right.

👤 Who this is for: J. Read before your first XER import or your first multi-calendar schedule.

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First, let's be honest about why this page exists

A subcontractor sends an XER. Their schedule showed a 10-day activity; on your screen it says 8 days, or 12.5. The dates match exactly. Panic follows, then an hour of checking, then someone finds the hours-per-day setting. This happens on every project, to every planner, at least once.

There is no bug. There are three settings that need to agree, and one of them lives in Admin Preferences where juniors don't go.

🟨 The standard — what "good" looks like

Source	What it says (paraphrased)
P6 Professional User Guide — Admin Preferences, Time Periods	Hours per day, week, month and year used to convert stored hours to displayed units; option to use each calendar's own hours per day instead.
P6 Professional User Guide — Calendars	Each calendar carries its own hours per day, week, month and year, set via the Time Periods button in the calendar dialog.
P6 Professional User Guide — User Preferences, Time Units	Controls the display unit (hours, days, weeks) and decimal places; does not change stored values.
PMI Practice Standard for Scheduling, 3rd ed.	Duration units and calendar assignments must be consistent and documented in the schedule basis.
AACE RP 38R-06, Documenting the Schedule Basis	The hours-per-day basis for durations is part of the schedule basis and must be stated.

🟢 Rule: tick "Use assigned calendar to specify the number of work hours for each time period", set each calendar's hours per day to its real working day, and state both in the SBM.

How it actually works

1. Where the number comes from.

Setting	Path	What it does	What to set
Global hours per time period	Admin → Admin Preferences → Time Periods	Default conversion for the whole database: Hours/Day, Hours/Week, Hours/Month, Hours/Year	8/40/172/2000 is the installation default; match it to the project's main calendar if you can't use the tick box
Use assigned calendar	Admin → Admin Preferences → Time Periods → tick box	Overrides the global figure with each activity's calendar hours per day	Ticked. Always.
Calendar hours per day	Enterprise → Calendars → select calendar → Modify → Time Periods button	The figure used when the tick box is on	10 for a 10-hour site calendar, 24 for curing, 8 for office
Display unit and decimals	Edit → User Preferences → Time Units	Days vs hours; 0, 1 or 2 decimals	Days, 1 decimal for review; 0 for client output

2. The arithmetic behind the surprise. An activity is 80 working hours. On a 10-hour calendar that's 8 days. If the tick box is off and the global setting is 8 hours per day, P6 displays 80 ÷ 8 = 10 days. Change the global setting to 10 and it reads 8 again. The dates never changed; the activity still occupies 8 working days on its calendar. Only the label moved.

3. Why it matters beyond cosmetics.

Where the wrong figure bites	Consequence
Client reports	"12.5-day activity" on a page invites questions
Duration entry	Typing "10d" on a 10-hour calendar with the global at 8 stores 80 hours = 8 real days. You've under-planned by 20%.
Resource loading	Units per time period are converted with the same figure; manhour totals come out wrong
DCMA Metric 8 (High Duration)	An activity that is genuinely 40 days on a 10-hour calendar may show as 50 days and fail the 44-day threshold
XER exchange	The receiving database applies its own Admin Preferences; the sender's display never travels with the file

4. When you receive an XER. Check Enterprise → Calendars → the imported calendars' Time Periods button. Subcontractor calendars often arrive with 8 hours per day even when the workweek is set to 10-hour days. Fix the calendar hours, not the durations.

5. Calendar workweek hours vs calendar Time Periods. Two different things in the same dialog. The workweek (Standard tab, or Detailed Work Hours) defines which hours are working — 07:00 to 18:00 with a break. The Time Periods button defines the conversion figure. They should agree; P6 doesn't force them to.

🟥 Where people go wrong

Editing durations to "fix" the display. The dates were right. Now they're wrong.

Leaving the tick box off on a multi-calendar job. Every activity not on an 8-hour calendar displays incorrectly.

Setting the global to 10 and forgetting the office calendar. Consultant reviews show as 4 days when they're 5.

Forgetting the calendar's own Time Periods after building the workweek. Workweek says 10-hour days; Time Periods still says 8. The tick box faithfully applies 8.

Assuming the subcontractor's file reads the same on your screen. It won't unless their calendars carry the right hours. Check before you comment on their durations.

Not stating the basis in the SBM. Three years later nobody knows whether "10d" meant 80 or 100 hours.

⚖️ When you're challenged

"Your programme says 8 days, ours says 10. Which is it?" Both files hold 80 hours. Our site calendar is 10 hours a day, so 8 days; your database is displaying at 8 hours a day. The dates are identical in both. I'll send a screenshot of the calendar Time Periods setting so your admin can align it.

"Why does the curing activity show 0.3 days?" It's 7 hours on a 24-hour calendar, which is a real curing time for that product. If the decimal confuses the report I'll show durations in hours for the curing activities, but I won't round it to a day — that would add 17 hours to the path.

"Can you just make everything display at 8 hours a day so it matches the consultant's?" Then every site activity reads 25% longer than it is and the High Duration check fails on activities that are fine. Better that the consultant's database uses assigned calendars, which is the setting the P6 guide recommends anyway.

📄 Related pages

Calendar Setup and Holidays — building calendars with the right hours.
Multiple calendars and float — the float version of the same conversion problem.
What Is an XER File — why everything is stored in hours.
Cleaning an XER Before Import — checking calendars on receipt.
Schedule Basis Memorandum — where the hours-per-day basis is recorded.
Activity Duration Limits — DCMA Metric 8 and why display matters.

✏️ Worked example

A mall fit-out in Dubai. Subcontractor XER imported into a sandbox. Ceiling grid activity: their transmittal says 12 days; P6 shows 15 days.

Check	Value
Stored duration (Hours column added)	120 h
Subcontractor calendar workweek	Sat–Thu, 07:00–18:00 with 1 h break = 10 h/day
Subcontractor calendar Time Periods	8 h/day (left at default when calendar was created)
Sandbox Admin Preferences tick box	Ticked
Displayed duration	120 ÷ 8 = 15 d
Fix	Calendar Time Periods set to 10 h/day
Displayed duration after	120 ÷ 10 = 12 d
Dates	Unchanged throughout

Noted in the review comment sheet: "Calendar SUB-10h Time Periods corrected from 8 to 10 h/day; no change to dates or logic." Original XER archived unedited alongside.

📖 References

Oracle Primavera P6 Professional User Guide — Admin Preferences, Time Periods; Calendars, Time Periods; User Preferences, Time Units.
PMI, Practice Standard for Scheduling, 3rd ed. — schedule model components.
AACE International RP 38R-06, Documenting the Schedule Basis.
DCMA 14-Point Schedule Assessment — Metric 8, High Duration.

From the field

Experience from working planners. Unreviewed — read it as experience, not guidance.

Add what you know about hours per day and duration display. What worked, what the consultant pushed back on, what you would do differently next time. A paragraph is plenty.

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