Conversion of Manhours into Durations

If you have the manhours, you also need the number of men. There is no need to convert manhours to duration. Just start with an estimate. Say you have 100 manhours/1000 metres of cable. Make a task, "install 1000 m of cable", estimate 40 (or 60 or 80, your initial estimate is not important) hours duration, assign two men, $100/hour at 100%, ie 8 hours per day. The work is 2 x 40 = 80 manhours, but your rate is 100 manhours/1000. You need more duration, about 5/4 or 1.25 times what you have now. Make duration 50 hours. Now you have 2 x 50 = 100 manhours. Then you also know the cost (because manhours is more or less the same as cost anyway) so 100 manhours x $100/manhour = $10000. Problem solved in one iteration.

This is not straightforward. 

The crew may be complex and some resources may be used part time.

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Sir Trevor Rabey,

Thanks for your reply, During Scheduling in Primavera, We have to put estimated duration of each activities, thats why duration is to be calculated and i have fixed Duration/Units. but the confusion is that, manhour would be hours/unit or unit/hours? means we have to multiply it with the total quantity or have to divide with it? 

if manhour is Unit/Hour productivity then have to divide to get the duration...

For example: 

Buried Steel Duct 100x600 (4"x 24")

Unit of Time: "Hours";

Unit of Qty "Meter";

Manhour Factor: 2.16

If BOQ Quantity of Steel Duct is 800 Meter, So what would be the correct Answer? Number 1 or Number 2 ???

Number 1:

800 x 2.16 = 1728 Hours = 1728/10 (Working Hours) = 172.8 Days further Divided by Size of Crew to get the estimated duration.

Number 2:

800 / 2.16 = 370.3 Hours = 370.3/10 (Working Hours) = 37 Days further Divided by Size of Crew to get the estimated duration.

Kindly, Help me out to understand how can i use standard manhours (Productivity) to calculate the activity durations?

Do you use machines, or all work you do is without machines?

What will be the production rate to move dirt if by hand or if using an excavator.  What if the excavator used is replaced by another more productive?

Construction estimators do not rely on isolated man-hours production rate but instead consider the crew production rate.

Without few exceptions to adjust activity duration without taking into consideration the crew as a whole is in abominable horror.

Multi-resources and skills modeling can become handy when in need to allow the software to make such adjustments, otherwise you might be better off if using fixed composition crews along with their production rates as a team. 

You must consider the production rate as per different crew sizes; 10 workers and x rollers might do the job in 8 hours or 80 manhours, but it is not the same as 40 workers and 2x rollers in 2 hours for same 80 manhours, it is not the same as 80 workers and 8x rollers pulling cables for 1 hour or 160 workers and 16x rollers for 1/2 hour.

That a woman can bear a child in 9 months it does not means 9 women can bear a child in 1 month. 

This is not how PMs manage their activities and crew composition at the jobsite.

We use our crew production records, our experience and for the few activities where there are no records we use industry references such as RS Means where production rates consider crew composition.  Reference books that only provide man-hours I consider a joke.  With very few exceptions modeling activity duration based on a fixed production rate is in error.  Use this functionality with caution. 

• By the same token it might be that going from 4 workers to 6 workers pulling cables on same conduit it is too much. 
• At times such extreme examples are good as to make the point on how such simplified model can be so wrong.
• Only in very few cases and within certain range that it makes sense.  If the functionality does not provide to prevent falling outside the range you might be better off if not using it at all. 
• Most of the abuse of such linear assumption probably happens not in the obvious extremes but in the frequency wrong assumption is used. 

divide the manhours per actiivity by the number of men you want to use to do that task 

example power panel installation is 48 hours

so using 1 man per day 48 hours / 1 man @ 8 hours =6 days duration

using 2 men per day 48 hours / 2 men per day is 16 hours = 3 days duration

so 48/8=6 days for 1 man

or  48/16= 3days for 2 men 

There are two ways to do this. Firstly, you may determine your crew size and then you may calculate the duration based on the man hour you have assigned. Secondy way is to determine the duration of the activity and estimate your crew size. Second method can be used if your activitiy is not on the critical path. If your activiy is on critical path, you have to determine your crew size according to the duration. In both situations, you need to take into consideration the max. crew size that can work on the specific activity in order not to decrease the productivity at the construction site.