Accurate time estimation is a skill essential for good project management. It is important to get time estimates right for two main reasons:

1. Time estimates drive the setting of deadlines for delivery and planning of projects, and hence will impact on other peoples assessment of your reliability and competence as a project manager.
2. Time estimates often determine the pricing of contracts and hence the profitability of the contract /project in commercial terms.

Often people underestimate the amount of time needed to implement projects. This is true particularly when the project manager is not familiar with the task to be carried out. Unexpected events or unscheduled high priority work may not be taken into account.

Project managers also often simply fail to allow for the full complexity or potential errors and stuff ups, involved with a project. The 2004-2006 Wembley Stadium project in London is often used as an example, although there are countless others of less profile.

Time estimates are important as inputs into other techniques used to organise and structure all projects. Using good time estimation techniques may reduce large projects to a series of smaller projects.

Step 1 - Understand the Project Outcome
First you need to fully understand what it is you need to achieve. (Refer to my article; Project Management - Begin with the end in mind). Review the project /task in detail so that there are no unknowns. Some difficult-to-understand, tricky problems that take the greatest amount of time to solve. The best way to review the job is to just list all component tasks in full detail.

Step 2 - Estimate time
When you have a detailed list of all the tasks that you must achieve to complete the project then you can begin to estimate how long each will take.

Make sure that you also allow time for project management administration, detailed project, liaison with outside bodies resources and authorities, meetings, quality assurance developing supporting documentation or procedures necessary, and training.

Also make sure that you have allowed time for:
Other high urgency tasks to be carried out which will have priority over this one
Accidents and emergencies
Internal/external meetings
Holidays and sickness in key staff/stakeholders
Contact with other customers, suppliers and contractors.
Breakdowns in equipment
Missed deliveries by suppliers
Interruptions by customers, suppliers, contractors, family, pets, co- workers etc
Others priorities and schedules e.g. local government planning processes
Quality control rejections etc.
Unanticipated events (e.g. renovating the bathroom finding white-ants/termites in the walls)

These factors may significantly lengthen the time and cost needed to complete a project.

If the accuracy of time estimates is critical, you will find it effective to develop a systematic approach to including these factors. If possible, base this on past experience. In the absence of your own past experience, ask someone who has already done the task or project to advise what can go wrong; what you need to plan for; and how long each task took previously.

You can lose a great deal of credibility, and money, by underestimating the length of time needed to implement a project. If you underestimate time, not only do you miss deadlines, you can also put other people under unnecessary stress.

Step 3 Plan for it Going Wrong
Finally, allow time for all the expected and unexpected disruptions and delays to work that will inevitably happen. Sickness, strikes, materials not available, poor quality work, bureaucratic bungling etc.