The Basic Schedule Equation

 A rule of thumb is that you can estimate schedule early in a project using the Basic Schedule Equation:
ScheduleInMonths = 3.0 * StaffMonths ^ 1/3

Sometimes the 3.0 is a 2.0, 2.5, 3.5, 4.0 or similar number, but the basic idea that schedule is a cube-root function of effort is almost universally accepted by estimation experts. 

Description

These models were developed to estimate custom, specification-built software projects. The Basic model expresses development effort strictly as a function of the size and class of software being developed.
The schedule equation is the reason that the uncertainty ranges in the Figure are much broader for efforts than they are for schedules. Effort increases in proportion to scope, whereas schedule increases in proportion to the cube root of effort. The schedule equation is the reason that the uncertainty ranges in the Cone of Uncertainty are much broader for efforts than they are for schedules. Effort increases in proportion to scope, whereas schedule increases in proportion to the cube root of effort. The Basic Schedule Equation is also not intended for estimation of small projects or late phases of larger projects. 

Example

To use the equation, suppose you've estimated that you will need 80 staff months to build your project. The schedule computed from this formula ranges from 8.6 to 17.2 months depending on what coefficient from 2.0 to 4.0 is used. The nominal schedule will be (3.0 x 801/3), which is 12.9 months.