You should give more information on the pipe you wish to test and the service it will be in.  All important pipework is designed, installed and tested to a reputable code these days and the relevant code (of which there are many) normally specifies the pressure that the pipe must be hydrotested to.  If you give us details of the system you are dealing with we can point you to codes that are relevant.  We need to know for example if you are dealing with steel, cast iron, plastic or concrete pipe. Also, what is the service?  Is it water, oil, gas or steam?. Also, what industry are you dealing with?.  Is it building services, high pressure oil and gas, power generation?.

In general you will find that the hydrostatic test pressure is a percentage over the intended design pressure of the line. Formulas are used to determine the design pressure of the line, not normaly the hysrostatic test pressure.  Long cross country oil and gas lines  for example are tested to 25% over their design pressure while refinery and power station piping is tested to 50% over the design pressure.  Depending on the relevant code the hydrostatic test  pressure may have to be adjusted for the design temperature of the line, and possibly the corrosion allowance used in the design of the line.

So as you can see there is no simple formula for determining the hydrostatic test pressure of a pipeline.  Please feel free to email me with details and I will do my best to point you in the right direction.

The piping system must be pressure tested after it has been
completely fabricated, erected, and inspected. The pressure test demonstrates the mechanical integrity of the system before it is placed into operation. The following highlights several test requirements.
1.A hydrostatic test must be used unless otherwise approved  for special situations.
2.The hydrostatic test pressure at any point in a metallic piping system shall be as follows:

a) Not less than 1½ times the design pressure.

b) For design temperatures that are above the test
temperature, the minimum test pressure shall be calculated
as follows, except that the value of (ST)/S shall not exceed
6.5:


     1.5*P(ST)
PT=-----------
        S
PT = Minimum hydrostatic test pressure, psig

P = Internal design pressure, psig
ST = Allowable stress at test temperature,psi
S = Allowable stress at design temperature,psi

c) If the test pressure as defined above would produce a
stress in excess of the yield strength at test
temperature, the test pressure may be reduced to the
maximum pressure that will not exceed the yield
strength at test temperature.