Standard decimal numbers in the Wolfram Language are internally represented with machine precision*:

In[1]:= $MachinePrecision
Out[1]= 15.9546

The InputForm of a given number displays all available digits:

In[2]:= InputForm[200./7]
Out[2]//InputForm= 28.57142857142857

Yet by default, only six digits of such machine-precision numbers are shown:

In[3]:= 200./7
Out[3]= 28.5714

More digits can be displayed either for an individual number or all machine-precision numbers.

Individual Number

NumberForm displays the desired number of digits of a particular number:

In[4]:= NumberForm[200./7, 12]
Out[4]//NumberForm= 28.5714285714

NumberForm cannot return more digits than the number has:

In[5]:= NumberForm[200./7, 24]
Out[5]//NumberForm= 28.57142857142857

All Numbers

  1. Navigate to the menu item Edit ▶ Preferences on Windows or Mathematica ▶ Preferences on Mac.
  2. Select the tab Appearance ▶ Numbers ▶ Formatting.
  3. In the line “Number of digits displayed in output”, set the preferred precision.

* It is of noninteger value since the precision of a number is defined in terms of the logarithm of the uncertainty in the corresponding exact value.