gnucash/doc/sgml/C/xacc-calculator.sgml

  <article id="xacc-calculator">
    <artheader>
      <title>Loan and Interest Calculations</title>
    </artheader>
    

    <sect1>
      <title>
	Using the Financial Calculator
      </title>

      <para>
	Given that you know the payment interval and compound interval, given
	any four of Present Value, Future Value, Interest Rate, Periodic Payment, 
	and number of Payment Periods, you can instantly find the missing figure
	using GnuCash's financial calculator.
	If you need an explanation of those concepts, see the 
	<link linkend="xacc-compound-interest">compound interest tutorial</link>.
      </para>

      <para>
	To use the financial calculator, find it on the "tools" menu.
	</para>

      <screenshot>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="image/financial-calculator.png">
	  </imageobject>
	</mediaobject>
      </screenshot>

      <para>To do a calculation, simply enter the figures that you <emphasis>do</emphasis>
	have into the text boxes on the left, and select the appropriate compounding and
	payment intervals on the right.  Then, to calculate the figure that you <emphasis>want</emphasis>, simply click on the appropriate "calculate" button.  
	</para>

      <para>For instance, say you have a loan of $5,000 at %5, compounding monthly, and are
	paying back $200 at the end of each month.  
	How many payment periods is it going to take to pay
	back the loan?  Easy.  Enter "5000" as the present value, the interest rate as "5", 
	the periodic payment as <emphasis>-200</emphasis> - note that if you are paying money
	back, this number should be negative, and the future value "0", select the appropriate
	compounding and payment options (make sure that the "discrete compounding" radio 
	button is selected), and then press "calculate" for the "payment periods" box.  
	You should find that "26" appears in the "Payment Period" box.  Therefore, you'll need
	to make 26 monthly payments to pay back the loan.
       </para>
    </sect1>
    <sect1 id="xacc-compound-interest">
      <title>
	Explaining Compound Interest
      </title>
      <sect2>
	<title>
	  Introduction
	</title>
      <para>
	  GnuCash offers the facility to do simple interest calculations
	  for loans and interest-bearing deposits, giving you the ability
	  to answer questions like "How long will it take me to pay off
	  this loan?", "Which of these two term deposits generate better
	  returns?", and "What loan payments do I need to make to pay
	  off this loan in, say, five years?"  GnuCash's financial
	  calculator gives you that information, and more.
      </para>
      </sect2>
      <sect2>
	<title>
	  How Interest Works
	</title>
	<para>
	  There's two angles for talking about interest - 
	  interest-bearing accounts such as savings accounts
	  and term deposits, when financial institutions pay
	  you for the use of your money.  Alternatively, 
	  we can discuss interest in the context of loans, where
	  you pay your financial institution for using their money.
	  To make things simple, we'll talk about loans first.
	</para>
	
	<para>
	  Firstly, let's consider the simplest possible loan
	  where with a simple interest rate and we pay it
	  back at the conclusion  of the term.  For example, 
	  let's say we borrow $10,000 (in financial terms, 
	  the <emphasis>Present Value</emphasis> is $10,000)
	  for one year at an interest rate of 10% per annum
	  that doesn't compound.  How much do we owe at the end
	  of it?
          In this case, calculating the <emphasis>Future Value</emphasis> 
	  that we owe is easy: it's just
	</para>

	
	<programlisting>
	  Future Value = Present Value + (10 % * Present Value)
	               = 10,000 + (10% * 10,000)
	               = 11,000
	</programlisting>

	<para>
	  Now, let's make things a bit more interesting.
	  Let's make the interest <emphasis>compound</emphasis> 
	  monthly.  After the first month, then, you'll owe
	</para>

	<programlisting>
	  Future Value = 10,000 + (10% * 1/12)
	               ~= 10,083.33
	</programlisting>
	   
	<para>Now, for the second month, you'll pay interest
	  on the 10,083.83, so the value after the second month 
	  is:
	  </para>
	<programlisting>
	  Future Value = 10,083.83 + (10% * 1/12)
	              ~= 10167.36
	</programlisting>
	<para>
	  Eventually, after 12 months, you'll owe $11,047.13, rather
	  than the $11,000 you would have paid with simple interest.
	</para>
	<para>
	  Next, we'll make things more complex again - let's say
	  we'll make a <emphasis>periodic payment</emphasis> of 
	  $200 at the end of every month?  Well, let's see.  After
	  one month, you'd owe:
	</para>
	<programlisting>
	  Future Value  = Present Value + Interest - Periodic Payment
          after month 1 = 10,000 + (10% * 1/12) - 200 
	                = 9,883.33
	</programlisting>

	<para>
	  After one year, if you do the calculations, you'll still
	  owe $8534.02 (if I've done my maths right . . .)
	</para>

	<para>
	  For our final example, consider <emphasis>continuous compounding</emphasis>,
	  where interest is charged continuously.  Explaining this calculation in
	  detail requires a little bit of university-level maths, but, roughly,
	  imagine that the payment periods are, really, really, really small!  
	  I can't easily show you the calculations for this, but that doesn't
	  matter.  GnuCash's financial calculator can do them!
	</para>

	<para>
	  So, to summarize, the following bits of information are used in interest
	  calculations:
	</para>
	<itemizedlist>
	  <listitem><para>Present Value</para></listitem>
	  <listitem><para>Future Value</para></listitem>
	  <listitem><para>Interest Rate</para></listitem>
	  <listitem><para>Number of Payment Periods</para></listitem>
	  <listitem><para>Periodic Payment</para></listitem>
	  <listitem><para>Compounding Type (discrete or continuous)</para></listitem>
	  <listitem><para>Compounding Interval (if discrete)</para></listitem>
	  <listitem><para>Payment Time (beginning or end of period)</para></listitem>
	</itemizedlist>
      </sect2>
    </sect1>
  </article>

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