The calculator uses "compound interest", that is; the interest you pay is based
on the amount you currently owe, which is the fairest, by far the most common
and unfortunately more mathematically complex way to work out what you owe.
The alternative method "Simple interest" is easier to calculate but it is not
as fair. You pay interest based on how much you originally borrowed rather than
how much you currently owe. Simple interest loans are illegal in some
juristicions.
To use the calculator fill in all except one of the input boxes, select the
Annual or Monthly options as required. When you have filled in all but one of
the boxes the remaining value will be calculated and displayed to the right of
the empty box.
You could also press the calculate button. All results are given to two decimal
places except the number of repayments which is an integer.
There are soe reality and calculation checks in the system, so not all
combinations of inputs will give a result.... but realistic ones will.

If you are calculating the number of repayments then it is most likely that the
last loan payment will be less than the amount you specified. If so this "pay
out" value is reported next to the number of repayments.

If you are calculating the residual, then it is not included in the total
payments

Numbers you enter should use . as the decimal separator and should not include
currency symbols or thousand separators

Although the currency display is dollars this is only for diaplay, the
calculated result is for any currency convention.
