The Excel ISPMT function calculates the interest paid during a specific period of a loan or investment.
The syntax of the function is:
Where the arguments are as follows:
rate    The interest rate, per period. 
per    The period for which the interest is to be calculated (must be an integer between 1 and nper). 
nper    The number of periods over which the loan or investment is to be paid. 
pv    The present value of the loan/investment. 
Cash Flow Sign Convention:
Note that, in line with the general cash flow sign convention, cash outflows are represented by negative numbers and cash inflows are represented by positive numbers. This is seen in the example below.In the following spreadsheet, the Excel Ispmt function is used to calculate the interest paid, during months 1 and 2, of a loan of $50,000 which is to be paid off over 5 years. Interest is charged at a rate of 5% per year.
Formula:

Result:

Note that in this example:
For further details and examples of the Excel Ispmt function, see the Microsoft Office website.
If you get an error from the Excel Ispmt function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 
Also, the following problem is encountered by some users:
The result from the Excel Ispmt function is much higher or much lower than expected.
Many users, when calculating monthly or quarterly payments, forget to convert the interest rate or the number of periods to months or quarters.
Solve this problem by ensuring that the rate and the nper arguments are expressed in the correct units. I.e.:
months = 12 * years quarters = 4 * years 
monthly rate = annual rate / 12 quarterly rate = annual rate / 4 