The Excel PMT function calculates the constant periodic payment required to pay off (or partially pay off) a loan or investment, with a constant interest rate, over a specified period.
The syntax of the function is:
Where the arguments are as follows:
rate    The interest rate, per period. 
nper    The number of periods over which the loan or investment is to be paid. 
pv    The present value of the loan/investment. 
[fv]   
An optional argument that specifies the future value of the loan/investment, at the end of nper payments. If omitted, [fv] has the default value of 0. 
[type]   
An optional argument that defines whether the payment is made at the start or the end of the period. This can have the value 0 or 1, meaning:
0  the payment is made at the end of the period; 
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 examples below.In the following spreadsheet, the Excel Pmt function is used to calculate the monthly payments on a loan of $50,000 which is to be paid off in full after 5 years. Interest is charged at a rate of 5% per year and the payment to the loan is to be made at the end of each month.
Formula:

Result:

Note that in this example:
In the spreadsheet below, the Excel Pmt function is used to calculate the quarterly payments required to increase an investment from $0 to $5,000 over a period of 2 years. Interest is paid at a rate of 3.5% per year and the payment into the investment is to be made at the beginning of each quarter.
Formula:

Result:

Note that, in this example:
For further examples of the Excel Pmt function, see the Microsoft Office website.
If you get an error from the Excel Pmt function, this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 
Also, the following problem is encountered by some users:
The result from the Excel Pmt function is much higher or much lower than expected.
When calculating monthly or quarterly payments, users sometimes forget to convert annual interest rates 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 