Purpose
Return value
Arguments
 rate  Interest rate.
 per  Period (starts with zero, not 1).
 nper  Number of periods.
 pv  Present value.
Syntax
Usage notes
The ISPMT function calculates the amount of interest in a given period of an investment where principal payments are equal. The given period is specified as a zerobased number instead of a 1based number. For example, to calculate the interest amount in payments for a loan where the amount is $10,000, the interest rate is 10%, and there are 5 periods in which the principal payment is constant (even), you can use:
=ISPMT(10%,0,5,10000) // interest in period 1
=ISPMT(10%,1,5,10000) // interest in period 2
=ISPMT(10%,2,5,10000) // interest in period 3
=ISPMT(10%,3,5,10000) // interest in period 4
=ISPMT(10%,4,5,10000) // interest in period 5
In the example shown, the formula in H11, copied down, is:
=ISPMT($C$6,B111,$C$7,$C$5)
Note ISPMT assumes principal amounts are equal, but the payment is variable. For a loan where the payment is a fixed amount, see the IMPT function.
Notes:

Be consistent with the units. For a 3 year loan with monthly payments and an annual interest rate of 10%, enter rate as 10%/12. Enter nper as 3*12.

ISPMT uses a zerobased index for period (per). Use 0 for period 1, 1 for period 2, etc.

The PPMT function is for loans with even principal payments. For a loan with even periodic payments, use the IPMT function.