A fixed payment stays the same every month. Early payments are mostly interest. Later payments are mostly principal. With declining payments, principal repaid each month is the amount borrowed divided by the number of months, and interest is charged on the remaining balance.
Payment = principal × i × (1 + i)ⁿ ÷ ((1 + i)ⁿ − 1)
A loan of 1,000 over two months at 10% a month has a fixed payment of 576.19. Declining payments are 600 and then 550. Total interest is 152.38 on the fixed schedule and 150 on the declining schedule.
What is left out
Fees, insurance and a balance that is indexed to inflation change the real installment. Enter the monthly contract rate when you have it. The annual rate on this page is the effective yearly rate, not a nominal rate divided by twelve.