How it works
Payments are calculated with the standard amortization formula M = P × r(1 + r)n / ((1 + r)n − 1), where r is the monthly rate (annual % ÷ 1200) and n is the number of months. If you choose years, n = years × 12. Total cost = M × n; total interest = total cost − loan amount. A 0% rate is handled as simply P ÷ n.
Extra payments (an extra amount every month and/or a one-time lump sum in a month you choose) go straight to principal. The loan is then re-amortized month by month at the same payment until it is paid off, and the results show the months and interest saved.
Worked examples
$15,000 personal loan at 9% for 5 years
Monthly payment: $311.38. Total interest: $3,682.52; total cost: $18,682.52.
$5,000 at 12% for 18 months
Monthly payment: $304.91. Total interest: $488.38; total cost: $5,488.38.
$15,000 at 9% for 5 years with $50 extra each month
Paying $361.38 instead of $311.38 clears the loan in 50 months instead of 60, saving $644.39 in interest.
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