Amortization Calculator
First-month interest/principal split and total interest for an amortized loan.
How it works
First interest = principal × monthly rate; first principal = payment − first interest
With level payments, interest dominates early and principal grows over time.
Worked example
Loan 300,000 at 4.5% over 15 years (180 payments): r = 0.00375, payment ≈ 2,294.
Month one: interest = 300,000 × 0.00375 = 1,125, principal only 1,169. Total paid ≈ 413,000, interest ≈ 113,000.
Notes and limitations
With level payments the interest share falls and the principal share rises; they cross near the midpoint of the schedule. To pay off faster, direct extra payments to shorten the term rather than lower the payment — that saves more interest.