Every month, interest equals the outstanding balance times the monthly rate; the rest of the payment reduces principal. The calculator runs this amortization month by month to find the first-year interest exactly, and uses M×n−P for lifetime interest.

Because the balance is largest early on, the first years are interest-heavy — which is why extra payments made early save the most.

Given: Loan $300,000, rate 6.5% p.a., 30 years

M ≈ $1,896/month  ·  Total paid = 1,896 × 360 ≈ $682,600

Total interest ≈ $382,600 — more than the original loan!

First-month interest = 300,000 × 0.005417 = $1,625  ·  First-year interest ≈ $19,370

• Assumes a fixed rate and no prepayments; extra principal payments cut lifetime interest sharply.
• Mortgage interest may be tax-deductible in some jurisdictions — check local rules.
• Shorter terms and lower rates both reduce total interest dramatically on large balances.

Why is total interest so large?
Long terms mean interest is charged on a big balance for decades — on a 30-year loan it can exceed the amount borrowed.

How can I pay less interest?
A shorter term, a lower rate, a larger down payment, or regular extra principal payments.

Is the monthly interest constant?
No — it falls each month as the balance falls, while the principal portion grows.