See the monthly payment, lifetime interest, and a year-by-year breakdown of principal versus interest for any fixed-rate loan — and how extra monthly payments shorten it.
An amortizing loan charges interest only on what you still owe. Each payment first covers that month's interest (balance × monthly rate); the remainder reduces the balance. Because the payment is fixed while the balance falls, the interest share shrinks every month — that's why a 30-year mortgage builds equity slowly at first and quickly at the end. Roughly half the principal of a 30-year loan is still owed after year 19 or 20 at today's rates.
| Rate | Monthly P&I | Total interest (30 yr) |
|---|---|---|
| 5.0% | $1,610 | $279,767 |
| 5.5% | $1,703 | $313,212 |
| 6.0% | $1,799 | $347,515 |
| 6.5% | $1,896 | $382,633 |
| 7.0% | $1,996 | $418,527 |
| 7.5% | $2,098 | $455,152 |
A single percentage point of rate on this loan changes lifetime interest by roughly $70,000 — which is why rate shopping and points math matter.
Extra principal payments are most powerful early in the loan, when every prepaid dollar would otherwise accrue interest for decades. One extra $200/month on the $300,000 example cuts about 6 years off the term; a single annual lump sum of $2,400 does nearly the same. If you're choosing between prepaying and investing, compare the loan's rate against your expected after-tax return — prepaying a 6.5% loan is a guaranteed 6.5% return. See also our Mortgage Payoff Calculator for lump-sum payoff scenarios.
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