How paying off your mortgage early works
A mortgage is an amortized loan: each payment is split between interest (the cost of borrowing) and principal (what you owe). Early on, most of each payment is interest because the balance is large. An extra payment skips the interest and goes straight to principal.
Interest is charged on whatever balance is left, so a lower balance today means less interest every month after. That is why a modest extra payment adds up to large savings over hundreds of remaining months.
The formula
The regular monthly payment is M = P × r / (1 − (1 + r)^−n), where P is the balance, r is the annual rate ÷ 12, and n is the number of months left. The calculator then runs the loan month by month: it adds that month's interest, subtracts your payment plus any extras, and repeats until the balance reaches zero. Your required payment never changes — extra payments simply make the last payment arrive sooner.
A worked example
A $300,000 balance at 6.5% over 30 years has a payment of about $1,896 and costs about $382,633 in interest. In the very first month, interest is 300,000 × 0.065 ÷ 12 = $1,625, so only about $271 of the payment reduces the balance. Adding $200 lifts that first month's principal paydown to about $471.
Keep paying $200 extra every month and the loan is gone in 23 yr 1 mo instead of 30 years — 6 yr 11 mo sooner — saving roughly $103,449 in interest.
Comparing ways to pay extra
Same $300,000 loan at 6.5% with 30 years left, computed with this calculator:
| Strategy | Paid off sooner by | Interest saved |
|---|---|---|
| $200 extra every month | 6 yr 11 mo | $103,449 |
| $2,400 once a year | 6 yr 9 mo | $99,670 |
| One extra payment ($1,896.20) a year | 5 yr 8 mo | $83,985 |
| $10,000 lump sum today, nothing after | 2 yr 9 mo | $53,917 |
Two lessons: the same yearly dollars save slightly more when paid monthly (the balance falls sooner), and money paid early in the loan is worth the most. The “one extra payment a year” row is roughly what a biweekly plan does — 26 half-payments add up to 13 full payments a year.
Pay extra or invest the difference?
Every extra dollar on the mortgage earns a guaranteed return equal to your interest rate — 6.5% in the example — with no market risk. Investing the same money might earn more or less over time, and it stays easier to reach in an emergency. A simple way to decide: grab any employer 401(k) match first, pay off debts with higher rates than your mortgage, keep an emergency fund, and then split the rest based on how much you value being mortgage-free versus keeping money flexible.
Common mistakes
- Not marking the extra as principal. Some servicers hold extra money or apply it to next month's payment. Choose “apply to principal” and check your next statement.
- Paying a fee for a biweekly program. You can get the same effect free by adding one-twelfth of a payment each month.
- Expecting the monthly bill to drop. Extra payments shorten the loan; they don't lower the required payment unless the lender agrees to re-amortize (sometimes called a recast).
- Draining your emergency fund. Home equity is hard to tap quickly; keep cash for surprises first.
Prepayment penalties and PMI
Most mortgages made today have no prepayment penalty. Under the CFPB's Regulation Z, a penalty is only allowed on certain fixed-rate qualified mortgages that aren't higher-priced loans, it is capped at 2% of the amount prepaid in the first two years and 1% in the third, and it can't apply after three years. Your closing disclosure says whether your loan has one.
Extra payments can also end private mortgage insurance sooner on a conventional loan. You can ask to cancel PMI once the balance reaches 80% of the home's original value, and it must end automatically when the balance is scheduled to reach 78%, as long as you're current.
When this calculator doesn't fit
It assumes a fixed rate. For an adjustable-rate mortgage, results are only accurate until the rate resets. It also leaves out escrow (taxes and insurance), and it doesn't weigh the trade-off of investing instead — if your mortgage rate is low, compare with the compound interest calculator. Carrying credit card or car debt? That usually costs more than your mortgage, so check the debt payoff calculator first.
Smart ways to pay extra
- Add a fixed amount to every payment (the “Extra every month” field).
- Put part of a yearly bonus or tax refund toward principal (“Extra once a year”).
- Round your payment up to the next $100 so the extra happens automatically.
- Pay half the payment every two weeks: that adds up to one extra monthly payment a year.
- Got a raise? See what it adds to each check with the paycheck calculator and send part of it to principal.
Before you overpay, keep an emergency fund, clear higher-interest debt, and check your loan has no prepayment penalty.