On a 30-year mortgage, your early payments go mostly to interest. Understanding why, and how a small extra payment changes the whole schedule, can save tens of thousands over the life of a loan. Here's the math with a full example.
The monthly payment formula
For a fixed-rate loan:
Payment = P × i ÷ (1 − (1 + i)^−N)
- P = loan amount (principal)
- i = monthly interest rate = annual rate ÷ 12
- N = number of monthly payments = years × 12
Worked example: $300,000 at 6.5% for 30 years
- i = 6.5% ÷ 12 ≈ 0.5417%
- N = 360
- Payment ≈ $1,896.20 a month (principal and interest only)
Over 30 years you pay 360 × $1,896.20 ≈ $682,633, of which $382,633 is interest, more than the original loan.
How amortization works
Each month:
- Interest = Remaining balance × i
- Principal paid = Payment − Interest
- New balance = Balance − Principal paid
In month one: interest = $300,000 × 0.5417% ≈ $1,625, so only about $271 of the $1,896 payment reduces the balance. As the balance falls, the interest share shrinks and the principal share grows.
Year-by-year snapshot
| Year | Principal paid that year | Interest paid that year | Balance at year end |
|---|---|---|---|
| 1 | $3,353 | $19,401 | $296,647 |
| 5 | $4,346 | $18,409 | $280,833 |
| 10 | $6,009 | $16,745 | $254,328 |
| 15 | $8,310 | $14,445 | $217,677 |
| 20 | $11,491 | $11,263 | $166,996 |
| 25 | $15,890 | $6,864 | $96,912 |
| 30 | $21,973 | $781 | $0 |
After 10 years of payments (about $227,500 paid), you still owe about $254,000. Principal payments only overtake interest around year 20.
See the full schedule for any loan with the free loan & mortgage amortization calculator.
What extra payments do
Extra money paid toward principal reduces the balance immediately, so every later month charges interest on a smaller amount. On the same loan:
| Extra per month | Payoff time | Total interest | Interest saved |
|---|---|---|---|
| $0 | 30.0 years | $382,633 | — |
| $100 | 26.0 years | $321,639 | $60,995 |
| $200 | 23.1 years | $279,185 | $103,449 |
| $500 | 17.5 years | $202,874 | $179,759 |
An extra $200 a month, about 10% more than the regular payment, cuts almost 7 years and over $100,000 of interest.
15-year vs 30-year
The same $300,000 at 6.5% over 15 years costs $2,613.32 a month and about $170,398 in interest, versus $382,633 on the 30-year (in reality, 15-year loans often come with a lower rate too). The trade-off is a higher required payment. A 30-year loan with voluntary extra payments keeps the flexibility to pay less in a tight month.
Before you prepay, check
- Prepayment penalties. Some loans charge fees for paying early. Read your loan terms.
- How the lender applies extra money. Make sure extra payments go to principal, not to next month's payment.
- Higher-interest debt first. Paying down a 24% credit card beats prepaying a 6.5% mortgage. See debt snowball vs avalanche.
- Emergency savings. Money paid into a mortgage is hard to get back out. Keep a cash buffer, and use the savings goal calculator to plan it.
- Other goals. Whether prepaying beats investing depends on your rate, taxes and risk tolerance. There's no single right answer.
What the payment doesn't include
Many mortgage payments also include property tax, homeowners insurance and sometimes mortgage insurance, often through an escrow account. The amortization math covers only principal and interest. Budget for the full monthly cost.
Lump sums vs monthly extras
A one-time lump sum early in the loan has a similar effect: the earlier principal is repaid, the more interest it avoids. A windfall paid in year two saves far more than the same amount paid in year twenty-five, because it removes decades of interest charges on that money.
Works for other loans too
The same formula and schedule apply to car loans, student loans and personal loans with a fixed rate and monthly payments. Just change the amount, rate and term.
Frequently asked questions
Is it better to make one extra payment a year or a bit each month? Monthly extra payments save slightly more because the balance falls sooner, but both work. Choose what fits your cash flow.
Does refinancing beat extra payments? If you can get a meaningfully lower rate, refinancing lowers the interest on the whole balance, but closing costs and a new term matter. Compare the total cost of both options.
Why is my lender's schedule slightly different? Lenders may calculate interest daily, round payments differently or have a different first payment date. Differences of a few dollars are normal.
Does an extra payment lower my monthly payment? Usually not. On most fixed-rate loans, the required payment stays the same and the loan simply ends sooner. Some lenders offer a "recast" that lowers the payment after a large prepayment.
What about biweekly payments? Paying half the monthly payment every two weeks gives 26 half-payments, the equivalent of 13 monthly payments a year. That one extra payment a year works like an automatic prepayment, as long as your lender applies it to principal right away.
Key takeaways
- Early mortgage payments are mostly interest.
- Extra principal payments save interest on every later month.
- $200 extra a month on a $300k, 6.5% loan saves about $103k in interest.
- Check for prepayment penalties and pay high-interest debt first.
Plan it in your budget
Finding an extra $100–$200 a month usually comes down to a clear budget. The 2027 Budget Planner template includes a monthly dashboard, savings goals and a debt payoff tracker, so you can see where extra payments come from and how fast the balance falls.
Educational example. Your lender's figures may differ slightly due to rounding and payment dates.