Mortgage Calculator
Estimate the monthly principal-and-interest payment, total interest, and amortization for a home loan.
Principal & interest over 360 payments.
The balance falls slowly at first — early payments are mostly interest — then drops faster as more of each payment goes to principal.
A mortgage payment is M = P · r(1+r)ⁿ ÷ ((1+r)ⁿ − 1), where P is the loan, r the monthly rate, and n the number of months. A $300,000 loan at 6.5% over 30 years (r ≈ 0.0054167, n = 360) costs about $1,896.20 a month — roughly $382,633 in total interest.
What the payment covers
This calculator finds the principal-and-interest portion of a mortgage — the fixed amount that fully repays the loan over its term. It’s an amortizing loan: every month the same payment is split between interest on the balance still owed and principal that pays the loan down. Early on most of the payment is interest; as the balance shrinks, more of each payment goes to principal, even though the payment itself never changes. Real bills often add property tax, homeowners insurance, and PMI (together “PITI”); those escrow items are not included here.
r = annual% ÷ 100 ÷ 12 (monthly rate); n = years × 12 (months). If r = 0, M = P ÷ n. Total interest = M × n − P.
Worked example
You borrow $300,000 at 6.5% annual interest for 30 years.
- 1 Find the monthly rate. r = 6.5 ÷ 100 ÷ 12 ≈ 0.0054167.
- 2 Find the number of payments. n = 30 × 12 = 360 months.
- 3 Raise (1 + r) to the n. (1.0054167)³⁶⁰ ≈ 6.9918.
- 4 Apply the payment formula. M = 300,000 × 0.0054167 × 6.9918 ÷ (6.9918 − 1) ≈ $1,896.20.
- 5 Get the totals. Total paid = 1,896.20 × 360 ≈ $682,633; interest = 682,633 − 300,000 ≈ $382,633.
How term and rate change a $300,000 loan
A shorter term or lower rate cuts total interest sharply. A longer term lowers the monthly payment but you pay far more interest overall.
| Rate | Term | Monthly payment | Total interest |
|---|---|---|---|
| 5.0% | 30 years | $1,610.46 | $279,767 |
| 6.5% | 30 years | $1,896.20 | $382,633 |
| 8.0% | 30 years | $2,201.29 | $492,466 |
| 6.5% | 15 years | $2,613.32 | $170,398 |
Why 30-year loans cost so much interest
Over 30 years you carry the balance for 360 months, and interest is charged on whatever is still owed each month. Because the balance starts high and falls slowly at first, the interest piles up: on the $300,000 example above the interest — about $382,633 — actually exceeds the amount borrowed. A 15-year term at the same rate more than doubles the monthly payment but cuts total interest to roughly $170,398, because the balance is paid down far faster and spends less time accruing.
Paying extra toward principal helps disproportionately. Any amount above the scheduled payment reduces the balance directly, so every future month’s interest is charged on a smaller number. That shortens the term and trims total interest by more than the extra dollars you put in — the earlier you do it, the bigger the effect.