Mortgage Calculator
Runs locallyInput stays in your browserFree, no sign-up
A mortgage payment is not the loan amount divided by the number of months. The lender first converts the annual rate into a monthly rate (annual rate divided by 12); each payment covers that month's interest first, and only the remainder reduces the principal — which is why interest dominates the early payments. With equal installments, total principal plus interest is spread into one fixed monthly payment: payment = P × r × (1+r)^n ÷ ((1+r)^n − 1). With equal principal, you repay a fixed slice of principal (P ÷ n) every month while interest is charged on the shrinking balance, so payments decrease month after month. Both methods repay exactly the same principal; they differ only in total interest and cash-flow timing: equal installments are gentler early on but cost more interest overall, while equal principal costs less interest but demands higher payments in the first years.
Three scenarios cover most uses. First, the affordability check before buying: after the down payment you borrow $300,000 at 6.5% for 30 years, the calculator shows about $1,896 a month — compare that against household income and the familiar 28–36% housing-cost guideline. If the ratio breaks the ceiling, the price or the down payment has to move. Second, choosing a repayment method: put both methods side by side and the trade-off is concrete — on that same $300,000 loan, equal principal saves roughly $55,000 in total interest but the first payment runs about $700 higher than the fixed alternative. Whether you can stomach the early years becomes a numbers question, not a gut feeling. Third, planning an early payoff: open the amortization schedule, watch the remaining-balance column, and combine it with your prepayment-penalty clause to decide how much to pay early and when.
A few edge cases deserve a clear statement. First, rate basis: enter the annual rate (for example 6.5), not a monthly rate; the actual rate you pay is whatever your loan contract says, and this tool cannot predict rate resets. Second, day-count differences: this calculator uses annual rate ÷ 12 as the monthly rate, while lenders often accrue interest daily, so expect a few dollars of difference plus per-period rounding — that is normal. Third, zero rate: with a 0% annual rate the equal-installment payment degrades gracefully to principal divided by the number of periods. Fourth, input limits: the term accepts 1–30 years and the rate 0–36%; anything outside is rejected with a prompt. Fifth, the figures exclude property tax, insurance, PMI and fees — the monthly payment is not the full cost of owning the home, and prepayment penalties or rate-adjustment clauses in your contract are outside the calculation. For large decisions, the lender's official amortization statement governs.
Using this page is straightforward: enter the loan amount, annual rate and term above, switch between equal installments and equal principal, and the payment, total interest and total repayment update live. Click “Show amortization schedule” for the month-by-month breakdown of payment, principal, interest and remaining balance; download it as CSV when you want to keep a copy. The “Load example” button fills in a sample loan so you can explore the interface, and everything runs locally in your browser — nothing is uploaded.
How to use
- Enter the loan amount, annual rate (%) and loan term (years) above — figures update live, no button needed.
- Switch between “Equal installments” and “Equal principal” to compare monthly payment, total interest and total repayment.
- Click “Show amortization schedule” to expand the month-by-month breakdown of payment, principal, interest and remaining balance; use “Download schedule” to save it as CSV.
- Use “Copy result” to share the key figures with family or your agent; “Clear” resets the inputs.
FAQ
Equal installments vs equal principal — which costs less?
Equal installments keep every payment identical, with interest dominating the early years; equal principal repays a fixed principal slice plus shrinking interest, so payments fall each month. For the same loan, equal principal always costs less total interest — about $55,000 less on a $300,000, 6.5%, 30-year loan — but its first payments are noticeably higher. There is no universally cheaper choice: pick equal installments when cash is tight now and income is expected to rise; pick equal principal when you can handle higher early payments and want to minimize interest.
How is the monthly payment calculated?
The annual rate is divided by 12 to get the monthly rate r, and n = years × 12. Equal installments: payment = P × r × (1+r)^n ÷ ((1+r)^n − 1). Equal principal: monthly principal = P ÷ n, month k interest = (P − principal repaid so far) × r, and that month's payment = monthly principal + month k interest. The amortization schedule on this page is computed period by period with exactly this logic, so every split reconciles.
Why does my bank's payment differ by a few dollars?
Three reasons: lenders usually accrue interest daily from a daily rate rather than annual ÷ 12; per-period rounding differs; and when the first payment covers a partial month, its interest is prorated by actual days. A few dollars of difference is normal — the loan contract and the lender's official statement are authoritative.
Which method is better if I plan to pay off early?
It comes down to how much interest you have already paid. With equal installments, early payments are mostly interest, so prepaying early saves the most; after seven or eight years most of the interest is already paid and early payoff matters less. Equal principal behaves more linearly since the principal slice is constant. Before deciding, expand the schedule on this page, check the remaining-balance column, and factor in your contract's prepayment-penalty clause.
Is my loan amount uploaded anywhere?
No. Every calculation runs locally in your browser; your inputs never touch a server, are never recorded, and vanish when you close the page. The tool works offline too.