A smaller monthly payment is not automatically a less expensive loan. Extending the repayment term spreads principal across more installments but can increase the amount paid in interest. A useful comparison keeps the borrowed amount and rate consistent, then looks at payment size, total cost, and the assumptions behind both.
The Loan and Mortgage Calculators model fixed-rate, fully amortizing monthly payments. Their new repayment planners show how each installment is divided and compare a regular schedule with a constant extra principal payment. These are mathematical scenarios, not lender quotes or a recommendation to borrow.
1. Put the borrowing scenarios on the same basis
Start with financed principal: the amount that actually accrues interest. For a 400,000 home with an 80,000 cash down payment, principal is 320,000. A down payment is an amount, not the number 20 merely because it represents 20% of that price. When comparing loans, do not change the principal inadvertently.
Enter the nominal annual interest rate used to calculate interest. An advertised APR can include fees and is not always interchangeable with that rate. This calculator does not reconstruct an APR from fees. Collect the lender's interest rate, fee disclosure, term, and payment schedule before comparing a quoted installment with a modeled one.
2. Understand the fixed-rate payment formula
The monthly payment is P × r / (1 − (1 + r)^−n), where P is principal, r is annual interest rate / 100 / 12, and n is the number of months. At zero interest the expression is replaced by P / n. This is the principal-and-interest payment; it is not necessarily the total amount debited by the lender.
For a 10,000 loan at 7% for five years, the modeled payment is approximately 198.01 and total interest is approximately 1,880.72. Intermediate values are not rounded to cents. Multiplying the displayed 198.01 by 60 can differ slightly from the internally precise total, which is why the tool shows the total separately.
3. Read the first row of an amortization schedule
For that 10,000 loan, the first month's interest is 10,000 × 0.07 / 12, approximately 58.33. Subtract it from the installment to find approximately 139.68 of principal repayment. The outstanding balance becomes approximately 9,860.32. Next month's interest is calculated on that smaller balance.
Interest is generally a larger share early in the schedule because the outstanding principal is higher. The modeled installment remains constant, but its split changes. At the end, the last payment may be smaller than a regular installment so that the schedule does not pay more principal than remains.
| Column | Interpretation |
|---|---|
| Paid | Principal and interest paid that month, including selected extra principal |
| Principal | Amount reducing the outstanding balance |
| Interest | Charge computed from the opening balance and monthly rate |
| Balance | Principal still outstanding after the installment |
4. Test extra principal without assuming lender rules
The planner applies the same extra amount in every month starting with month one. It first pays monthly interest, then applies the rest to principal. Compare total interest and payoff month with the no-extra baseline. The regular installment is not automatically recalculated downward; the original loan is instead paid off sooner.
Before using an overpayment strategy in a real contract, ask how your lender applies extra money, whether it must be marked principal-only, and whether a prepayment charge applies. This planner excludes those charges and does not model recasting, a rate reset, missed payments, or refinancing.
5. Keep modeled borrowing cost separate from the full budget
A mortgage payment estimate excludes property taxes, insurance, mortgage insurance, association fees, and closing costs. Those can materially change the cash needed for housing. A lender's statement may bundle several items into one payment, while the calculator reports only principal and interest.
CSV export includes every modeled month, not just the first twelve visible rows. Use it to inspect repayment timing or compare scenarios in a spreadsheet. The file is created on your device; avoid sending a file containing your real financial assumptions to anyone you do not intend to share them with.
Before using the result
- Use the same financed principal when comparing terms or rates.
- Separate nominal interest from a fee-inclusive APR.
- Compare total interest as well as the monthly installment.
- Confirm extra-payment and prepayment rules with the lender.
- Add taxes, insurance, fees, and other costs outside this model.
Try the tools in this guide
- Loan Calculator — Estimate monthly loan payments, total repayment, and total interest. Open the advanced planner.
- Mortgage Calculator — Estimate a principal-and-interest mortgage payment from price, down payment, rate, and term. Open the advanced planner.
Sources and calculation notes
Examples and comparisons are original illustrative calculations, not copied case studies. Reference links explain standards or concepts; they do not endorse this site. See the calculation methodology for engine behavior and precision.
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