How it works
Every time you borrow money through an installment loan, you lock in a mathematical contract: the lender calculates a fixed monthly figure that will retire your debt precisely by the final month of the term. That figure is your repayment. While the installment amount stays constant, what happens inside it shifts every single month. Early on, most of your cash disappears into interest charges. By the end, nearly all of it chips away at the principal balance.
This calculator reveals both the baseline math and the impact of overpayments. You enter how much you owe, the annual interest rate, and the remaining term in months. The tool returns your required monthly installment and the total interest cost over the life of the loan. It also models a powerful scenario: what happens if you pay extra toward principal every month?
Unlike a mortgage-specific payoff tool focused on home loans with escrow, this is a general-purpose "what if I pay extra" engine. It works identically for a personal loan, auto financing, student debt, or a business credit line. The math of amortization does not care what asset the borrowing funded.
The formula
M = P × [r(1 + r)^n] / [(1 + r)^n - 1]
Where P is the loan principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of months in the term. The result M is your fixed monthly repayment.
To find total interest, multiply the monthly repayment by the number of months, then subtract the original principal.
Worked example
Consider an existing $18,000 loan. The borrower decides to add an extra $100/month.
Let us assume the loan carries a 9% annual interest rate with a 48-month remaining term. First, we calculate the required installment:
- P = $18,000
- r = 0.09 / 12 = 0.0075
- n = 48
Plugging into the formula:
- (1 + r)^n = (1.0075)^48 ≈ 1.4314
- M = 18,000 × [0.0075 × 1.4314] / [1.4314 - 1]
- M = 18,000 × 0.0107355 / 0.4314
- M ≈ $447.74
Your required monthly repayment is $447.74.
Total interest without extra payments: (447.74 × 48) - 18,000 = $3,491.52.
Now the borrower adds an extra $100/month toward principal, making the actual monthly outlay $547.74. Because that extra $100 reduces the principal faster, interest accrues on a smaller balance each subsequent month. The loan no longer lasts 48 months. Running the amortization iteration:
| Scenario | Monthly Outlay | Term | Total Interest |
|---|---|---|---|
| Minimum payment only | $447.74 | 48 months | $3,491.52 |
| With $100 extra | $547.74 | ~39 months | ~$2,786 |
Adding $100/month shortens the loan by roughly 9 months and saves about $705 in interest. That is the direct mechanical result of principal reduction shrinking the base on which future interest compounds.
Things to watch
Two practical points matter when running these numbers. First, confirm that extra payments actually go toward principal. Some lenders apply overpayments to next month's bill, which simply pushes your due date forward rather than shortening the term. Second, verify whether your loan agreement includes prepayment penalties. These are rare in consumer personal loans but can appear in auto financing and some business credit agreements. A penalty could erode the savings shown in the calculator.
This tool provides a mathematical estimate based on a fixed rate and level payments. It is not professional financial advice.