How Our Calculators Work

Our calculators are meant to be trusted, so here is exactly what they do. Everything you type stays in your browser; we don’t collect or store it.

The monthly payment

For a fixed-rate loan with equal monthly payments, we use the standard amortization formula, the same one most installment lenders use. Interest each month is charged on the balance still owed; the rest of the payment reduces the balance. We handle the zero-interest case separately: the payment is simply the principal divided by the number of months.

Fees and the money you receive

If a fee is deducted from the loan, you repay the full amount but receive less. If it’s added to the balance, you receive the full amount and pay interest on the fee too. Either way, we define the cost of borrowing as everything you repay minus the money you actually got.

Estimated APR

When a tool shows an APR, it’s our estimate: the yearly rate at which your scheduled payments are worth exactly what you received, solved numerically. A lender’s Truth in Lending disclosure is the official figure and may include charges we don’t know about. We label our APR as an estimate everywhere it appears. See APR versus interest rate for the worked example.

Comparisons and break-evens

The consolidation and refinance tools compare your current situation with a proposed one by total cost in dollars, accounting for fees and any difference in term. A lower monthly payment is not treated as a saving on its own; a longer term that raises total interest is shown as a cost. The refinance tool also reports a break-even: how long before the savings outweigh the fees.

Validation and edge cases

The math library is pure and tested against an independent reference implementation, so the on-screen result matches the reference to the cent. Invalid or empty inputs are caught with clear messages; we avoid false precision by rounding only at display and stating assumptions in every result.

What the calculators can’t tell you

• The rate you’d actually be offered, which depends on the lender, your credit, income and the loan.
• Daily-interest accrual, late fees, or a lender’s exact rounding, which can shift totals by a few dollars.
• Variable-rate loans, where the payment can change over time.

The numbers here are estimates for planning, not offers or credit decisions.