How the rate changes the cost

The monthly sequence

Interest is calculated from the opening balance and periodic rate. The payment first covers that interest; the remainder reduces principal. Next month begins with a slightly smaller balance.

Why extra principal works

A principal-only payment reduces every later interest calculation. Confirm that the servicer applies it to principal and does not merely advance the next due date.

$30,000 at 8% for 60 months

Rate input
Payment: about $608
Rate input
First-month interest: $200
Rate input
First-month principal: about $408
Rate input
Total interest if held: about $6,498

Illustrative calculation. Rounded values can differ from a lender or card issuer schedule.

This decision belongs in The Real Cost of Borrowing. Use Extra Loan Payments: Calculate the Interest and Time Actually Saved to check the next assumption, then test the numbers in the Loan Total Cost Calculator.

If a large principal payment is followed by a lower required payment, understand what a mortgage recast changes and what it leaves unchanged.

Interpret the result in context

The comparison that matters

For refinancing, compare remaining cash flows from today - Not interest already paid. Past interest is sunk; the new closing costs and the future schedule determine the decision.

Inputs that change the answer

The payment alone cannot show how quickly debt is disappearing. Track the opening balance, interest charged and principal reduction at the same point in each schedule.

Numbers needed to rebuild the balance

  • Original principal, rate and first payment date.
  • Scheduled payment and any recurring required charge.
  • Current payoff balance rather than the statement principal alone.
  • Obtain the rate, payment frequency, term, and opening balance.
  • Calculate periodic interest before subtracting principal.
  • Check the balance at the dates you may sell or refinance.
  • Verify how the servicer applies extra payments.
  • Use a payoff statement for a real transaction rather than an estimate.

Rebuild the calculation with your terms

Loan Total Cost Calculator

Separate the advertised rate from the dollars that leave your account.

Where rate comparisons go wrong

  • Multiplying the payment by months and calling the difference interest when fees are included.
  • Assuming half the term means half the principal is repaid.
  • Counting already-paid interest in a forward-looking refinance decision.
  • Sending extra money without a principal-allocation instruction.

Questions this decision usually raises

Why is early interest so high?

Interest is calculated on the largest balance at the beginning; the principal share grows as the balance falls.

Does a biweekly plan always save money?

Only if it causes earlier or additional principal payments without offsetting fees.

Can my schedule differ from a calculator?

Yes. Day-count rules, posting dates, rounding, fees, and rate changes can create differences.

Check the rules and definitions

This page explains a calculation, not a personal recommendation. Contract rules and consumer protections vary by product and jurisdiction. Check the current agreement and official guidance before acting.