What an extra payment does
Extra principal reduces the balance that next month's interest is calculated on. The gain from an extra payment is the interest saved minus any early-repayment fee. Looking only at the interest saved overstates the gain when the contract charges a fee. Write the fee down before you compare it with the loan page. The sample amounts on the page are not a quote from a bank, a card issuer, or an employer. Replace them with the figures on your statement and run the page twice to see which field moves the result.
Where the money goes
On a level loan, the scheduled payment already covers that month's interest. Anything extra reduces principal.
The loan page shows the interest saved against the same loan with no extra.
On a 20,000 loan at 6% for 5 years, an extra 100 each month reduces principal above the scheduled payment. Next month's interest is calculated on the smaller balance. The loan page compares that with the same loan and no extra, and shows the interest saved.
Timing
An extra paid early saves more than the same extra paid near the end, because fewer months of interest remain on that amount.
A one-time extra and a monthly extra are different. Use the monthly extra field for the second.
The same 1,000 paid in the first year saves more interest than the same 1,000 paid in the last year, because fewer months of interest remain on it. A one-time amount and a monthly amount are different. Use the extra field for the monthly one.
Read the contract
Some loans charge a fee for paying early. The calculator does not know that fee.
If the fee is larger than the interest you save, the extra is not a gain.
If an early-repayment fee is larger than the interest saved, the extra is not a gain. The calculator does not know the fee. Subtract it from the interest saved before you send the money.
Related tools
These figures are estimates you can recompute from the formula on the page. They are not a lender quote, a tax bill, a payroll statement, or financial advice.