Methodology
Every number on this site comes from the standard amortization schedule. No shortcuts, no rounding tricks — here is the exact math.
The monthly payment formula
For a loan of principal P, monthly rate r = APR ÷ 12, and term of n months, the level payment is:
M = P · r / (1 − (1 + r)^−n)
This is the standard level-payment annuity formula used by lenders for simple-interest loans. If r = 0 the payment is simply P ÷ n.
Month-by-month schedule
- Interest for the month = balance × r.
- Principal reduction = payment + extra payment − interest.
- New balance = old balance − principal reduction.
- Repeat until balance ≤ 0; the final month's payment is truncated to the remaining balance plus interest.
Extra payments and lump sums
Extra monthly payments increase the principal-reduction step on every month after they start. One-time lump sums are applied as an additional principal reduction on the month you specify — mirroring how servicers credit a windfall payment.
Biweekly acceleration
Biweekly mode models 26 half-payments per year (one every 14 days), which equals 13 full monthly payments annually. The 13th payment is applied entirely to principal once per year, so the loan behaves like a monthly schedule with roughly +8.3% extra principal each year.
Snowball vs. avalanche (multi-loan)
When you add multiple loans, freed-up minimums from paid-off loans roll onto the remaining loans each month — snowball targets the smallest balance first, avalanche targets the highest rate first. Both use identical rollover mechanics; only the targeting order differs.
Why our numbers may differ slightly from your servicer
- Servicers accrue interest daily; we use monthly periods. On a 60-month loan the difference is typically under $5.
- Payment dates, leap months, and rounding conventions vary by lender.
- Precomputed-interest loans (rare) do not follow this schedule — check your contract.
Educational tool, not financial advice. Your servicer's official amortization schedule governs your actual loan.