Where the lump sum can come from
- Employer SLR contribution under IRC Section 127, up to $5,250/year, tax-free
- Year-end performance bonus redirected to principal
- Federal or state tax refund
- Sign-on bonus as part of a job offer
- Inheritance, gift, or asset sale
How the calculator works
Standard monthly amortization. Each month: interest = balance × (APR ÷ 12); principal portion = payment − interest; new balance = old balance − principal. The lump sum is applied to principal in month one, instantly reducing the balance against which interest accrues.