📖 Overview
Use this calculator to measure how extra principal payments can shorten payoff time.
⚙️ How It Works
This iterative amortization model estimates payoff months and interest when you add extra monthly principal payments.
The Formula
Each Month: Interest = Balance × (Rate ÷ 12) | Principal Paid = Min Payment + Extra − Interest | Balance = Balance − Principal Paid
| Balance | Remaining loan balance after each payment |
| Rate ÷ 12 | Monthly interest rate |
| Extra | Additional principal payment above minimum |
💡Adding just $50/month extra on a $10,000 credit card at 18% can save over $1,400 in interest and pay off the debt 14 months sooner.
Quick Reference
| $10k Loan @ 18% APR | Min only ($250) | +$50/mo | +$100/mo | +$200/mo |
|---|---|---|---|---|
| Months to payoff | 62 mo | 48 mo | 41 mo | 31 mo |
| Total interest | $5,445 | $3,953 | $3,157 | $2,166 |
| Interest saved | — | $1,492 | $2,288 | $3,279 |
Practical Tips
💡 Even small extra payments can shorten payoff materially.
💡 Confirm lender applies extra payment to principal.
Frequently Asked Questions
❓ Why can payoff fail in the model?
If payment is too low to cover monthly interest, balance will not decrease.
❓ Is this exact to my lender schedule?
It is a planning estimate; lender compounding and fees can differ.