About the Loan Amortization Calculator (with Extra Payments)
Most amortization calculators stop at “this is your payment.” This one answers the follow-up question people actually have: what if I add a bit every month?
Because interest is charged on the live balance, every extra dollar immediately stops compounding the rate against itself — and it does so for every remaining month of the loan. Early extra payments therefore compound their own benefit in reverse, and the savings surprise people every time.
The formula behind it
Two schedules are simulated, the only difference being the payment size:
Standard principal = M − balance × (APR/12) • Accelerated principal = (M + extra) − balance × (APR/12)The simulation repeats monthly, in each case applying interest first, then all remaining funds to principal, until the balance reaches zero. M is the standard payment from the annuity formula; extra is what you volunteer each month.
Worked example
The defaults above model a $32,000 debt at 5.8% over 10 years, plus $100 extra per month:
- Standard payment ≈ $352.06; payoff takes the full 120 months and about $10,247 of interest.
- With the $100 extra ($452.06 total), payoff completes in about 87 months (7.3 years).
Takeaway: ≈ 33 months (over 2.7 full years) eliminated and roughly $2,990 of interest avoided — 100 extra dollars per month effectively “earning” 5.8% risk-free.
What moves the result
- Timing — extra dollars sent in year 1 buy more months of avoided interest than the same dollars in year 8.
- Rate type — the higher the loan’s rate, the more valuable extra principal becomes.
- Lender posting rules — some servicers must be told to apply extra as principal, not to hold it as “future payment.”
- Payment size — one larger annual payment (e.g., a tax refund) replaces 12 smaller extras for a fraction of the monthly habit.
Practical tips
- Before accelerating any loan, check whether a提前 payment would reduce your emergency cushion — the first buffer beats the best schedule.
- Autopay enrollment usually trims student-loan rates by 0.25 percentage points; that discount applies to every month of the life of the loan.
- Compare this tool’s savings with what the same extra would earn in a high-yield savings account when deciding whether to prepay or invest.
- Against mortgages costing 6–7%, an extra principal every month beats most HYSA yields right now — but inflation-adjust spending too, not just the loan.
- Print both schedules before and after the lender’s payoff quote so you know the servicer applied the volunteers correctly.
Frequently asked questions
Is it better to pay extra monthly or save for a bigger lump payment?
Earlier wins. Interest is calculated each month on the current balance, so monthly extras outrun a same-total lump sum paid later.
Can I use this on a car loan or amortizing student loan?
Yes — it is loan-type-neutral: any fixed-rate, level-payment loan reproduces this math.
Will my lender automatically apply the extra as principal?
Not always. Bills marked “extra to principal” are usually honored, but cataloging it in writing with your servicer removes microwaiting risk.