Monster Amortization Calculator
See every payment for the life of a loan — principal, interest and remaining balance — and exactly how extra payments pull your payoff date forward.
Payment schedule
| Period | Payment | Principal | Interest | Extra | Balance |
|---|
The schedule starts next month and assumes every payment is made on time. Extra payments go straight to principal.
How it works
Each month the lender charges interest on the remaining balance (balance × annual rate ÷ 12); the rest of your payment reduces principal. Early on, most of the payment is interest; near the end, most is principal. Extra payments skip the interest step entirely — they cut principal directly, so every later month charges less interest. The "interest saved" figure compares your schedule against the same loan with no extra payments.
Formula & worked example
Formula: Monthly interest = remaining balance × annual rate ÷ 12. The rest of the payment reduces principal, so the interest share shrinks every month.
Worked example: On a $250,000 loan at 6% over 30 years (payment $1,498.88), month one charges $1,250.00 of interest and only $248.88 comes off the principal; in the final year almost the whole payment is principal.
When to use this
- You want the month-by-month truth of a loan: balance, interest and principal over time.
- You are deciding whether extra payments beat investing the difference.
- A lump sum (bonus, tax refund) is coming and you want its payoff effect dated.
- You are near payoff and want the exact month the balance dies.
How to use it, step by step
- Enter amount, rate and term.
- Add a monthly extra if you pay ahead regularly.
- Optionally add a one-time lump and the month it lands.
- Read the payoff date and interest with and without extras.
- Use the year-by-year table to watch the principal share overtake interest.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Loan amount / rate / term | The loan as written | Works for mortgage, auto, personal |
| Extra per month | Added to principal every month | Even $50–$150 moves years on long loans |
| Lump sum + month | One-time extra and when it lands | Earlier lumps save more interest |
Another worked example
A $180,000 loan at 5% over 15 years has a payment of $1,423.43 and $76,217.14 of lifetime interest. Add $150 a month and the loan dies in 156 months instead of 180 — two full years early — with interest cut to $65,033.93, saving $11,183.21. Extra payments are worth most on long, high-rate loans.
Frequently asked questions
Why is so much of my early payment interest?
Interest is charged on the outstanding balance, which is largest at the start. On a $250,000 loan at 6.5%, the first payment is about $1,354 interest and only $226 principal. The split flips as the balance falls.
Is it better to pay extra monthly or one lump sum?
Earlier money works harder, because it stops interest from being charged on that principal for every remaining month. A lump sum in month 12 saves more than the same total spread later — compare both above.
What does "amortization" actually mean?
It is the process of paying a loan off in fixed installments over time, with each payment split between interest and principal according to the remaining balance.
Does this schedule include taxes and insurance?
No — this is the loan itself (principal and interest). If your lender collects escrow for taxes and insurance, your real monthly outlay is higher.
🔑 Already have a mortgage?
The Mortgage Payoff Calculator starts from your current balance and shows your debt-free date with extra payments.
When does principal start beating interest in my payment?
When the remaining balance falls enough that balance × rate ÷ 12 drops below half the payment. On a 30-year mortgage that crossover can take a decade or more, which is why early extra payments — made while the balance is big — do the heaviest lifting.