Loan Amortization & Early Payoff Calculator
Loan amortization and early payoff: what this calculator shows
An amortizing loan (like most mortgages, auto loans, and personal loans) is paid down with regular payments that include both interest (the cost of borrowing) and principal (the amount that reduces your balance). Because interest is calculated from the remaining balance, the interest portion of each payment is usually highest at the beginning of the loan and declines over time as the balance falls.
This calculator generates an amortization schedule for your base loan and then estimates how your schedule changes when you add an extra monthly payment and/or aim for a target payoff year. The primary outputs to interpret are your monthly payment, total interest paid, the payoff date, and the interest savings from paying down principal faster.
Core formulas used in amortization
For a standard fixed-rate loan with payments made monthly, the fixed monthly payment is determined by the principal, interest rate, and number of payments.
Monthly payment
Let:
- P = loan principal (starting balance)
- i = monthly interest rate (annual rate ÷ 12)
- N = total number of monthly payments (term in years × 12)
The monthly payment (M) is:
Interest and principal split each month
For a given month, if your balance at the start of the month is B:
- Interest = B × i
- Principal paid = Payment − Interest
- New balance = B − Principal paid
When you make an additional principal payment, it reduces the balance sooner, which reduces future interest because future interest is computed from a smaller balance.
How to interpret the results
- Monthly payment (base): the required payment to fully amortize the loan over the selected term.
- Payoff date: when the balance reaches $0 under the chosen strategy (standard vs. extra payments).
- Total interest: sum of all monthly interest charges over the life of the loan.
- Interest savings: standard total interest minus early-payoff total interest.
In general, extra payments earlier in the schedule tend to save more interest than the same total extra paid later, because they reduce the balance for a longer period of time.
Worked example (fixed-rate mortgage)
Assume:
- Loan amount: $300,000
- Annual interest rate: 6.5%
- Term: 30 years (360 payments)
- Extra monthly payment: $200 (applied to principal)
First compute the monthly rate: i = 0.065 ÷ 12 ≈ 0.0054167. The base monthly payment from the formula above is about $1,896 (rounded). In the first month, interest is roughly 300,000 × 0.0054167 ≈ $1,625, leaving about $271 going to principal (again, rounded). If you add $200 extra to principal, you effectively reduce the balance by about $471 in month one instead of $271.
That faster balance reduction compounds over time: the loan typically pays off several years earlier, and the total interest paid drops materially. Your exact payoff date and savings depend on rounding, payment timing, and whether the extra payment is applied consistently every month.
Comparing strategies
| Strategy | What changes? | Typical impact | Best when |
|---|---|---|---|
| Standard amortization | Pay the required monthly payment only | Highest total interest; payoff at end of term | You need maximum monthly flexibility |
| Extra monthly principal | Add a fixed amount to principal each month | Earlier payoff; lower total interest | You want a simple, consistent early-payoff plan |
| Target payoff year | Choose a payoff deadline; calculator can infer the needed extra | Aligns payments to a goal date; may increase required cash flow | You’re planning around retirement, moving, or other milestones |
| Refinancing (conceptual comparison) | Replace the loan with a lower rate (plus closing costs) | Can lower payment and/or interest, but costs may offset savings | Rates dropped and you’ll keep the loan long enough to break even |
Assumptions and limitations (important)
- Estimates only: Results are illustrative and may not match a lender statement exactly due to rounding, payment posting rules, and escrow conventions.
- Monthly compounding/payment frequency: Assumes interest accrues and payments are applied monthly. Some loans use daily interest accrual or different conventions.
- Payment timing: Assumes payments are made on schedule. Late payments, partial payments, or irregular extra payments can change outcomes.
- Extra payments: Assumes extra amounts are applied directly to principal (not to future interest) and do not trigger prepayment penalties.
- Excludes escrow and fees: Property taxes, homeowners insurance, mortgage insurance (PMI), HOA dues, and lender fees are not included unless explicitly modeled.
- Refinancing not fully modeled: If you compare a refinance scenario, you must account for closing costs, new term length, and how long you’ll keep the loan to evaluate break-even.
- Rate type: Intended for fixed-rate amortizing loans. Adjustable-rate mortgages (ARMs) require rate-change assumptions that can materially alter schedules.