Monthly Payment
Total Interest
Total Amount Paid
Table of Contents
Full Amortization Schedule, Calculated Instantly
Every monthly payment on an amortized loan is split between interest and principal — and the split shifts month by month. This calculator builds the full schedule so you can see exactly where each dollar goes from the first payment to the last.
How the amortization calculation works
Every fully amortizing fixed-rate loan uses the same standard formula to determine the monthly payment that will reduce the balance to zero by the end of the term:
M = P × [ r(1+r)n ] ÷ [ (1+r)n − 1 ]
- M — monthly payment (principal + interest)
- P — loan principal (the amount borrowed)
- r — monthly interest rate (the annual rate divided by 12)
- n — total number of monthly payments (term in years × 12)
Worked example using the calculator's default values ($25,000 loan, 5.5% APR, 5-year term):
- P = $25,000
- r = 0.055 ÷ 12 = 0.004583
- n = 5 × 12 = 60 payments
- (1 + r)n ≈ 1.3157
- M = 25,000 × (0.004583 × 1.3157) ÷ (1.3157 − 1) ≈ $477.53 per month
Total paid over 5 years: $28,651.79. Total interest: $3,651.79. The schedule shows that month 1 splits roughly $114.58 interest and $362.95 principal, while month 60 is nearly all principal.
Reading the amortization schedule
Each row of the schedule shows one month and four numbers: total payment, principal portion, interest portion, and remaining balance. Three patterns are worth understanding:
- The total payment stays fixed for a fixed-rate loan — it is the M from the formula.
- The interest portion shrinks every month because it is calculated on a smaller balance each time. The principal portion grows by the same amount the interest portion shrinks.
- The crossover point (when principal first exceeds interest) happens later than most people expect — for a typical 30-year mortgage, it is around year 18 to 20.
How the interest/principal split shifts across loan types
The longer the term and the higher the rate, the more front-loaded the interest is. Comparing a $25,000 loan at 5.5% across terms:
| Term | Monthly payment | Total interest | Interest as % of principal |
|---|---|---|---|
| 3 years | $754.85 | $2,174.66 | 8.7% |
| 5 years | $477.53 | $3,651.79 | 14.6% |
| 7 years | $359.97 | $5,237.51 | 20.9% |
| 10 years | $271.27 | $7,552.40 | 30.2% |
The monthly payment falls sharply with longer terms, but the total interest cost rises much faster. This is the core trade-off in any installment loan: cash flow now versus total cost later.
Extra payments and prepayment
Any payment above the scheduled monthly amount — if applied to principal — reduces the balance faster than the schedule predicts. Because interest is calculated on the outstanding balance, every dollar of extra principal saves interest on all subsequent months. Three common strategies:
- Round up: if your payment is $477.53, pay $500 each month. The extra $22.47 may seem trivial but, on a 5-year loan, shaves a few months off the term.
- Bi-weekly payments: pay half the monthly payment every two weeks. Because there are 26 bi-weekly periods per year, you end up making the equivalent of 13 monthly payments instead of 12 — one full extra payment per year.
- Lump sums: applying a tax refund or bonus directly to principal early in the loan has the largest effect, because there are more remaining months for the saved interest to compound.
Always specify "apply to principal" when making extra payments — some lenders default to applying overpayments toward future scheduled payments instead, which does nothing to reduce interest.
Limitations of this calculator
The schedule above models a clean amortization with one assumption: fixed rate, fixed payment, no fees, no escrow. Real-world loans can deviate in several ways:
- Adjustable-rate loans recalculate the schedule each time the rate resets (typically every 6 or 12 months after the fixed period). The schedule shown here is only valid while the rate stays constant.
- Escrow items (property taxes, homeowners insurance, PMI) are typically added to the actual monthly payment on mortgages but are not part of amortization.
- Daily-interest loans charge interest based on the actual days in each month, which produces slight variations from the standard monthly-period calculation.
- Late fees, prepayment penalties, and origination fees are not modeled here. Check your loan agreement — some loans charge a penalty for paying down principal too aggressively.
Sources & references
- Consumer Financial Protection Bureau — Owning a Home — US government guidance on mortgage amortization and disclosure requirements.
- Federal Reserve — Consumer Information — explanatory resources on installment loans and how interest is calculated.
- SEC investor.gov — background on compound interest math underlying the amortization formula.
FAQs
Interest is charged on the outstanding balance. At month one your balance is at its peak, so the interest portion is also at its peak. On a $25,000 loan at 5.5% over 5 years, the first payment of about $478 splits roughly $115 to interest and $363 to principal. By the final payment, the same $478 is almost entirely principal because the balance has shrunk.
On a $25,000 loan at 5.5% over 5 years, adding $100/month to principal pays the loan off about 11 months early and saves around $700 in interest. On longer loans the effect compounds: an extra $100/month on a $300,000 30-year mortgage at 6.5% saves around $90,000 in interest and finishes the loan 7 years early. Earlier in the schedule means bigger impact.
It works for any fully amortizing fixed-rate installment loan — mortgages, auto loans, personal loans, and student loans on standard repayment. It does not apply to credit cards (revolving), interest-only loans (no principal repayment in the initial period), balloon loans (large lump sum at end), or graduated-payment plans. Adjustable-rate mortgages use the formula too, but the schedule is recalculated each time the rate resets.
Amortization spreads loan repayment over time so each payment includes both interest and principal until the balance reaches zero. Depreciation spreads the cost of an asset (a car, equipment, a building) over its useful life for accounting purposes. The math feels similar but they describe opposite things — amortization tracks what you owe going down; depreciation tracks what an asset is worth going down.
Three usual causes. First, lenders may round each month's payment to the nearest cent, then apply a small adjustment to the final payment — this calculator uses a single rounded payment. Second, some lenders charge interest daily based on actual days in the month rather than the simple monthly-rate convention used here. Third, escrow components for taxes and insurance are separate line items in lender statements but excluded from a pure principal-and-interest amortization.