Key Takeaways
- The payment is constant. Its composition is not. Interest is charged on whatever balance remains, so the split between interest and principal moves every month without the payment changing at all.
- Early payments are mostly interest, and the longer the term, the more extreme that is. On a thirty-year schedule the first payment can be more than three-quarters interest.
- An extra payment is pure principal. It never buys down interest directly; it removes balance, and the interest saving is everything that balance would have accrued for the rest of the term.
- Extra payments are worth most at the start. The saving is proportional to how long the removed balance would otherwise have sat there, so the same dollar in month 6 does far more than in month 200.
- Term length is the largest single driver of total interest. Extending a loan lowers the payment and raises the total cost, and the schedule makes the size of that trade explicit.
- The final payment is smaller than the others. Lenders round the level payment up to the cent, so the last row of a real schedule is a partial payment rather than a full one.
What Is an Amortization Schedule?
An amortization schedule is the row-by-row record of what each payment on an installment loan does. Every row contains four things: the payment, how much of it was interest, how much reduced the balance, and what the balance became.
The arithmetic behind a row is short:
- Interest for the month equals the current balance multiplied by the monthly rate, which is the annual percentage rate divided by twelve.
- Principal for the month is the payment minus that interest.
- The new balance is the old balance minus the principal.
Because interest is charged on a shrinking base, the interest column falls every month and the principal column rises by the identical amount. The payment total never moves. This is what makes a loan feel like it is going nowhere in the first years and then suddenly collapse in the last ones.
The Consumer Financial Protection Bureau describes amortization as the process by which loan payments are applied to both principal and interest, with the balance reducing to zero at the end of the term. That last condition is what defines the payment: it is the exact amount that lands the balance on zero after the agreed number of payments, no more and no less.
Why Does an Extra Payment Save So Much?
Because it does not reduce this month's interest. It reduces every future month's interest.
An extra $100 sent in month one is $100 that is no longer in the balance for months two through the end of the term. Every one of those months charges interest on a base that is $100 smaller. On a five-year loan the saving is that $100 compounding at the loan's rate for four and a bit years; on a thirty-year loan it is nearly three decades. Nothing about the payment changes, so the loan simply reaches zero earlier and the payments that would have followed never happen.
This is also why the effect is so front-loaded. An extra payment in the final year removes a balance that had only months left to accrue, so it saves almost nothing. The same dollar in the first year saves the most the loan can offer. Anyone deciding whether an extra payment is worth making should compare the interest it removes, which the calculator reports directly, against what that money would otherwise do.
Two practical cautions the arithmetic cannot see. Some loans carry a prepayment penalty, which changes the sum. And an extra payment normally has to be designated as principal-only, or the servicer may apply it to the next scheduled payment instead, which advances the due date without reducing the balance any faster than the schedule already would.
Loan Amortization Calculator
Enter the loan and, optionally, an extra amount to add to every monthly payment. The schedule is produced in full, and the summary reports what the extra payment removed.
Illustrative arithmetic on the figures entered above. It assumes a fixed rate, monthly compounding, on-time payments and no fees, insurance, escrow or prepayment penalty. It is not a loan offer, a quote or advice.
The loan
What the extra payment changed
Year-by-year summary
| Year | Interest | Principal | Total paid | Closing balance |
|---|
First twelve payments
| Month | Payment | Interest | Principal | Closing balance |
|---|
The full schedule runs to the payoff month; the table above shows the first twelve rows, which is where the interest share is highest and the shape of the loan is clearest.
Worked Example: $25,000 Over Five Years at 6%
Take a $25,000 loan at 6% APR over 60 months. The monthly rate is 6 divided by 100 divided by 12, which is 0.005. The payment formula gives:
P = 25,000 × 0.005 / (1 − 1.005^−60) = 125 / 0.2586278 = 483.320
Rounded up to the cent, the payment is $483.33. Total interest over the full term is $3,999.08 and total repaid is $28,999.08.
Now look at the first payment. Interest is $25,000 multiplied by 0.005, which is $125.00. Principal is $483.33 minus $125.00, which is $358.33. So 25.9% of the first payment is interest. By the final payment, interest is a couple of dollars and essentially the whole amount is principal.
Add $100 a month. The loan clears in 49 months instead of 60, and total interest falls to $3,206.22, a saving of $792.86. The extra payments total $4,900 across those 49 months, and they eliminated eleven payments of $483.33, which is $5,316.63 that never has to be found. The gap between those two figures is the interest that was never charged.
One arithmetic detail worth naming: the principal column of the full schedule sums to exactly $25,000.00, not to $24,999.97 or $25,000.04. This calculator holds every balance as a whole number of cents rather than as a decimal, which is what stops rounding error accumulating across sixty rows. On a 360-row mortgage schedule the difference between doing that and not doing it is visible in the total.
The Term Is the Expensive Decision
Lengthening a loan lowers the payment, which is why it is offered. What it does to the total is easy to state and hard to feel until it is on the screen: a longer term means the balance sits there longer, and interest is charged on time as much as on money.
Running the same $25,000 at 6% through different terms shows the shape of it. The payment falls steadily. The total interest does not; it climbs faster than the payment falls, because the extra years each carry a full year of interest on a balance that is now being retired more slowly. Comparing 36, 60 and 84 months in the calculator takes under a minute and is the single most informative thing it can be used for before signing anything.
The CFPB's guidance on comparing auto loan offers makes the same point about monthly payment being the wrong comparison: two offers with the same monthly payment can differ by thousands in total cost if their terms differ. The figure to compare across offers is total repaid, which this calculator reports directly, alongside the rate.
What This Calculator Does Not Model
- Fees and closing costs. Origination fees, documentation fees, points and closing costs are excluded. Where a fee is financed into the loan, entering the total borrowed rather than the purchase price gets it into the schedule.
- Escrow, insurance and taxes. A mortgage payment usually includes property tax and insurance alongside principal and interest. Only principal and interest are modelled here.
- Variable rates. The rate entered is held for the whole term. An adjustable-rate loan changes payment when the rate resets, which this schedule does not represent.
- Precomputed interest. Some loans, including some auto loans, calculate the total interest at the outset rather than on the declining balance. On those, paying early saves less than this schedule suggests, and the CFPB material linked below explains the distinction.
- Prepayment penalties. Where one applies, the interest saved from an extra payment is reduced by the penalty, which is not deducted here.
- Payment timing. Every payment is assumed to arrive exactly on schedule. Loans with daily interest accrual charge slightly more or less depending on the day a payment posts.
Reading the Schedule Rather Than the Payment
Most loan decisions are made on one number, the monthly payment, because it is the number the seller leads with and the only one that appears in the budget. The schedule contains three others that describe the loan far better.
Total repaid. This is the sum of every payment across the term. Comparing it against the amount borrowed states the cost of the loan in dollars rather than in a percentage, and it is the figure that makes a long term look different from the way its low payment makes it feel.
The interest share of the first payment. On a short loan at a modest rate this is a quarter or less. On a long loan at a high rate it can be three-quarters. That single ratio predicts how the loan behaves: a high figure means the balance will barely move for years, so early repayment or refinancing has a large effect, and being underwater on a depreciating asset is a live risk.
The closing balance at the point you expect to exit. Cars are traded and houses are sold long before month 360. The year-by-year table shows what would still be owed at each anniversary, which is the figure that matters if the asset is likely to be sold before the loan ends. On a long term against a fast-depreciating asset, that balance can exceed what the asset is worth for a substantial part of the schedule.
Used this way, the schedule stops being a formality and becomes the document that answers the questions the payment cannot: what this loan costs in total, how quickly it becomes yours rather than the lender's, and what happens if it does not run to term.
Frequently Asked Questions
How is a monthly loan payment calculated?
The level payment on an installment loan is P = L * i / (1 - (1 + i)^-n), where L is the amount borrowed, i is the annual rate divided by twelve, and n is the number of monthly payments. That formula produces the exact amount that reduces the balance to zero on the final payment. When the rate is zero the formula reduces to the amount borrowed divided by the number of payments. Lenders round the result up to the nearest cent, which is why the final payment of a real schedule is slightly smaller than the others.
Why is most of my early payment going to interest?
Because interest each month is charged on the balance that is still outstanding, and at the start of a loan that balance is at its largest. The payment stays fixed, so the interest portion is largest in month one and falls every month as principal comes off. The principal portion rises by exactly the same amount the interest portion falls. On long terms this effect is extreme: the first payment on a thirty-year loan at a high rate can be more than three-quarters interest.
How much does an extra monthly payment save?
An extra payment is applied entirely to principal, so it removes that amount from the balance for every remaining month of the loan. The interest saved is everything that removed balance would have accrued for the rest of the term, which is why the calculator reports the saving as a total rather than as a monthly figure. On the worked example on this page, $100 a month against a $25,000 loan at 6% over five years removes eleven months and $792.86 of interest.
Is it better to make extra payments early or later in the loan?
Early, and the difference is large. The interest an extra payment saves is proportional to how long the removed balance would otherwise have stayed on the loan, so a payment in the first year saves the maximum the loan can offer and one in the final year saves almost nothing. Anyone planning to accelerate a loan gets the most from doing it at the start of the term rather than treating it as something to begin once other things are settled.
Does a longer loan term cost more even at the same interest rate?
Yes, and usually by more than people expect. A longer term lowers the monthly payment by spreading the same principal over more months, but each of those extra months charges interest on a balance that is now being retired more slowly. Running the same amount and rate through different terms in this calculator shows the trade in dollars: the payment falls steadily while the total repaid climbs.
What is negative amortization?
Negative amortization is what happens when a payment is smaller than the interest charged for that period. The unpaid interest is added to the balance, so the amount owed grows despite payments being made. This calculator refuses to produce a schedule for inputs where that occurs, and reports the condition as an error instead, because there is no payoff date to report. The Consumer Financial Protection Bureau describes the same mechanism in the context of loans that permit it by design.
Does this calculator include taxes, insurance or fees?
No. It models principal and interest only. A mortgage payment normally also includes property tax and homeowners insurance held in escrow, and many loans carry origination or documentation fees that are not part of the interest calculation. Where a fee is financed into the loan itself, entering the total amount borrowed rather than the purchase price brings it into the schedule as principal.
Why does the last payment differ from the others?
Because the calculated payment is rounded up to the nearest cent so that the loan is certain to reach zero within the agreed number of payments. Rounding up means a fraction of a cent extra is repaid each month, and by the end that accumulation leaves slightly less owed than a full payment. The final row is clamped to whatever is actually outstanding, which is what makes the principal column sum to exactly the amount borrowed.
Can I use this for a mortgage or a car loan?
Yes for any fixed-rate installment loan repaid in equal monthly payments, which includes most mortgages, auto loans, personal loans and student loans on standard repayment. It is not suitable for a credit card or any other revolving balance, where the required payment is recalculated from the balance each month rather than fixed. It also does not represent an adjustable-rate loan after its first reset, or a precomputed-interest loan.
Is any of the information I enter sent anywhere?
No. The entire schedule is computed in the browser from the numbers typed into the form. Nothing is transmitted to Swoopr Investment, to a lender or to any third party, nothing is stored between visits, and no loan account is contacted or affected. The calculator is a simulation with no connection to any real account.
References
- CFPB: What Is Amortization and How Could It Affect My Auto Loan?
- CFPB: What Is Negative Amortization?
- CFPB: How Much Can I Afford to Borrow for a Car or Auto Loan?
- CFPB: What Is the Difference Between a Fixed APR and a Variable APR?
- CFPB: What Is a Payoff Amount and Is It the Same as My Current Balance?
Every source above was retrieved and its document title confirmed on 23 August 2026. Rules, disclosure requirements and product terms change, so a figure taken from any of them should be re-checked against the current version before it is relied on. Swoopr Investment quotes no rate, fee or product term as a current fact anywhere on this page: every rate, fee and term in the calculator is a value the reader supplies.