In a fixed-rate car loan, each payment splits into interest on the remaining balance and principal reduction; the interest share shrinks and the principal share grows every month. On the $24,000, 6.9% APR, 60-month loan from the payment-formula guide, payment one is $138.00 interest and $336.06 principal, while payment two is $136.07 interest and $337.99 principal.
How does a fixed payment split between interest and principal?
Start with a $24,000 balance and a 0.00575 monthly rate. Payment 1 interest: 24,000 × 0.00575 = 138.00; principal: 474.10 − 138.00 = 336.10; new balance: 24,000 − 336.10 = 23,663.90. Payment 2 interest: 23,663.90 × 0.00575 = 136.07; principal: 474.10 − 136.07 = 338.03; new balance: 23,663.90 − 338.03 = 23,325.87. Payment 3 interest falls again to 134.12, and principal rises to 339.97. The monthly payment itself never changes; only its split does.
How do I build the schedule month by month?
For each month, multiply the balance carried from the previous month by the monthly rate to get that month’s interest. Subtract interest from the fixed payment to get the principal portion, then subtract principal from the balance to get the new balance. Repeat for every remaining month; that running table is the amortization schedule referenced in the payment-formula guide.
How do I check one row of the schedule?
Each month’s interest should be smaller than the one before it as long as no extra principal payment was skipped, and the principal portion should keep growing. Add interest and principal for any month: they must equal the fixed monthly payment, 474.10 in this example, to the cent.
What do people get wrong about amortization?
A common misreading is assuming the interest-to-principal split stays constant across the loan, when it actually shifts every month as the balance falls. Another is thinking a missed month simply defers that payment with no cost, when unpaid interest and a lower principal reduction typically follow. Extra payments toward principal reduce future interest and shorten the schedule; they do not reduce the size of the next scheduled payment unless the lender recasts the loan.
When does this schedule not apply?
This schedule assumes on-time payments with no late fees, no extra principal payments, and a simple-interest, fixed-rate contract. Precomputed-interest loans allocate finance charges differently and do not reward early payoff the same way. Refinancing resets the schedule against a new balance, rate, and term, and any deferred or skip-payment agreement changes the sequence shown here.
Source for the external fact
The Consumer Financial Protection Bureau states that more of each early auto-loan payment is applied toward interest, with the principal share increasing as the loan matures, in its amortization guidance.
A second loan at a different rate and balance
The same recurrence applies to any loan. An $18,000 loan at 4.5% APR over 48 months has a monthly rate of 0.045 ÷ 12 = 0.00375 and a level payment of $410.46. Payment 1: interest 18,000 × 0.00375 = $67.50, principal $410.46 − $67.50 = $342.96, new balance $17,657.04. Payment 2: interest $66.21, principal $344.25, new balance $17,312.79. Payment 3: interest $64.92, principal $345.54, new balance $16,967.25. The pattern is identical to the $24,000 example — shrinking interest, growing principal — only the dollar amounts differ.
A snapshot of the schedule at several points
Following the $24,000, 6.9% APR, 60-month loan all the way through shows the split continuing to shift long after the first two payments.
| Payment | Interest | Principal | Balance after |
|---|---|---|---|
| 1 | $138.00 | $336.06 | $23,663.94 |
| 15 | $109.91 | $364.15 | $18,751.06 |
| 30 | $77.21 | $396.85 | $13,030.71 |
| 45 | $41.57 | $432.49 | $6,796.63 |
| 60 | $2.73 | $471.33 | $2.66 |
By payment 45, principal makes up more than 91% of the payment; by payment 60, interest has shrunk to a few dollars.
Why the last payment is rarely identical to the others
Carrying a payment rounded to the cent through all 60 rows rarely lands on exactly $0.00. The unrounded payment here is $474.0959…, and the schedule uses $474.10; a few hundredths of a dollar a month accumulates into a final balance a dollar or two away from zero. Lenders resolve this by adjusting the last payment slightly up or down. Watch the direction of the rounding error: a payment rounded DOWN leaves money still owed at the end, which is why a schedule that finishes with a positive balance is usually a rounding artefact rather than a missed payment. If the leftover runs to tens of dollars, the cause is not rounding — check the rate conversion and the number of periods first.
How the split changes with the interest rate alone
Keeping the $24,000 principal and 60-month term fixed and changing only the APR shows how much the rate — not the balance — drives the early interest-to-principal split. At 3.9% APR, payment 1 is $78.00 interest and $362.91 principal, 82.3% principal. At 6.9% APR it is $138.00 interest and $336.06 principal, about 70.9% principal. At 12.9% APR it is $258.00 interest and $286.85 principal, about 52.6% principal. A higher rate does not just raise the payment; it also slows how quickly principal is repaid in the early months.
How to check a schedule with the total-interest shortcut
Instead of re-adding every row, check a finished schedule against the total figure from the payment-formula guide: 60 payments of $474.10 total $28,445.84, and $28,445.84 minus the $24,000 principal is $4,445.84 in interest. Summing the 60 individual interest columns from a schedule built with a fixed $474.10 payment gives a close but not identical figure, differing by a dollar or two, because of the same cent-level rounding described above — a useful reminder that a schedule’s summed total and the formula’s total interest estimate can differ by a few dollars without either one being wrong.
How much of each year’s payments goes to interest
Grouping the same 60-row schedule into 12-month blocks shows the front-loading pattern at a coarser, yearly scale.
| Year | Interest paid | Principal paid |
|---|---|---|
| Year 1 | $1,525.99 | $4,162.73 |
| Year 2 | $1,229.50 | $4,459.22 |
| Year 3 | $911.89 | $4,776.83 |
| Year 4 | $571.67 | $5,117.05 |
| Year 5 | $207.21 | $5,481.51 |
Year 1 alone accounts for roughly a third of all the interest this loan will ever charge, even though it is only one-fifth of the payments.
Why the first month’s interest does not depend on the term
The $18,000, 4.5% APR loan from the second example has payment-1 interest of $67.50 whether the term is 48 months or 60 months, because interest for the first month is simply the starting balance times the monthly rate, with no reference to how many payments remain. Only the payment itself changes with the term — $410.46 at 48 months versus $335.57 at 60 months — so the first month’s principal portion differs ($342.96 versus $268.07) even though the interest charge is identical. Term length reshapes the payment and the principal split; it does not touch the interest on the very first payment.
Payment 1 does not always favour principal over interest
In the $24,000, 6.9% APR, 60-month example, principal already exceeds interest in payment 1 ($336.06 versus $138.00). That is not guaranteed for every loan. The same $24,000 at 15% APR over 84 months has a $463.12 payment where payment 1 is $300.00 interest and only $163.12 principal — interest-dominated from the start — and principal does not overtake interest until payment 30. A higher rate and a longer term both push the crossover point later in the schedule; “principal is always the bigger share” is a feature of the specific numbers in the worked example, not a rule that holds for every fixed-rate loan.
Related calculators
Recalculate the underlying payment in the payment-formula guide, compare how term length changes total interest in the loan-amount guide, and convert a stated rate with the percentage calculator.