How Loan Payments Are Calculated
Every fixed-rate loan uses the same formula. Your lender takes the principal, divides the annual interest rate by 12 to get a monthly rate, then plugs both into the amortization formula: M = P × [r(1+r)^n] / [(1+r)^n – 1], where M is your monthly payment, P is the loan amount, r is the monthly interest rate, and n is the total number of payments.
On a $25,000 loan at 6% for 60 months, your monthly rate is 0.5% (6% ÷ 12). The formula yields $483.32/month. Over 60 months you pay $28,999—that's $3,999 in interest on top of the $25,000 principal.
Early in the loan, most of each payment goes toward interest. By month 30, the split roughly reverses. This is why extra payments in the first year have the biggest impact—they reduce the principal that future interest is calculated on.
How Extra Payments Save You Money
Every dollar you pay above the minimum goes directly to principal. That means next month's interest is calculated on a smaller balance, which means more of your regular payment goes to principal too. The effect compounds over time.
Take that same $25,000 loan at 6% for 60 months. Adding $100/month in extra payments:
| Scenario | Monthly Payment | Payoff Time | Total Interest |
|---|---|---|---|
| Minimum only | $483.32 | 60 months | $3,999 |
| +$50/month extra | $533.32 | 52 months | $3,557 |
| +$100/month extra | $583.32 | 46 months | $3,155 |
| +$200/month extra | $683.32 | 38 months | $2,569 |
An extra $100/month saves you $844 in interest and eliminates 14 months of payments. The key insight: you don't need to double your payment to see meaningful savings. Even $50/month makes a real difference.
Auto Loan vs Personal Loan vs Student Loan
Interest rates, terms, and tax treatment vary dramatically by loan type. Here's how they compare in 2026:
| Feature | Auto Loan | Personal Loan | Student Loan |
|---|---|---|---|
| Typical APR | 5% – 12% | 8% – 24% | 5% – 8% (federal) |
| Typical Term | 36 – 72 months | 24 – 60 months | 10 – 25 years |
| Collateral | The vehicle | None (unsecured) | None |
| Tax Deduction | No | No | Up to $2,500/yr |
| Prepayment Penalty | Rare | Some lenders | Never (federal) |
| Best Strategy | Shortest term you can afford | Pay off aggressively | Income-driven + PSLF if eligible |
Auto loans have the lowest rates because the car itself serves as collateral. Personal loans carry higher rates since they're unsecured. Student loans are unique—federal loans offer income-driven repayment plans and potential forgiveness that can change the math entirely on whether to pay early.
When to Pay Off Loans Early vs Invest
The decision comes down to one comparison: your loan's interest rate vs your expected investment return after taxes.
Pay off early when: Your loan rate exceeds 7%. Personal loans at 15%+ should always be attacked first—no investment reliably beats that. Credit card debt at 20%+ is an emergency.
Invest instead when: Your loan rate is below 5% and you have decades of investing ahead. A 4% auto loan costs you less than the historical 10% average stock market return. But this only works if you actually invest the difference—not spend it.
The hybrid approach: Split extra cash 50/50 between extra loan payments and investing. You reduce interest costs while building wealth. This works especially well for loans in the 5%–7% gray zone.
One factor people overlook: the psychological value of being debt-free. If carrying debt stresses you out and affects your spending habits, paying it off may be worth more than the theoretical investment return you're giving up.
To see how a mortgage specifically breaks down with property taxes, insurance, and PMI, use our mortgage calculator. If you're splitting debt with a partner or ex, the debt split calculator can help you divide balances fairly. And to see how your loan payments fit into your overall savings plan, try the compound interest calculator.