Installment loans are calculated with a single formula called amortization: the lender takes your principal, converts your annual interest rate to a monthly rate, and spreads the balance across a set number of monthly payments so that each payment is the same dollar amount. Inside each of those identical payments, the split between interest and principal shifts every month. Everything else you’ll see on a loan disclosure — total cost, finance charge, APR — flows from that one calculation.
The Three Inputs Behind Every Calculation
Three numbers drive the math. The principal is the amount you actually borrow, stated separately in your loan agreement from interest and fees. The interest rate is what the lender charges annually for the use of that money; federal law requires it to be disclosed clearly so you can compare offers.1Federal Deposit Insurance Corporation. V-1 Truth in Lending Act (TILA) The term is how long you have to repay, usually stated in months.
A shorter term raises the monthly payment but cuts total interest. A longer term does the opposite: smaller monthly bill, more interest over time. Those three numbers are all a lender needs to build your full payment schedule.
The Amortization Formula
Lenders use the standard amortization formula to produce your fixed monthly payment:
M = P × [r(1 + r)n] / [(1 + r)n − 1]
- M is your monthly payment.
- P is the principal.
- r is the monthly interest rate, which is the annual rate divided by 12.
- n is the total number of monthly payments.
Work it through with a concrete example. Borrow $20,000 at 7% for 60 months. The monthly rate is 0.07 ÷ 12 = 0.005833. Raise 1.005833 to the 60th power and you get about 1.4176. Multiply that by the monthly rate (0.005833 × 1.4176 = 0.008269), then divide by 1.4176 minus 1, or 0.4176. The result is 0.01980. Multiply by the $20,000 principal and the monthly payment lands at roughly $396.
The formula guarantees every payment is identical. What changes each month is what’s happening inside that payment.
How the Interest and Principal Split Shifts Each Month
Interest is charged on the balance you still owe, not on the amount you originally borrowed. Early in the loan the balance is at its highest, so interest takes the biggest bite. As you pay down principal, the interest charge shrinks and more of each identical payment goes toward the balance.
Take a $10,000 loan at 10% annual interest. Month one interest is about $83 ($10,000 × 0.10 ÷ 12). Once the balance has dropped to $5,000, the monthly interest charge is about $42. The payment amount hasn’t budged, but the $41 that used to go to interest is now knocking down principal. Borrowers often feel like they’re making no progress in the first year of a long-term loan; the math is working, just slowly at first, and it accelerates near the end.
Your lender must give you a payment schedule showing the number, amount, and timing of every payment before you sign.2Consumer Financial Protection Bureau. 12 CFR 1026.18 – Content of Disclosures That schedule is how you can see the interest-to-principal ratio for every single month of the loan.
Turning the Monthly Payment Into a Total Cost
Once you know the monthly payment, total cost is multiplication. Payment times number of months equals what you’ll pay over the life of the loan. Federal disclosure rules call this figure the Total of Payments, and the lender must show it before closing.3Consumer Financial Provider Bureau. 12 CFR 1026.18 – Content of Disclosures
Using the $20,000 loan at 7% for 60 months: $396 × 60 = about $23,760. Subtract the original $20,000 and you get the finance charge, roughly $3,760 in interest across five years. Change only the rate to 14% and the monthly payment climbs to about $465, the total becomes $27,900, and the finance charge jumps to about $7,900. The rate is the single biggest lever on what a loan costs you.
Why the Rate You’re Quoted Depends on Your Credit
The interest rate a lender offers you is tied directly to how risky your credit profile looks. Based on 2024 offer data, average personal loan rates by credit tier ran roughly:
- Excellent credit (720–850): around 12% APR
- Good credit (690–719): around 14.5% APR
- Fair credit (630–689): around 18% APR
- Poor credit (below 630): around 22% APR or higher
On the $20,000, 60-month example, the gap between 12% and 22% works out to about $5,700 in extra interest. Some lenders in early 2026 are quoting rates under 7% for top-tier borrowers, which shows how much the same borrowed amount can cost different people.
Fees and APR: Why the Interest Rate Alone Understates the Cost
Interest isn’t the only charge. Many personal loan lenders take an origination fee, typically 1% to 10% of the loan amount, and usually deduct it from your proceeds. On a $15,000 loan with a 5% origination fee, you’d receive $14,250 but still owe the full $15,000 principal.
That’s why the Annual Percentage Rate matters. APR combines the interest rate with origination fees and certain other finance charges into a single yearly figure. A loan advertising a 9% interest rate might carry an 11% APR once fees are folded in. Federal law requires the APR to be disclosed prominently.1Federal Deposit Insurance Corporation. V-1 Truth in Lending Act (TILA) When you’re comparing offers from different lenders, APR is the honest number.
Older Calculation Methods That Change the Result
The amortization formula above is how most mainstream lenders calculate installment loans. Two older methods still appear, and both cost borrowers more if they pay off early.
Add-On Interest
With add-on interest, the lender computes total interest up front as principal × annual rate × years. That entire interest amount is added to the principal and the sum is divided into equal payments. On a $10,000 loan at 10% for three years, add-on interest is $3,000, the total becomes $13,000, and the payment is $361. Under standard amortization, the same loan totals about $12,748 with payments near $323. Add-on is more expensive because it charges interest on the full original principal for the entire term, regardless of the fact that you’re paying it down.
The Rule of 78s
The Rule of 78s front-loads interest to the early months of a loan. If you pay on schedule for the full term, the total is the same as add-on interest. If you pay off early, the lender keeps a disproportionate share of the interest because so much of it was assigned to the first months. Federal law prohibits lenders from using the Rule of 78s to calculate interest refunds on any consumer credit transaction with a term longer than 61 months.4Office of the Law Revision Counsel. 15 USC 1615 – Prohibition on Use of Rule of 78s in Connection with Mortgage Refinancings and Other Consumer Loans Shorter-term loans can still use it, so check the agreement before assuming early payoff will save proportional interest.
How Early Payoff, Biweekly Schedules, and Missed Payments Change the Math
On a standard amortized loan, paying early always saves interest, because you’re eliminating future months where interest would have been charged on the remaining balance. On the $20,000 loan at 7% for 60 months, paying it off at month 36 instead of month 60 saves about $1,100 in interest.
The catch is prepayment penalties. Some loan agreements charge a fee for paying off ahead of schedule, which can erase part or all of the savings. There is no blanket federal prohibition on prepayment penalties for personal installment loans; some states restrict them and many lenders waive them, but the specific agreement controls. For residential mortgages, federal law is stricter, restricting or barring these fees depending on the loan type.5Office of the Law Revision Counsel. 15 USC 1639c – Minimum Standards for Residential Mortgage Loans Ask about prepayment terms before signing.
Payment frequency changes the numbers too. Interest on most consumer loans accrues daily, so paying every two weeks instead of once a month knocks the principal down slightly sooner and cuts the daily interest charged for the rest of the cycle. There are 26 biweekly periods in a year, which works out to 13 monthly payments’ worth instead of 12. That extra payment goes straight to principal. On a $20,000 loan at 7%, switching to biweekly can save several hundred dollars in interest and clip a few months off the term. On a mortgage the same idea can save tens of thousands.
Missed payments push the math the other way. Late fees add to the balance, and interest keeps accruing on whatever remains unpaid, so the longer a delinquency runs, the more the loan ends up costing. Most agreements also contain an acceleration clause that lets the lender demand the full remaining balance at once if you breach the contract.
The Disclosures That Show You the Numbers
Before you finalize any installment loan, the lender has to give you a written disclosure with the amount financed, the finance charge in dollars, the APR, the payment schedule, and the total of payments.2Consumer Financial Protection Bureau. 12 CFR 1026.18 – Content of Disclosures Those five items are how you check the calculation against what you were quoted, and how you compare one lender’s offer to another on equal terms. The finance charge and the total of payments are the two numbers that tell you what the loan costs in full, not just what it costs each month.