Whether you're pricing out a mortgage, a personal loan, or an auto loan, the payment quote you get from a lender is built from the same amortization math underneath. Here's how that number is actually calculated, what fees do to your true cost, and how much extra payments can save — with a worked example you can adapt to your own numbers.
Quick answerA loan payment is calculated with the standard amortization formula: Payment = P × i × (1+i)ⁿ ÷ ((1+i)ⁿ − 1), where P is the loan amount, i is the monthly interest rate (APR ÷ 12 ÷ 100), and n is the number of monthly payments. A $200,000 loan at 6.5% APR over 30 years (360 payments) comes to about $1,264.14 per month in principal and interest.
This same formula applies whether you're looking at a personal loan, a mortgage, or an auto loan — what changes between them is the typical rate, the typical term, and whether the loan is secured by collateral (a mortgage by the home, an auto loan by the vehicle; a personal loan is usually unsecured, which is part of why its rates tend to run higher). Every month, interest is charged on whatever principal balance remains, and the rest of that fixed payment chips away at the balance. Early in the loan, most of the payment is interest; by the final years, most of it is principal.
Amortization, in plain terms: a fixed monthly payment sized so that, after the very last scheduled payment, both the remaining interest and the remaining principal balance land at exactly zero. The rate and the term are the two levers that decide how large that fixed payment has to be.
How term length changes your $200,000 payment
Quick answerA longer term lowers the monthly payment but raises the total interest paid, because interest keeps accruing on a balance that falls more slowly. On the same $200,000 loan at 6.5% APR, stretching from a 10-year term to a 30-year term roughly cuts the monthly payment in half — but more than triples the total interest.
Here's that same $200,000 loan at 6.5% APR, shown across a few common term lengths, as an illustrative example:
Example only: $200,000 loan at 6.5% APR, by term
| Term | Monthly payment | Total repayment | Total interest |
|---|
| 120 mo (10 yr) | $2,270.96 | $272,515 | $72,515 |
| 180 mo (15 yr) | $1,742.21 | $313,599 | $113,599 |
| 240 mo (20 yr) | $1,491.15 | $357,875 | $157,875 |
| 360 mo (30 yr) | $1,264.14 | $455,089 | $255,089 |
There's no universally "right" term — a shorter one builds equity faster and pays far less interest overall, but the higher required payment has to fit your monthly budget. A useful way to work backward from your budget is to flip the formula around: Loan amount = Payment × ((1+i)ⁿ − 1) ÷ (i × (1+i)ⁿ). As an example, a $1,000-per-month budget at 6.5% APR over 60 months supports a loan of roughly $51,109 — useful for sanity-checking what a monthly number actually buys you before you go loan shopping.
True APR: what fees actually add to your cost
Quick answerBeyond the interest rate itself, loans often carry an origination fee, closing costs, or monthly insurance (like PMI on a mortgage with less than 20% down) that don't show up in the scheduled payment but do raise the true APR — the real cost of borrowing once fees are spread across the loan.
A personal loan or auto loan may deduct an origination fee from the amount you actually receive, meaning you're paying interest on money you never got to use in full. A mortgage typically layers on appraisal fees, title and closing costs (commonly 2%-5% of the loan amount), and monthly PMI if your down payment is under 20%. None of these change the size of your scheduled payment, but they do mean the effective rate you're paying is higher than the advertised APR suggests — which is exactly what a true-APR calculation is meant to reveal.
It's also worth keeping the APR versus APY distinction straight when comparing offers: APR is a nominal annual figure (monthly rate × 12), while APY (annual percentage yield) accounts for monthly compounding, using APY = (1 + APR ÷ 12)² − 1. A 6.5% APR, for instance, works out to roughly a 6.70% APY — a small but real difference once you're comparing rates carefully.
Quick answerAny amount you pay above your required monthly payment goes straight to reducing principal, which lowers every future interest charge and shortens the loan. Switching to biweekly payments (half the monthly amount every two weeks) works out to 26 half-payments a year — the equivalent of 13 monthly payments instead of 12 — which can shave years off a long-term loan.
Because interest is calculated on the remaining balance each period, knocking that balance down sooner means every subsequent month's interest charge is a little smaller, and the loan reaches zero balance faster than the original schedule called for. As an illustrative example, on a 30-year, $200,000 loan at 6.5% APR, consistently adding even a modest amount to the monthly payment — or making one or two extra lump-sum payments a year — can meaningfully cut both the payoff timeline and the total interest paid, though the exact savings depend on the size and timing of the extra payments. If you're weighing prepayment, it's also worth checking whether your specific loan carries a prepayment penalty; these are uncommon on personal loans and most conforming mortgages today, but not universal.
⚠ Worth noting: the biweekly-payment effect isn't a rate trick — it works purely because 26 half-payments a year adds up to one extra full payment annually compared with a standard monthly schedule, and that extra payment goes entirely to principal.
Frequently asked questions
What is the monthly payment on a $200,000 loan at 6.5% APR for 30 years?
A $200,000 loan at 6.5% APR over 30 years (360 monthly payments) has a monthly payment of about $1,264.14 (principal and interest only). Over the full term you'd pay about $455,089 in total, of which about $255,089 is interest. A shorter term lowers total interest but raises the monthly payment — enter your own numbers in the calculator above to see the full amortization schedule.
How is a loan payment calculated?
Loans in this calculator use the standard amortization (equal-payment) formula: Payment = P x i x (1+i)^n / ((1+i)^n - 1), where P is the loan amount, i is the monthly interest rate (APR / 12), and n is the number of monthly payments. Each month, interest is charged on the remaining balance, and the rest of the payment reduces the principal; over time the interest portion shrinks and the principal portion grows.
What's the difference between APR and the interest rate?
Your interest rate is the base rate used to calculate interest each period. The APR (annual percentage rate) is meant to reflect the total yearly cost of borrowing, including certain fees, expressed as a single nominal rate. Because APR is a nominal annual figure (monthly rate times 12), it differs from the APY (annual percentage yield), which accounts for monthly compounding: APY = (1 + APR / 12)^12 - 1. When comparing loan offers, compare APRs (or true APRs including fees), not just the advertised interest rate.
How much do extra payments or biweekly payments actually save?
Any extra amount you pay above your required monthly payment goes straight to principal, which lowers every future interest charge and shortens your term. Switching to biweekly payments (half your monthly payment every two weeks) results in 26 half-payments per year — the equivalent of 13 monthly payments instead of 12 — which alone can cut several years off a 30-year mortgage and save tens of thousands in interest, as an illustrative example.
How much loan can I afford based on my monthly budget?
Divide your budget into a maximum loan amount using the reverse amortization formula: Loan amount = Payment x ((1+i)^n - 1) / (i x (1+i)^n). As an example, a $1,000-per-month budget at 6.5% APR over 60 months supports a loan of about $51,109. Many lenders also cap your total monthly debt payments at roughly 36%-43% of your gross income (the debt-to-income ratio), so your true affordable amount may be lower.