Loan Amortization Explained: How Your Monthly Payment Is Calculated
Every time you make a mortgage payment, something counterintuitive happens: most of the money goes to interest, not principal — especially in the early years. A $300,000 mortgage at 7% interest has a monthly payment of about $1,996, but in the first month, only $246 of that reduces your loan balance. The remaining $1,750 goes straight to interest. This is loan amortization, and understanding it can save you tens of thousands of dollars.
What Is Loan Amortization?
Amortization is the process of paying off a debt through regular, scheduled payments over a fixed period. Each payment is split between interest (the cost of borrowing) and principal (the amount that reduces your balance). Over time, as your balance decreases, less interest accrues — so more of each payment goes to principal.
By the final payment, nearly all of your payment is principal. The loan is "amortized" — fully paid off — at the end of the term.
The Amortization Formula
The monthly payment for a fixed-rate loan is calculated with this formula:
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where:
- M = Monthly payment
- P = Principal loan amount (the amount borrowed)
- r = Monthly interest rate (annual rate ÷ 12). For 7% annual rate: r = 0.07 ÷ 12 = 0.005833
- n = Total number of payments (loan term in years × 12). For 30 years: n = 360
Example: $300,000 loan, 7% interest, 30-year term
- P = 300,000
- r = 0.07 / 12 = 0.005833
- n = 30 × 12 = 360
- M = 300,000 × [0.005833 × (1.005833)^360] / [(1.005833)^360 − 1]
- M = 300,000 × [0.005833 × 8.1165] / [8.1165 − 1]
- M = 300,000 × 0.04734 / 7.1165 = $1,996/month
Use the Loan Calculator on UtilDaily to calculate monthly payments and generate a full amortization schedule for any loan instantly.
Why You Pay Mostly Interest at the Start
Each month, interest accrues on your current outstanding balance:
Monthly Interest = Remaining Balance × Monthly Rate
With a $300,000 balance at 7%: $300,000 × 0.005833 = $1,750 in interest in month 1. Your total payment is $1,996, so only $246 goes to principal. Your new balance: $299,754.
Month 2: interest on $299,754 = $1,748.57. Principal paid: $247.43. And so on — each month, the balance drops slightly, interest decreases slightly, and more goes to principal.
By month 180 (halfway through a 30-year mortgage), your balance is about $229,000 — you've paid $180 × $1,996 = $359,280 but reduced the loan by only $71,000. You've paid $288,280 in interest in 15 years.
How to Read an Amortization Schedule
An amortization schedule is a table showing every payment broken down by interest and principal:
| Payment # | Payment | Interest | Principal | Remaining Balance |
|---|---|---|---|---|
| 1 | $1,996 | $1,750 | $246 | $299,754 |
| 2 | $1,996 | $1,748 | $248 | $299,506 |
| 60 | $1,996 | $1,668 | $328 | $285,854 |
| 180 | $1,996 | $1,335 | $661 | $228,484 |
| 300 | $1,996 | $652 | $1,344 | $110,538 |
| 360 | $1,996 | $12 | $1,984 | $0 |
The total interest paid over 30 years on this $300,000 loan at 7%: $418,527. You paid $718,527 to borrow $300,000.
Extra Payments: The Most Powerful Lever
Making extra principal payments is the most effective way to reduce total interest paid and shorten your loan term. Because interest accrues on your remaining balance, reducing the balance early has a compounding effect.
Example: Pay $100 extra per month on a $300,000, 7%, 30-year mortgage
- Without extra payments: 360 payments, $418,527 total interest
- With $100 extra/month: pay off in about 321 months (26.75 years), saving roughly $47,000 in interest
Pay $500 extra/month: Pay off in about 233 months (19.4 years), saving approximately $170,000.
The earlier in the loan you make extra payments, the more you save — because you're reducing the principal that interest accrues on for all future months.
15-Year vs. 30-Year Mortgage: Real Numbers Compared
The most common mortgage decision: 15-year or 30-year? Here's a side-by-side comparison for a $300,000 loan at market rates (30-year at 7%, 15-year at 6.5%):
| 30-Year at 7% | 15-Year at 6.5% | |
|---|---|---|
| Monthly payment | $1,996 | $2,613 |
| Total paid | $718,527 | $470,343 |
| Total interest | $418,527 | $170,343 |
| Interest savings | — | $248,184 |
| Higher monthly cost | — | $617/month more |
The 15-year mortgage saves a quarter million dollars in interest — but requires $617 more per month. Whether that tradeoff makes sense depends on your income, other financial goals, and whether you can earn more than 6.5% by investing that $617 instead.
Fixed vs. Variable Rate Loans
Fixed-rate loans lock in your interest rate for the life of the loan. Your payment never changes. Best when rates are low or you value payment predictability.
Adjustable-rate mortgages (ARMs) start with a lower fixed rate for an initial period (e.g., 5/1 ARM = fixed for 5 years, then adjusts annually). After the fixed period, your rate adjusts based on a market index (like SOFR) plus a margin. Your payment can increase significantly if rates rise.
For planning purposes, the Loan Calculator uses a fixed rate — ideal for modeling fixed mortgages, auto loans, student loans, and personal loans.
How to Use the Loan Calculator
- Enter the loan amount (principal)
- Enter the annual interest rate
- Enter the loan term in years
- Click Calculate to see your monthly payment, total interest, and full amortization schedule
- Try different scenarios — change the term or rate to see how your payment and total cost change
The calculator runs entirely in your browser — no data is sent to any server.
Frequently Asked Questions
What is the difference between amortization and depreciation?
Amortization refers to paying off a debt (loan) over time through scheduled payments. Depreciation refers to allocating the cost of a tangible asset over its useful life for accounting purposes. Both spread costs over time, but they apply to different things — loans vs. assets.
Does paying bi-weekly instead of monthly help?
Yes. Bi-weekly payments (every two weeks) result in 26 half-payments per year — equivalent to 13 full monthly payments instead of 12. That one extra payment per year can reduce a 30-year mortgage by 4–5 years and save tens of thousands in interest.
Can I pay off my loan early without penalty?
Most modern mortgages and personal loans have no prepayment penalty, but check your loan agreement. Some older mortgages and auto loans include prepayment penalty clauses that charge a fee for paying off early.
What happens to my amortization schedule if I make a lump-sum payment?
A lump-sum extra payment reduces your principal immediately. If your loan terms allow it, this shortens the total loan term and reduces all future interest charges. Some loans require you to re-amortize (recalculate payments) after a large principal reduction — ask your lender.
How does the interest rate affect total cost?
Interest rate has a massive impact. On a $300,000 30-year mortgage: at 5% you pay $279,767 in interest; at 7% you pay $418,527; at 9% you pay $579,191. Each 1% rate increase on a $300,000 loan adds roughly $60,000 in total interest over 30 years.