A Complete Tutorial — From Interest Basics to Amortization Schedules
Interest is the price you pay to borrow money — or the reward you earn for lending it. Understanding interest is the foundation of every financial decision, from taking a mortgage to choosing a savings account.
Calculated only on the original principal amount. If you borrow $10,000 at 5% simple interest for 3 years, you pay $500/year × 3 = $1,500 in total interest. The formula is straightforward: I = P × r × t.
Calculated on both the principal and accumulated interest. That same $10,000 at 5% compounded annually for 3 years grows to $11,576, meaning $1,576 in interest. Compounding more frequently (monthly, daily) increases the total further. This is Einstein's "eighth wonder of the world."
Most consumer loans — mortgages, auto loans, personal loans — use simple interest applied to the declining balance. This means you're charged interest only on what you still owe, not on the original loan amount. Credit cards, however, use compound interest, which is why credit card debt can spiral so quickly.
The nominal (stated) interest rate doesn't tell the whole story. A 6% loan compounded monthly actually costs more than 6% annually because interest compounds 12 times per year. The effective annual rate accounts for this compounding:
For a 6% nominal rate compounded monthly: (1 + 0.06/12)12 - 1 = 6.17% effective rate. This difference seems small but becomes significant on large loans over long periods.
These two abbreviations appear everywhere in finance, and confusing them can cost you money.
APR represents the total cost of borrowing, including the interest rate plus fees. For a mortgage, APR includes origination fees, discount points, closing costs, and mortgage insurance. For personal loans, it includes origination fees (typically 1-8% of the loan amount).
Example: A mortgage might advertise a 6.5% interest rate but carry a 6.8% APR after including $4,500 in closing costs on a $300,000 loan. The APR gives you a more accurate basis for comparing loans from different lenders.
APY represents the true return on your savings or investments, accounting for compound interest. Banks advertise APY on savings accounts, CDs, and money market accounts because it shows what you actually earn.
Key insight: For the same stated rate, more frequent compounding means a higher APY. A 5% rate compounded daily yields 5.13% APY, while the same rate compounded annually yields exactly 5.00% APY.
Not all loans are structured the same way. Understanding the repayment method affects how quickly you build equity and how much total interest you pay.
The most common type for mortgages, auto loans, and personal loans. Your monthly payment stays the same for the entire term, but the composition changes over time. In the early years, most of your payment goes toward interest; in later years, most goes toward principal.
For a $200,000 mortgage at 6.5% over 30 years:
| Year | Principal Paid | Interest Paid | Balance Remaining |
|---|---|---|---|
| Year 1 | $2,384 | $12,972 | $197,616 |
| Year 5 | $14,740 | $61,080 | $185,260 |
| Year 15 | $71,500 | $155,700 | $128,500 |
| Year 30 | $200,000 | $255,089 | $0 |
You'll notice that in year 1, only about $2,384 of your $15,356 in total payments goes toward the principal — the rest is interest. By year 15, you're paying significantly more principal than interest. This front-loaded interest structure is why making extra payments early in the loan has an outsized impact on total interest savings.
During the interest-only period (typically 5-10 years), you pay only the interest charges, not the principal. Your monthly payment is lower, but you build zero equity. When the interest-only period ends, payments jump significantly because you must now repay the full principal over a shorter remaining term.
For a $300,000 interest-only loan at 6.5%: the interest-only payment is $1,625/month. When the 10-year interest-only period ends, the payment jumps to approximately $2,455/month for the remaining 20 years.
Each payment includes the same principal amount plus interest on the remaining balance. This means payments start high and decrease over time. On a $20,000 loan at 7% over 5 years, you'd pay $333.33 in principal each month plus declining interest — starting at $116.67/month interest in month one and ending at $3.89 in month 60.
An amortization schedule is the most powerful tool for understanding your loan. It shows every single payment over the life of the loan, broken into principal and interest components, plus the remaining balance after each payment.
Here's a sample for a $25,000 personal loan at 7% for 5 years:
| Payment # | Payment | Principal | Interest | Balance |
|---|---|---|---|---|
| 1 | $495.03 | $349.03 | $145.83 | $24,650.97 |
| 6 | $495.03 | $358.95 | $136.08 | $23,924.50 |
| 12 | $495.03 | $370.79 | $124.24 | $23,068.96 |
| 24 | $495.03 | $397.49 | $97.54 | $21,124.33 |
| 36 | $495.03 | $426.22 | $68.81 | $18,967.23 |
| 48 | $495.03 | $457.15 | $37.88 | $16,578.65 |
| 60 | $495.03 | $492.14 | $2.89 | $0.00 |
Total paid: $29,701.80 ($25,000 principal + $4,701.80 interest). Our loan calculator generates complete amortization schedules so you can see exactly where your money goes.
🔑 Generate your own amortization schedule instantly
Use Our Free Loan Calculator →The standard loan payment formula calculates your fixed monthly payment for an amortized loan:
For a $30,000 loan at 8% for 4 years (48 months):
While the formula is useful to understand, you don't need to calculate by hand. Our loan calculator handles the math instantly and produces detailed amortization tables.