How Loan Amortization Works

Updated July 2026 · Mathematical breakdown with real examples

The Amortization Formula

Every fixed-rate loan uses the same mathematical formula to calculate your monthly payment. The formula ensures that after the final payment, your balance is exactly zero.

Step-by-Step Calculation Example

Let's calculate the monthly payment for a $20,000 loan at 12% APR over 5 years:

1. P = $20,000 (loan principal)
2. r = 0.12 ÷ 12 = 0.01 (monthly interest rate = 1%)
3. n = 5 × 12 = 60 (total payments)
4. (1+r)^n = (1.01)^60 = 1.8167
5. Numerator: r(1+r)^n = 0.01 × 1.8167 = 0.018167
6. Denominator: (1+r)^n - 1 = 1.8167 - 1 = 0.8167
7. Fraction: 0.018167 ÷ 0.8167 = 0.022244
8. M = $20,000 × 0.022244 = $444.89/month

Your fixed monthly payment is $444.89 for all 60 months. But the split between principal and interest changes every single month.

Why Early Payments Are Mostly Interest

Interest is calculated on your remaining balance—not the original loan amount. When your balance is high (early in the loan), the interest portion is high. As your balance decreases, so does the interest, and more of each payment goes toward principal.

Payment Breakdown: Month 1 vs. Month 60

Using our $20,000 / 12% / 5-year example:

Component	Month 1	Month 12	Month 24	Month 36	Month 48	Month 60 (Final)
Starting Balance	$20,000.00	$17,391.72	$14,563.28	$11,496.96	$8,174.12	$440.49
Interest	$200.00	$173.92	$145.63	$114.97	$81.74	$4.40
Principal	$244.89	$270.97	$299.26	$329.92	$363.15	$440.49
Ending Balance	$19,755.11	$17,120.75	$14,264.02	$11,167.04	$7,810.97	$0.00
% to Interest	45.0%	39.1%	32.7%	25.8%	18.4%	1.0%

In Month 1, 45% of your payment goes to interest. In Month 60, only 1% goes to interest. The total payment never changes—only the split does.

How to Read an Amortization Schedule

An amortization schedule is a table showing every payment for the life of the loan. Each row contains:

• Payment #: The sequential number (1, 2, 3... 60)
• Date: When the payment is due
• Payment: The fixed monthly amount ($444.89 in our example)
• Principal: The portion reducing your loan balance
• Interest: The portion going to the lender as profit
• Extra: Any additional payment above the minimum
• Balance: Remaining loan balance after this payment
• Cumulative Interest: Total interest paid to date

Our calculator generates this full schedule for any loan. You can toggle between showing the first 12 payments (quick view) and the full schedule (complete breakdown).

The Impact of Loan Term on Amortization

Loan term dramatically changes both your monthly payment and total interest paid. Here's the same $20,000 loan at 12% across different terms:

Term	Monthly Payment	Total Interest	Total Cost	Interest as % of Principal
2 years	$941.47	$2,595	$22,595	13.0%
3 years	$664.29	$3,914	$23,914	19.6%
5 years	$444.89	$6,793	$26,793	34.0%
7 years	$352.28	$9,592	$29,592	48.0%
10 years	$286.94	$14,433	$34,433	72.2%

Notice the trade-off: a 10-year term cuts your monthly payment by 36% compared to 5 years, but you pay 112% more in total interest. The 7-year term costs 41% more interest than the 5-year term. This is the power of compound interest working against you.

How Extra Payments Change Amortization

Extra payments directly reduce your principal balance, which immediately reduces the interest calculated on every future payment. This creates a compounding effect in your favor.

Example: $20,000 / 12% / 5 Years with Extra Payments

Extra/Month	Monthly Payment	Total Interest	Interest Saved	Months Saved	Payoff Date
$0 (Base)	$444.89	$6,793	—	—	Aug 2031
$50	$494.89	$5,513	$1,280	8	Dec 2030
$100	$544.89	$4,395	$2,398	14	Jun 2030
$200	$644.89	$2,847	$3,946	24	Aug 2029
$500	$944.89	$1,287	$5,506	38	Jun 2028

Adding just $100/month saves $2,398 in interest and pays off the loan 14 months early. Adding $500/month saves $5,506 and cuts the loan by 38 months—more than 3 years. This is why financial advisors recommend paying extra on high-interest debt before investing in low-return assets.

Amortization vs. Simple Interest

Amortized loans are different from simple interest loans:

Feature	Amortized Loan	Simple Interest Loan
Payment Structure	Fixed monthly payment	Interest calculated on remaining balance
Interest Calculation	On declining balance	On original principal (some types)
Early Payoff Benefit	High—reduces all future interest	Variable—depends on loan terms
Common Examples	Mortgages, auto loans, personal loans	Some auto loans, short-term loans
Rule of 78s	Not used	Sometimes used (pre-1992 common)

Key Takeaways

1. Amortization is mathematical, not arbitrary. Every payment is calculated using the same formula. The lender does not "decide" how much interest you pay each month—it is determined by your remaining balance and rate.
2. Interest is front-loaded. Early payments are mostly interest because your balance is highest. This is not a trick—it is just math.
3. Extra payments have outsized impact. Because interest is calculated on the remaining balance, every extra dollar reduces future interest across all remaining payments. A $100 extra payment in Month 1 saves more than a $100 extra payment in Month 50.
4. Shorter terms save massive interest. A 3-year loan costs roughly half the interest of a 5-year loan on the same principal and rate. The monthly payment is higher, but the total cost is dramatically lower.
5. Your amortization schedule is a roadmap. It shows exactly where every dollar goes. Use it to plan extra payments, track progress, and verify your lender's calculations.