How to Calculate a Loan Payment: Complete Amortization Guide

Amortization of a loan is the process of repaying borrowed money together with interest through periodic payments. Understanding how it works is fundamental because it directly affects how much you pay each month, how much you pay in total interest, and how long it takes to become debt-free.

When you take out a loan, the bank or financial institution lends you a principal (the loan amount) and charges you interest for using that money. Your monthly payment consists of two parts:

• Principal repayment: The portion that reduces your outstanding debt
• Interest payment: The cost of using the bank's money

What many people do not understand is that at the beginning of the loan, most of your payment goes to interest, not to reducing the debt. As time progresses and the outstanding principal decreases, the proportion reverses: more goes to principal and less to interest.

Three key concepts you must know:

• Nominal Annual Rate: The interest rate expressed annually, without considering compounding
• Effective Annual Rate (EAR): The real rate you pay considering interest compounding. It is always greater than or equal to the nominal rate
• APR (Annual Percentage Rate): Includes the interest rate plus fees, insurance, and other costs. It is the most comprehensive indicator for comparing loans

Use our loan calculator to see the exact breakdown of your loan with the complete amortization table.

The French system is the most widely used amortization method in the world for personal loans, mortgages, and consumer credit. Its main characteristic is that the monthly payment is always the same throughout the life of the loan (assuming a fixed rate).

How it works:

• The payment is constant from start to finish
• At the beginning, most of the payment is interest with little principal repayment
• At the end, most is principal repayment with little interest
• Interest is calculated on the outstanding balance, which decreases over time

Monthly payment formula (French system):

Payment = Principal x [i x (1+i)^n] / [(1+i)^n - 1]

Where:

• Principal: Loan amount
• i: Monthly interest rate (annual rate / 12)
• n: Total number of payments (months)

Practical example:

• Principal: $100,000
• Annual rate: 12% (monthly rate: 1%)
• Term: 36 months
• Payment = 100,000 x [0.01 x (1.01)^36] / [(1.01)^36 - 1]
• Payment = 100,000 x [0.01 x 1.43077] / [1.43077 - 1]
• Payment = 100,000 x 0.014308 / 0.43077
• Monthly payment = $3,321.43

Loan totals:

• Total paid: $3,321.43 x 36 = $119,571.48
• Total interest: $119,571.48 - $100,000 = $19,571.48

The main advantage of the French system is predictability: you always know exactly how much you pay each month, making financial planning easier.

The German system (also called constant amortization system) is the second most widely used method. Unlike the French system, here the principal repayment is constant in each installment, but interest decreases over time, resulting in decreasing payments.

How it works:

• Principal repayment is the same in every installment (Principal / number of payments)
• Interest is calculated on the outstanding balance, which decreases with each payment
• The total payment (principal + interest) decreases over time
• The first payments are higher than in the French system, but the last ones are lower

German system formulas:

Constant amortization = Principal / n

Interest for month k = Outstanding balance x i

Payment for month k = Constant amortization + Interest for month k

Example with the same data:

• Principal: $100,000
• Annual rate: 12% (monthly: 1%)
• Term: 36 months
• Constant amortization: $100,000 / 36 = $2,777.78

First 3 payments:

Payment	Principal	Interest	Total	Balance
1	$2,777.78	$1,000.00	$3,777.78	$97,222.22
2	$2,777.78	$972.22	$3,750.00	$94,444.44
3	$2,777.78	$944.44	$3,722.22	$91,666.67

Last payment:

• Payment 36: Principal $2,777.78 + Interest $27.78 = $2,805.56

Totals:

• Total interest: $18,500.00
• Total paid: $118,500.00

Comparison with French system:

Item	French	German
First payment	$3,321.43	$3,777.78
Last payment	$3,321.43	$2,805.56
Total interest	$19,571.48	$18,500.00
Interest savings		$1,071.48

The German system generates less total interest because you repay more principal at the beginning. However, it requires greater payment capacity in the first months.

The amortization table (or amortization schedule) is the detailed payment-by-payment breakdown of your loan. It is the most important document for understanding exactly where your money goes each month.

Typical columns in an amortization table:

Column	What it shows
Payment number	The payment period (month)
Total payment	What you pay that month
Principal (amortization)	How much of your payment reduces the debt
Interest	How much you pay in financing cost
Outstanding balance	How much you still owe after the payment

Example table for a $50,000 loan at 12% annually over 12 months (French system):

Payment	Total	Principal	Interest	Balance
1	$4,442.44	$3,942.44	$500.00	$46,057.56
2	$4,442.44	$3,981.86	$460.58	$42,075.70
3	$4,442.44	$4,021.68	$420.76	$38,054.02
6	$4,442.44	$4,143.69	$298.75	$25,731.37
9	$4,442.44	$4,269.66	$172.78	$12,999.38
12	$4,442.44	$4,398.46	$43.98	$0.00

What you can observe:

• In payment 1, 11.25% of your payment goes to interest ($500 of $4,442)
• In payment 12, only 0.99% goes to interest ($44 of $4,442)
• The principal portion grows progressively from $3,942 to $4,398

Why the amortization table matters:

• It shows you exactly how much you have paid in interest vs principal at any point
• It helps you decide whether to make extra payments (and when)
• It lets you compare offers from different banks with real numbers
• It is useful for calculating the outstanding balance if you want to refinance

Generate your complete amortization table with our loan calculator.

Extra payments (additional payments toward principal) are one of the most powerful tools for reducing the total cost of your loan. Every extra dollar you pay goes directly to reducing the outstanding principal, which decreases future interest charges.

Types of extra payments:

• Extra payment with term reduction: You keep the same monthly payment but finish paying sooner. Generally saves more interest
• Extra payment with payment reduction: You keep the same term but your monthly payment decreases. Useful if you need to improve cash flow

Example of extra payment impact:

Loan: $200,000 at 10% annually, 60 months. Monthly payment: $4,249.49

Without extra payment:

• Total paid: $254,969.40
• Total interest: $54,969.40

With $20,000 extra payment at month 12 (term reduction):

• Remaining months: from 48 reduced to ~42
• Total interest: ~$46,200
• Interest savings: ~$8,769

With $20,000 extra payment at month 12 (payment reduction):

• New payment: ~$3,609
• Total interest: ~$48,500
• Interest savings: ~$6,469

When is term reduction better vs payment reduction:

• Reduce term if: you can comfortably maintain the current payment, want to minimize total cost, want to become debt-free sooner
• Reduce payment if: you need more monthly flexibility, your financial situation is uncertain, you have other significant expenses coming

Watch out for prepayment penalties: Some loans charge a penalty for early payment (typically 1% to 3% of the amount paid). Before making an extra payment, check your contract and calculate whether the interest savings exceed the penalty.

Refinancing means replacing your current loan with a new one, usually with better terms (lower interest rate, longer term, or both). But it is not always worthwhile, and calculating correctly is essential.

When to consider refinancing:

• Market interest rates have dropped significantly since you took out your loan
• Your credit profile has improved and you can access better rates
• You need to reduce your monthly payment (by extending the term)
• You want to consolidate multiple debts into one with a better rate

How to evaluate whether refinancing is worthwhile:

You must compare the total cost of keeping the current loan vs the total cost of the new loan, including all refinancing expenses:

• Prepayment penalty on the current loan
• Origination fee on the new loan
• Notarial expenses (if applicable, as with mortgages)
• Insurance required by the new loan

Refinancing analysis example:

Current loan: $150,000 outstanding, 12% annually, 36 months remaining. Payment: $4,982.14

Refinancing option: $150,000, 9% annually, 36 months. Payment: $4,769.55

• Refinancing costs: $3,000 (origination + penalty)
• Monthly savings: $4,982.14 - $4,769.55 = $212.59
• Total payment savings: $212.59 x 36 = $7,653.24
• Net savings: $7,653.24 - $3,000 = $4,653.24
• Break-even point: $3,000 / $212.59 = 14.1 months

In this case, refinancing is worthwhile if you plan to keep the loan for at least 15 more months. If you are going to pay it off early before that, it may not be worth it.

General rule: If the new rate is at least 2 percentage points lower than the current one and you have more than 12 months remaining, it is worth analyzing refinancing.

Avoid these frequent mistakes that can cost you thousands in unnecessary interest:

Mistake 1: Comparing only the monthly payment

A loan with a lower monthly payment is not always better. It may have a longer term, resulting in paying much more in total interest. Always compare the total cost (APR) of your options.

Mistake 2: Not considering the total cost of interest

On a 20-year mortgage at 10%, you can end up paying more than double the property's price. It is essential to calculate how much you will pay in total, not just the monthly payment. Use our compound interest calculator to see the effect of interest over time.

Mistake 3: Taking the longest available term

Extending the term reduces the payment but dramatically increases interest. Example with $100,000 at 12% annually:

Term	Monthly payment	Total interest
24 months	$4,707.35	$12,976.40
36 months	$3,321.43	$19,571.48
48 months	$2,633.84	$26,424.32
60 months	$2,224.44	$33,466.40

Going from 24 to 60 months reduces the payment by 53%, but interest increases by 158%.

Mistake 4: Ignoring variable rates

Variable rate loans may seem cheaper initially, but if rates rise, your payment can increase significantly. Only take a variable rate if you understand the risk and can absorb a 30-50% payment increase.

Mistake 5: Not reading the fine print

• Prepayment penalties
• Required insurance included in the APR
• Hidden fees (origination, administration, annual)
• Late payment interest rate clauses

Mistake 6: Taking on debt exceeding 30% of your income

The general rule is that your total debt payments should not exceed 30-35% of your monthly net income. Exceeding this limit puts your financial stability at risk.

Not all loans are the same. Each type has different characteristics, rates, and conditions:

1. Personal loan (consumer):

• Use: Any purpose (travel, appliances, emergencies)
• Typical rates: 8-25% annually depending on country and credit profile
• Terms: 6 to 60 months
• Collateral: Unsecured
• Characteristic: Higher rates due to lack of collateral

2. Mortgage:

• Use: Home or property purchase
• Typical rates: 5-12% annually
• Terms: 5 to 30 years
• Collateral: The property itself
• Characteristic: Lower rates due to collateral, but down payment required (10-30%)

3. Auto loan:

• Use: Vehicle purchase
• Typical rates: 7-18% annually
• Terms: 12 to 72 months
• Collateral: The vehicle (lien)
• Characteristic: The vehicle depreciates, unlike property

4. Credit card:

• Use: Day-to-day purchases, emergencies
• Typical rates: 25-60% annually (the most expensive in the market)
• Terms: Revolving (indefinite minimum payment)
• Characteristic: The worst form of long-term financing. Only ideal if you pay the full balance each month

5. Peer-to-peer (P2P) loan:

• Use: Variable
• Typical rates: 10-20% annually
• Terms: 6 to 36 months
• Characteristic: Digital platforms connecting lenders with borrowers

For any type of loan, calculate your exact payment and amortization table with our loan calculator. If you want to see how much that money would grow if you invested it instead of paying interest, use our compound interest calculator.