Simple Loan Calculator With Amortization

A simple loan calculator with amortization estimates your fixed monthly payment, total interest, and total paid from the loan amount, annual percentage rate (APR), and term. Enter those three figures to see the payment calculation; the schedule underlying it shows how each payment is divided between interest and principal.

Formula

The calculator uses the standard fixed-payment loan formula. It converts the annual APR to a monthly rate, then calculates the level payment that repays the starting balance over the full term:

Monthly payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the original principal, in dollars ($).
• r is the monthly interest rate as a decimal: APR ÷ 1,200. For an 8% APR, r = 0.08 ÷ 12.
• n is the number of monthly payments: term in years × 12.

Total interest is calculated as the monthly payment multiplied by the number of payments, minus the original principal: Total interest = payment × n − P. The calculator displays the payment to the nearest cent; its total-interest result uses that displayed, rounded payment. Actual loan schedules may use more detailed rounding or payment-date rules.

Worked Example

Using the calculator’s defaults: P = $20,000, APR = 8%, and term = 5 years.

1. Convert APR to a monthly decimal rate: r = 8 ÷ 1,200 = 0.0066667.
2. Convert the term to monthly payments: n = 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰] = $405.53 per month, rounded to the nearest cent.
4. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

Under these assumptions, the estimated total of principal and interest is $24,331.80. The final payment in a real schedule can differ slightly if the lender adjusts it to clear a remaining balance.

Assumptions

• The interest rate is fixed for the full term.
• Payments are equal monthly payments made at the end of each monthly period.
• Interest accrues monthly at APR ÷ 12; this is the calculator’s periodic-rate convention.
• Fees, taxes, insurance, private mortgage insurance, and escrow are not included.

How Amortization Works

Amortization is the process of repaying a loan through scheduled payments that cover interest and reduce principal. Interest for a period is calculated from the outstanding balance. Early in a level-payment loan, the balance is larger, so more of the payment goes to interest. As principal falls, the interest portion generally falls and more of each payment reduces principal.

For the example, the first month’s estimated interest is $20,000 × 0.0066667, or about $133.33. Subtracting that from the $405.53 payment leaves about $272.20 to reduce principal, taking the balance to about $19,727.80. The next month’s interest is calculated on that lower balance. This changing split is why a payment amount alone does not show how quickly principal is being repaid.

A full amortization schedule lists each payment date or period, payment amount, interest, principal reduction, and remaining balance. This calculator returns the fixed payment and total interest, not a row-by-row schedule. The formula can nevertheless explain the schedule’s basic pattern when the loan follows the stated assumptions.

When to Use the Estimate

Use the calculation to understand the arithmetic of a fixed-rate, fully amortizing loan or to see how changing the amount, APR, or term changes the estimated payment. It can also help you check whether figures in a quoted repayment illustration are internally consistent, provided you enter the same principal, annual rate, and duration and account for the quote’s rounding conventions.

Enter the principal you expect to borrow, not a desired monthly payment. Use the APR supplied for the specific loan and the stated repayment term; do not substitute a rate from another offer or assume the example APR applies to you. The displayed figures are not an approval decision, a borrowing limit, or an assessment of personal finances. Obtain actual loan terms and any additional charges from the lender.

Limits and Common Mistakes

• Confusing APR and monthly rate: APR is entered as a percentage per year. The formula divides it by 1,200 to get the monthly decimal rate. Do not enter 8% as 0.08 in the APR field; enter 8.
• Mixing years and months: This calculator asks for years and multiplies by 12. Entering a number of months as though it were years makes the term twelve times too long.
• Assuming all borrowing costs are included: The calculated payment covers principal and interest under the formula. A real bill may include separately charged items, while upfront charges may affect the cost without changing this calculation.
• Expecting exact lender figures: Payment dates, daily interest methods, rounding, variable rates, fees, and a final adjusted payment can produce differences. The calculator does not model those details.
• Applying it to an irregular balance: Interest-only periods, balloon payments, changing rates, skipped payments, and payments that vary over time do not follow this simple fixed-payment model.

A longer term spreads repayment over more months and changes both the payment and the total interest; a shorter term changes those figures in the other direction when the other inputs remain fixed. This describes the formula’s arithmetic, not what any particular borrower should choose. For a schedule that includes extra payments, fees, or rate changes, those features must be calculated separately using their actual terms.

Frequently Asked Questions

What does an amortization calculator show?
It estimates scheduled payments and how repayment is allocated between interest and principal over time. This simple calculator gives the fixed monthly payment and total interest, rather than a complete payment-by-payment table.

How much of my first payment goes to principal?
Under this formula, calculate first-period interest as the starting balance multiplied by the monthly rate. Subtract that interest from the monthly payment to estimate the principal reduction. In the example, that is about $272.20 before any lender-specific rounding.

Does this calculator include fees or insurance?
No. It estimates principal and interest only. Fees, taxes, insurance, mortgage insurance, and escrow are not inputs and are not included in its results.

Can I use it for a variable-rate loan?
Not as a complete forecast. The formula assumes one fixed APR for the whole term. If the rate changes, the payment or payoff path may change, so the calculator cannot represent the full schedule.

Why might a lender’s payment differ?
The lender may use different payment dates, rounding, interest accrual conventions, fees, or a final-payment adjustment. Compare the calculator’s assumptions with the loan disclosure rather than treating a small difference as an error.

This result is an estimate; the lender’s disclosure gives the exact figures.

Related

• APR Calculator: The True Rate of a Loan With Fees
• Simple Interest Loan Calculator: I = P × r × t
• Interest-Only Loan Calculator: Payment During and After the Interest-Only Period

in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2