Simple Interest Loan Amortization Table Calculator

This simple interest loan amortization calculator works out a fixed monthly payment and total interest for a loan with a fixed APR and equal monthly payments. Enter the loan amount, APR, and term to see the payment and the total interest; the table below explains how to build the month-by-month schedule.

Formula

The calculator uses the standard fixed-payment, fully amortizing loan formula. It assumes interest accrues at a constant monthly rate, with each payment made monthly:

• Monthly rate: r = APR / 1200, where APR is the annual percentage rate entered as a percentage.
• Number of payments: n = loan term in years × 12.
• Monthly payment: M = P × r / [1 − (1 + r)⁻ⁿ].
• Total interest: I = M × n − P.

P is the starting principal in dollars, r is the monthly rate as a decimal, n is the number of monthly payments, and M is the monthly payment in dollars. The calculator rounds M to cents for display; its total-interest output uses the unrounded formula value for payment, then rounds the result to cents. A real payment schedule can differ because a lender may round each payment and interest posting separately.

Worked Example

Using the defaults—$20,000 principal, 8% APR, and five years—the calculation is:

1. Convert APR to a monthly decimal rate: r = 8 / 1200 = 0.0066667.
2. Find the payment count: n = 5 × 12 = 60.
3. Substitute: M = 20,000 × 0.0066667 / [1 − (1 + 0.0066667)⁻⁶⁰].
4. The monthly payment is approximately $405.53.
5. Total interest is approximately (unrounded monthly payment × 60) − $20,000 = $4,331.84.

Displayed values are rounded to cents, so multiplying the displayed $405.53 by 60 gives $24,331.80 rather than the total based on the unrounded formula payment. The difference is a rounding effect, not extra principal or interest.

Assumptions

• The rate is fixed for the entire term.
• Payments are equal, made monthly, and occur at the end of each monthly period.
• Interest compounds monthly at APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, and late charges are not included.

What an Amortization Table Shows

An amortization table, also called an amortization schedule, breaks each payment into interest and principal. Interest is calculated on the outstanding balance, so the interest share is generally larger near the beginning of a level-payment loan. As the balance declines, less interest accrues each month and more of the same payment reduces principal.

For a basic monthly schedule, calculate each row in this order:

1. Start with the balance carried forward from the previous month.
2. Calculate that month’s interest as beginning balance × monthly rate.
3. Calculate principal paid as scheduled payment − interest.
4. Calculate ending balance as beginning balance − principal paid.

For example, the first month’s interest on the defaults is about $133.33 ($20,000 × 0.0066667). With an unrounded payment of about $405.53, roughly $272.20 goes toward principal, leaving a balance near $19,727.80. The cents shown are illustrative; actual statements can differ slightly with lender rounding and payment-posting rules.

Simple Interest and Amortization: What the Terms Mean

“Simple interest” usually means interest is calculated on the outstanding principal rather than adding unpaid interest to principal to create compound interest. In an installment loan, interest can still accrue over time and each payment can contain both interest and principal. The word simple does not necessarily mean that every month’s interest is the same: when the balance declines, interest calculated on that balance also declines.

This calculator is a fixed-rate monthly amortization model. It is not a daily simple-interest calculator. Some contracts calculate interest each day on the current principal, meaning the amount of interest can depend on the exact number of days between payments and when payments post. A formula using APR divided by 12 cannot reproduce that schedule exactly. Check the loan documents or lender’s payment breakdown to identify how interest is calculated.

How to Read and Use the Schedule

Use the payment amount as the scheduled installment under these assumptions, not as a payoff quote. The total-interest figure estimates the interest across all scheduled payments if the loan runs for the full term. An amortization table can also show the balance remaining after a particular payment, which is useful for understanding how much principal remains—not for calculating a lender’s exact payoff amount.

• Compare the same inputs: changing either the term or APR changes the payment and the total interest.
• Check the starting balance: the principal may not equal the amount received if financed fees or other charges are added to the loan.
• Use the contract’s payment frequency: this calculator assumes monthly installments, not weekly or biweekly payments.
• Verify the last row: rounding in a real schedule can require a slightly adjusted final payment to bring the balance to zero.

Common Limits and Misunderstandings

A level-payment table does not necessarily describe every loan called “simple interest.” Variable rates, irregular payment dates, skipped or late payments, daily accrual, fees added to principal, and extra payments can all change the schedule. An extra payment may reduce principal and future interest, but its effect depends on how the lender applies it and whether the contract has relevant conditions. This calculator does not model those details.

Another common confusion is treating APR as the monthly rate. For the formula here, divide the annual percentage by 12 and convert percent to decimal: 8% becomes 0.08 / 12, or 0.0066667 per month. Do not enter 8 as the monthly rate. The stated APR may also not describe all charges in the way a particular disclosure presents them, so use the rate and terms applicable to the specific loan.

Frequently Asked Questions

How do I make a simple interest loan amortization table?
Start with the opening balance, calculate period interest on that balance, subtract interest from the payment to find principal repaid, then subtract principal from the balance. Repeat for each payment period.

Is simple interest the same as an amortized loan?
No. Simple interest describes how interest is calculated; amortization describes how scheduled payments repay principal and interest over time. A loan can use simple interest and have an amortization schedule.

Why does the principal portion increase each month?
With a fixed payment and a declining balance, interest generally falls over time. The amount left after interest is therefore available to reduce more principal.

Does this calculator show my exact payoff amount?
No. It estimates scheduled payments and total interest under its assumptions. A payoff amount may include interest accrued since the last payment and contract-specific charges or adjustments; request the exact figure from the lender.

Why does my lender’s schedule differ by a few cents?
Payment rounding, daily rather than monthly interest accrual, payment dates, and the method used to apply payments can cause small differences. Compare the schedule’s assumptions with the loan documents.

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

Related

• Qualified Income Trust Form: What It Is and How It Works
• Qualified Income Trust Account: Medicaid Income Cap
• Qualified Income Trust: Medicaid Income Cap Calculator

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