Simple Interest Loan Calculator and Explanation

A simple interest loan calculates interest on the current unpaid principal, so the interest charged can fall as you repay the balance. Enter the loan amount, APR, and term to estimate a level monthly payment and total interest; this calculator uses a standard amortizing-payment formula, not a daily simple-interest schedule.

Formula

The calculator treats the loan as a fixed-rate loan with equal monthly payments. It first converts the annual percentage rate to a monthly rate and calculates the payment that amortizes the principal over the selected number of months.

• Monthly rate: r = APR / 1200, where APR is entered as a percentage per year and r is a decimal rate per month.
• Number of payments: n = 12 × years.
• Monthly payment: payment = P × r / [1 − (1 + r)⁻ⁿ], rounded to the nearest cent. P is the loan principal in dollars.
• Total interest: payment × n − P. The calculation uses the cent-rounded monthly payment shown above.

The rate must be above zero for this formula. Although a zero-interest loan can be calculated by dividing principal by the number of payments, this calculator’s APR input starts at 0.01% and does not model that case. The selected term is converted to whole monthly payment periods through years × 12.

Worked Example

With the defaults, P = $20,000, APR = 8% per year, and the term is 5 years.

1. Convert the rate: r = 8 / 1200 = 0.0066667 per month.
2. Convert the term: n = 5 × 12 = 60 monthly payments.
3. Substitute: payment = 20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰].
4. The monthly payment rounds to $405.53.
5. Total interest = $405.53 × 60 − $20,000 = $4,331.80.

This is an estimate under the calculator’s assumptions. A lender’s payment schedule may differ if it uses daily interest, different payment dates, fees, or other terms.

Assumptions

• The APR and payment amount remain fixed for the full term.
• Payments are equal, made monthly, and treated as occurring at the end of each monthly period.
• Interest compounds monthly at APR / 12.
• Fees, taxes, insurance, PMI, escrow, late payments, and extra principal payments are not included.

How Simple Interest Works

In a simple-interest loan, interest is calculated on principal that remains unpaid rather than on the original amount for the entire term. When a scheduled payment is applied, part generally covers accrued interest and the remainder reduces principal. If principal declines, later interest charges may also decline, assuming the rate and timing remain unchanged.

“Simple interest” describes how interest accrues; it does not by itself specify the payment schedule. Some loans calculate interest daily using the outstanding balance and the number of days between payments. The calculator above instead approximates a monthly amortizing loan: it divides APR by 12 and assumes regular monthly periods. Check the loan agreement or payment disclosure to learn how a particular loan accrues interest and applies payments.

How to Read the Results

The monthly payment is the calculated principal-and-interest payment under the assumptions shown. Total interest is the sum of those rounded payments minus the starting principal. It does not mean that this amount is due at once, and it excludes any charges not represented in the inputs.

Changing the term changes both payment size and the number of payments. A longer term spreads principal repayment across more months, which can reduce the calculated payment while increasing the number of periods on which interest accrues. The results show the mathematical effect of the inputs, not whether a loan is suitable or whether anyone would qualify for it.

Common Misconceptions and Limits

• Simple interest is not always “interest only on the original amount.” That description fits a flat-interest calculation, not necessarily a loan where interest is calculated on a declining balance.
• APR is not necessarily the exact periodic rate charged under every contract. This estimate uses APR / 12. Daily accrual, irregular payment dates, or other contract terms can change the schedule.
• Rounding affects totals. This calculator rounds each displayed monthly payment to cents and multiplies that payment by the number of months. A lender may adjust the final payment to clear a small remaining balance.
• Extra payments are not modelled. Additional principal payments can change later balances and interest, depending on how and when they are credited.

Use figures from a lender’s written disclosure when checking a specific offer. Rates, fees, and payment terms are inputs that must come from the lender; this calculator does not supply current rates or predict approval.

Frequently Asked Questions

What is a simple interest loan?
A loan where interest is calculated on unpaid principal. As the balance falls, interest may fall too, depending on the contract’s accrual method and payment timing.

How do I calculate a simple interest loan payment?
For a fixed-rate monthly amortizing estimate, convert APR to a monthly rate, set the number of monthly payments, then apply payment = P × r / [1 − (1 + r)⁻ⁿ]. This calculator performs those steps and rounds the payment to cents.

Is simple interest the same as compound interest?
Not necessarily. Simple interest generally refers to interest based on unpaid principal, while compound interest adds accrued interest to the balance on which later interest is calculated. A loan’s exact accrual and payment terms matter.

Does paying early reduce interest on a simple interest loan?
It can reduce interest when the lender credits the payment to principal earlier, but the effect depends on the contract, payment timing, and any applicable charges. This calculator does not model early or extra payments.

Why is my lender’s payment different?
The lender may use daily rather than monthly interest accrual, different dates or rounding, fees, or a different payment schedule. Compare the assumptions and figures in the loan disclosure.

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

Related

• Calculate a Monthly Loan Payment: Formula and Example
• How to Calculate Loan Payments: Formula and Example
• Basic Loan Calculator Payment and Formula

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2