mCalculator logo
Loan Calculators

Simple Interest Loan Calculator

Calculate simple interest, total repayment, and the equivalent monthly payment using the I = P × r × t formula common with auto and short-term loans.

Uses the textbook simple-interest formula. Most US auto loans actually use simple-interest amortization, which is different — see the related auto loan calculator for that.

$
%
yr
Total interest
$2,400
Total to repay $12,400 · ≈ $344/mo over 3 yr.
Principal
$10,000
Interest (I = P × r × t)
$2,400
Total repayment
$12,400
Equivalent monthly payment
$344
Overview

How the Simple Interest Loan Calculator Works

Simple interest accrues only on the original principal, never on previously earned interest. It's the formula behind many short-term personal loans, dealer-financed promotions, and the textbook 'I = P × r × t' you learned in school. This calculator applies the formula directly and shows the equivalent level monthly payment if the loan is repaid in equal installments.

Formula

The Math Behind the Calculator

Interest I = P × r × t, where P is principal, r is the annual rate as a decimal, and t is time in years. Total repayment = P + I. Equivalent monthly payment = Total repayment ÷ (years × 12).

Example

A Worked Example

$10,000 borrowed at 8% simple interest for 3 years: I = 10,000 × 0.08 × 3 = $2,400. Total to repay = $12,400. Spread over 36 months: about $344/mo. Compare that to a standard amortizing 8% loan of the same size, which would carry ~$1,281 in interest — simple-interest term loans cost more when held to maturity.

How to use

How to Use the Simple Interest Loan Calculator

  1. 1Enter the principal you'll receive.
  2. 2Use the annual rate from your contract — make sure it's labeled 'simple interest', not APR on an amortizing loan.
  3. 3Enter the term in years (or a fraction, e.g. 0.5 for six months).
  4. 4Compare the result to a standard amortizing loan calculator to see which structure costs less.
Interpretation

What the Results Mean

  • Interest is the total dollar cost of borrowing, computed only on the original principal.
  • Total repayment is principal + interest, the amount you'll have paid by the end of the term.
  • Monthly payment assumes the total is split evenly — actual contracts may use a different schedule.
Avoid

Common Mistakes to Avoid

  • Confusing simple-interest loans with simple-interest amortization (used by most auto lenders). The latter accrues interest daily on the remaining balance and is usually cheaper.
  • Treating a simple-interest rate as if it were an APR on an amortizing schedule.
  • Forgetting that paying early on a true simple-interest loan often doesn't reduce the interest charge unless the contract allows interest rebates.
Keep going

Related Calculators

Auto Loan Calculator

Estimate your monthly car payment with down payment, trade-in value, sales tax, and fees built in.

Personal Loan Calculator

Estimate the monthly payment, total interest, and total cost of a personal loan at any rate and term.

Loan Comparison Calculator

Compare two loan offers side by side — monthly payment, total interest, total cost including fees, and effective APR.

Compound Interest Calculator

Project the future value of savings or investments with monthly contributions and compound growth.

FAQ

Frequently Asked Questions

Is simple interest the same as APR?+

No. APR can apply to either simple-interest or compounding/amortizing loans. Always read the contract type — short-term notes and dealer promos often use true simple interest.

When is simple interest a good deal?+

On short-term loans where the savings from paying off early are passed back to you, or when the rate is meaningfully lower than the amortizing alternative.

Why is the total cost higher than a normal loan at the same rate?+

Because interest is charged on the full principal for the entire term, even as you pay it down — there's no benefit from a falling balance like on a standard amortizing loan.

Financial Disclaimer

This calculator is for educational and estimation purposes only. It does not provide financial, mortgage, tax, investment, or legal advice. Actual rates, payments, taxes, fees, insurance costs, eligibility, and loan terms vary by lender, location, credit profile, and market conditions. Always compare official offers and consult a qualified professional before making financial decisions.

Last updated June 2026 · Prepared by the mCalculator Editorial Team