# Simple Interest Calculator

> Calculate simple interest and total amount with interest using principal, annual rate, and time. Simple interest applies only to the original principal.

- **Canonical URL:** https://boringtoolskit.com/simple-interest-calculator/
- **Category:** Everyday Calculators
- **Evidence level:** authored-methodology
- **Reviewed:** August 2026
- **Last verified:** 2026-09-20
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Simple interest = principal × rate ÷ 100 × time in years. Total amount = principal + interest. Interest applies only to the original principal, not to accumulated interest.

## Worked example

For $1,000 at 5% annual interest over 3 years, interest = 1000 × 0.05 × 3 = $150, and the total amount = $1,150.

## Assumptions

- Interest is calculated on the original principal only; no compounding is applied.
- The annual rate and time in years are user-provided planning inputs.
- This is a math estimate and does not include fees, penalties, compounding, or loan-specific terms.

## Common questions

### What is the simple interest formula?

Simple interest I = P × r × t: principal times the annual rate as a decimal times time in years. Total amount A = P + I. At 5% for 3 years, a $1,000 principal earns 1,000 × 0.05 × 3 = $150 of interest, for a total of $1,150.

### How do I calculate simple interest on a loan?

Enter the loan principal, the annual interest rate, and the time in years. The calculator multiplies them and adds the interest to the principal to show the total amount due. For a term in months or days, convert to years first: 6 months is 0.5 years, 90 days is 90/365.

### What is the difference between simple and compound interest?

Simple interest is charged only on the original principal, so the interest amount is the same every year. Compound interest is charged on principal plus accumulated interest, so the balance grows faster each period. On $10,000 at 5% over 10 years, simple interest totals $15,000 while annual compounding totals about $16,289.

### Is simple interest better for borrowers or savers?

For borrowers, simple interest usually costs less than compound interest on the same rate and term, because interest never earns interest. For savers, compound interest earns more. A simple interest loan is generally the cheaper structure to borrow under, all else equal.

### Do banks use simple interest or compound interest?

Savings accounts, CDs, and credit cards use compound interest, and US banks disclose the compounded yield as APY. Many personal and auto loans accrue interest daily on the outstanding principal using simple interest arithmetic, but the required payment schedule amortizes the balance. The sticker math on a deposit account is compounding; a single-payment note is simple interest.

### How do I convert months or days to years for the formula?

Divide months by 12 and days by 365, unless the agreement specifies a 360-day year. Six months is 0.5 years, 18 months is 1.5 years, and 45 days is about 0.1233 years on a 365-day basis. Using the wrong convention changes the interest in proportion to the error.

### How do I find the rate or time if I know the interest?

Rearrange the formula. Rate r = I ÷ (P × t); time t = I ÷ (P × r); principal P = I ÷ (r × t). For example, $450 of interest on $5,000 over 1.5 years implies r = 450 ÷ 7,500 = 0.06, a 6% annual rate.

### Is this calculator suitable for mortgage or credit card debt?

No. Mortgages and credit cards amortize or compound and may include fees, minimum-payment rules, and rate changes. Use this tool for straight-line estimates on a fixed principal, and use a loan, credit card payoff, or amortization calculator for payment schedules. This page is financial education, not tax, legal, or investment advice.

## Related tools

- /savings-calc/
- /compound-interest/
- /auto-loan-calculator/

## Sources

- [What is simple interest?](https://www.consumerfinance.gov/ask-cfpb/what-is-simple-interest-en-3498/)

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