Quick answer
Simple interest is I = P × r × t, and the total is P + I. $1,000 at 5% for 3 years earns $150, for a total of $1,150. Compound interest at the same rate would give $1,157.63.
Enter the principal, annual rate and time. The calculator returns the interest and total amount.
Related tools: Compound Interest Calculator: Future value with compound interest and regular contributions. Interest Per Day Calculator: Daily interest on a balance at an annual rate. Real Interest Rate Calculator: Inflation-adjusted interest rate using the exact Fisher relation.
How to use the Simple Interest Calculator
- Enter principal ($) (0 or more).
- Enter annual rate (%) (0 or more).
- Enter time (years) (0 or more).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Principal | A number, in $ | ≥ 0 |
| Annual rate | A number, in % | ≥ 0 |
| Time | A number, in years | ≥ 0 |
Formula and method
Formula
I = P × r × t; total = P + I
IEEE-754 double precision; money shown to cents, rates to 4 decimals. Results are estimates, not financial advice.
Linear interest on the original principal.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Principal = 1000; Annual rate = 5; Time = 3.
| Result | Value |
|---|---|
| Interest | $150.00 |
| Total amount | $1,150.00 |
Example 2
Inputs: Principal = 2500; Annual rate = 4.5; Time = 0.5.
| Result | Value |
|---|---|
| Interest | $56.25 |
| Total amount | $2,556.25 |
The first line is the headline result (interest). Below it the simple Interest Calculator lists total amount, which describe the same case from other angles. IEEE-754 double precision; money shown to cents, rates to 4 decimals. Results are estimates, not financial advice.
Simple versus compound
Simple interest is calculated only on the original principal, so it adds the same $50 each year. Compound interest also earns interest on past interest: 1,000 × 1.05³ = $1,157.63, which is $7.63 more after 3 years, and the gap widens with time.
Where it is used
Some car loans, short-term loans and bonds use simple interest. Time must be in years, so 6 months is 0.5. A time of zero gives zero interest.
How the result changes with principal
Holding every other input at the first worked example, this table changes only principal. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Principal | Interest | Total amount |
|---|---|---|
| 500 | $75.00 | $575.00 |
| 1000 | $150.00 | $1,150.00 |
| 2000 | $300.00 | $2,300.00 |
| 4000 | $600.00 | $4,600.00 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Principal: entering "-1" shows "Principal must be at least 0." beside that field and no result.
Choosing between the Simple Interest Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Compound Interest Calculator](/finance/compound-interest-calculator/) | initial amount, annual interest rate and years | Future value |
| [Interest Per Day Calculator](/finance/interest-per-day/) | balance, annual rate and day-count basis | Interest per day |
| [Real Interest Rate Calculator](/finance/real-interest-rate-calculator/) | nominal interest rate and inflation rate | Real interest rate |
| [APY Calculator](/finance/apy-calculator/) | nominal annual rate and compounding | APY |
| [Continuous Compounding Calculator](/finance/continuous-compound/) | principal, annual rate and years | Future value |
Background: interest, saving and investing
A nominal rate (APR) does not include the effect of compounding; the annual percentage yield (APY) does: APY = (1 + r/n)^n − 1. A 5% nominal rate compounded annually is 5.000%, semi-annually about 5.063%, quarterly about 5.095% and monthly about 5.116%, so more frequent compounding helps, but with diminishing returns.
The rate you enter in a projection is an assumption, not a promise: real returns vary and can be negative. Fees and taxes are not modelled unless the page says so.
Assumptions and limitations
- Zero time gives zero interest
Results are estimates for planning, not financial advice. Lender terms, fees, taxes and rounding rules can change real figures, so confirm important numbers with your lender or a licensed adviser.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for principal to confirm this.
Frequently asked questions
How do I calculate interest for months?
Divide the months by 12 to get years.
Is the rate annual?
Yes, enter the annual rate.
Which is better for savers?
Compound interest, because it earns interest on previously earned interest.
What does the Simple Interest Calculator calculate?
Interest earned or owed without compounding. It returns Interest and Total amount.
What formula does the Simple Interest Calculator use?
I = P × r × t; total = P + I. Linear interest on the original principal.
What do I need to enter in the simple Interest Calculator?
You enter Principal ($), Annual rate (%) and Time (years).
How is the result rounded or limited?
IEEE-754 double precision; money shown to cents, rates to 4 decimals. Results are estimates, not financial advice. Zero time gives zero interest.
What is an example of a simple Interest result?
With principal 1000, annual rate 5 and time 3, the interest is $150.00.
Can I rely on this calculator for a decision?
Results are estimates for planning, not financial advice. Lender terms, fees, taxes and rounding rules can change real figures, so confirm important numbers with your lender or a licensed adviser. For this calculator in particular: Zero time gives zero interest.
Related calculators
- Compound Interest Calculator — Future value with compound interest and regular contributions.
- Interest Per Day Calculator — Daily interest on a balance at an annual rate.
- Real Interest Rate Calculator — Inflation-adjusted interest rate using the exact Fisher relation.
- APY Calculator — Convert a nominal rate (APR) into the effective annual yield (APY).
- Continuous Compounding Calculator — Growth with continuously compounded interest.
- Interest Rate Calculator — Annual interest rate implied by a loan amount, payment and term.
Sources
- Standard time-value-of-money relations (e.g. Brealey, Myers & Allen, Principles of Corporate Finance)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.