Simple Interest on $500 at 3%
Simple interest earned on $500 at 3% over different time periods. Adjust any field below to try your own numbers.
Interest Earned
$75.00
Total Amount
$575.00
Example
$500.00 at 3.00% simple annual interest for 5.0 years earns $75.00 in interest, for a total of $575.00.
Total amount, split between principal and interest
- Principal: $500.00
- Interest: $75.00
What is a Simple Interest Calculator?
A simple interest calculator computes interest that accrues only on the original principal — never on interest that has already been earned or charged. It's the most straightforward form of interest math, commonly used for short-term loans, certain promissory notes, car loans in some jurisdictions, and as a teaching example for how interest works before introducing compounding.
Because simple interest grows in a straight line rather than accelerating, it is always the cheaper option for a borrower (and the lower-yielding option for a saver) compared to compound interest at the same nominal rate over the same period — the gap between the two widens the longer the money is left in place.
Growth of $500.00 at 3% Simple Interest
Every row below is computed for your exact principal and rate, showing how interest accumulates in a straight line — unlike compound interest, each additional year adds exactly the same dollar amount of interest.
| Years | Interest | Total |
|---|---|---|
| 1 yrs | $15.00 | $515.00 |
| 2 yrs | $30.00 | $530.00 |
| 3 yrs | $45.00 | $545.00 |
| 5 yrs (current) | $75.00 | $575.00 |
| 10 yrs | $150.00 | $650.00 |
| 15 yrs | $225.00 | $725.00 |
| 20 yrs | $300.00 | $800.00 |
| 25 yrs | $375.00 | $875.00 |
| 30 yrs | $450.00 | $950.00 |
The Simple Interest Formula
Simple interest is calculated once, directly on the principal, scaled by the rate and the length of time — there is no compounding step at all.
- I — interest earned or owed
- P — principal
- R — annual interest rate (as a decimal)
- T — time in years
Simple Interest vs. Compound Interest
With simple interest, only the original principal earns interest — the interest itself never earns more interest. With compound interest, each period's interest is added to the balance, so future interest is calculated on a growing balance. Over short periods the difference is small, but it compounds (literally) over time: at 5% for 20 years, $10,000 grows to $20,000 under simple interest but to roughly $26,533 under annual compounding — over $6,500 more, purely from interest earning interest.
For interest that compounds, use the Interest Calculator instead, which supports annual, quarterly, monthly, and daily compounding frequencies.
Where Simple Interest Is Actually Used
Simple interest shows up in short-term promissory notes, certain personal loans between individuals, some auto loans (which use a "simple interest" method where interest accrues daily on the outstanding balance but doesn't compound), and treasury bills and other short-duration securities. It is also the standard teaching model for introducing interest math, since the linear growth is easier to reason about than the exponential curve of compound interest.
Example — Your Current Inputs
$500.00 at 3.00% simple annual interest for 5.0 years earns $75.00 in interest, for a total of $575.00.
Additional Example — A Short-Term Personal Loan
You lend a friend $2,000 at 6% simple annual interest, to be repaid in 9 months (0.75 years). The interest owed is $2,000 × 0.06 × 0.75 = $90, so your friend repays $2,090 total — regardless of whether they repay it in one lump sum or several installments, since simple interest doesn't recalculate based on a declining balance the way an amortized loan does.
About These Parameters
- Principal
- The starting amount — the sum deposited, lent, or borrowed before any interest. Simple interest is always calculated on this fixed original amount, no matter how much interest has already accrued.
- Annual Interest Rate
- The yearly rate as a percentage. Because simple interest never compounds, this rate directly and linearly scales the total interest earned over any time period.
- Time Period
- The length of time the principal accrues interest, in years. Use a decimal value for partial years — for example, enter 0.25 for 3 months or 0.5 for 6 months.
Frequently Asked Questions
Is simple interest better for a borrower or a saver?
It's better for a borrower and worse for a saver, compared to compound interest at the same nominal rate. As a borrower, you want interest that doesn't compound, since you pay less over time. As a saver or investor, you want compounding, since your interest starts earning its own interest, accelerating growth.
Do credit cards use simple interest?
No — credit cards almost universally compound interest daily on the outstanding balance, which is one reason credit card debt grows so quickly if left unpaid. Simple interest is far more common in short-term personal loans, some auto loans, and short-duration fixed-income securities.
How do I convert a monthly rate into the annual rate this calculator expects?
Multiply your monthly rate by 12 to get the equivalent simple annual rate — for example, a 0.5% monthly rate equals 6% annually under simple interest (since there's no compounding to account for, unlike with compound interest rate conversions).
Can the time period be in months or days instead of years?
Yes — just convert to a fraction of a year first. For months, divide by 12 (so 9 months = 0.75 years). For days, divide by 365 (so 45 days ≈ 0.123 years). The formula itself doesn't change, only the T value you plug in.