Simple Interest Calculator

Simple interest is charged only on the amount you start with — the principal — and never on the interest already earned. Because of that, the balance grows by the same dollar amount every year. This calculator shows the interest and the final value for any principal, rate, and number of years.

The formula

Interest equals principal times the annual rate times the number of years: I = P × r × t, where the rate is written as a decimal. So $1,000 at 5% for 3 years earns $1,000 × 0.05 × 3 = $150, leaving a total of $1,150. The ledger bar above shows how much of the final value is interest versus your original principal.

Because every term is multiplied, you can rearrange the same formula to solve for whatever you're missing:

Rate: divide the interest by principal times years. To earn $200 on $4,000 over 2 years, you need $200 ÷ ($4,000 × 2) = 0.025, or 2.5% a year.
Principal: divide the interest by rate times years.
Time: divide the interest by principal times rate.

For terms in months, convert to a fraction of a year first: 18 months is 1.5 years, and 9 months is 0.75 years. The math is straight-line, so $5,000 at 6% earns $300 in one year and exactly double that, $600, in two.

Simple vs compound interest: the real gap

With compound interest, each period's interest is added to the balance and then earns interest itself, so the total accelerates. Simple interest skips that step. Over a short term the difference is minor. Take $5,000 at 6% for 4 years: simple interest pays $1,200, while interest compounded once a year pays about $1,312 — a gap of roughly $112.

Stretch the same idea over decades and the gap becomes enormous. $10,000 at 7% for 30 years earns $21,000 in simple interest, for a total of $31,000. Compounded annually, that same deposit earns about $66,123 — more than three times as much. This is exactly why it matters which method applies: when you're earning, compounding is your friend; when you're borrowing, simple interest is the cheaper deal because the debt never snowballs.

Where simple interest is actually used

Simple interest shows up more often on the borrowing side than the saving side:

Most car loans. Auto lenders typically charge daily simple interest on the outstanding balance, which is why paying early in the month, or paying extra, reduces what you owe right away.
Short-term and personal loans. Many installment and payday-style loans quote a flat simple-interest charge rather than compounding.
Bonds. Most bonds pay simple interest as periodic coupons; the interest is paid out, not added back to the principal.

Savings accounts, credit cards, and most long-term loans use compounding instead, so confirm which method a quote uses before you rely on a figure. One caution: the result here is not the same as APR. APR can fold in fees and may assume compounding, so a simple-interest total and an advertised APR can describe the same loan with very different numbers.

Frequently asked questions

Can I solve for the rate or the principal instead of the interest?

This calculator solves for interest and final value. To find the rate, divide the interest you want by principal times years; to find the principal, divide the interest by the rate times years.

How do I handle a loan term measured in months?

Convert months to a fraction of a year before entering it. Eighteen months is 1.5 years, and nine months is 0.75 years.

Is simple interest the same as APR?

Not always. APR can fold in fees and may assume compounding. Simple interest here is just principal times rate times time, so compare carefully before quoting an APR.

Does a higher rate or a longer term add more interest?

With simple interest both have a straight-line effect, so doubling the rate and doubling the years each double the interest. Neither one accelerates the way compounding does.

Last reviewed June 2026. This tool is for education, not financial advice.