FinanceCalcs
Simple Interest Calculator
Calculate simple interest on any principal, rate, and time period — and compare the result directly against compound interest to see how much the compounding difference is worth over time.
Works for loans, savings, and any fixed-rate interest calculation — with a year-by-year comparison table showing simple vs. compound growth side by side.
Simple Interest Calculator
Principal: $0 | Rate: 0% | Time: 0
Simple Interest: $0
- Simple Interest Total
- Compound Total
"Simple interest grows in a straight line. Compound interest grows like a snowball — slowly at first, then faster and faster."
— Unknown
Simple vs. compound interest
Simple interest is calculated only on the original principal — never on accumulated interest. It grows in a straight line: the same dollar amount of interest is added every period regardless of how long the money has been invested. Formula: SI = P × R × T, where P is principal, R is the annual rate as a decimal, and T is time in years.
Compound interest is calculated on the principal plus all previously accumulated interest. Each period, interest earns its own interest — creating exponential growth that accelerates over time. This is why compound interest dramatically outpaces simple interest over long periods, even at the same stated rate.
Simple interest is still commonly used in short-term personal loans, some auto loans, U.S. savings bonds, and certain financial calculations where simplicity is preferred. Most mortgages, credit cards, savings accounts, and investment accounts use compound interest — which is why understanding both, and the difference between them, matters.
lightbulb Simple vs. Compound Interest Example
Suppose you invest $10,000 at a 6% annual rate for 20 years.
With simple interest: $10,000 × 0.06 × 20 = $12,000 in interest. Total value: $22,000. The same $600 in interest is added each year, every year.
With compound interest (annual compounding): the balance grows to approximately $32,071 — producing $22,071 in interest, nearly double the simple interest result. The gap widens with every passing year as accumulated interest generates its own returns.
Over 30 years, the gap becomes even more dramatic: $28,000 (simple) vs. $57,435 (compound) — a difference of over $29,000 on the same principal at the same rate.
Simple Interest Calculator FAQs
What is the simple interest formula?
Simple Interest = Principal × Rate × Time, or SI = P × R × T, where R is the annual interest rate expressed as a decimal (e.g., 6% = 0.06) and T is time in years. The total amount at the end of the period is Principal + SI. For partial years, convert months to a decimal fraction of a year (e.g., 9 months = 0.75 years).
When is simple interest used in real life?
Simple interest is used in some short-term personal loans, certain auto loans, U.S. Treasury bills, and informal lending arrangements. Some installment loans also use a simple interest structure where your payment reduces the principal directly and interest is recalculated on the remaining balance each period. Most long-term financial products — mortgages, credit cards, savings accounts — use compound interest instead.
Why is compound interest so much better for investing?
Because compound interest earns returns on returns. In the early years the difference is modest, but over decades the gap becomes enormous. A $10,000 investment at 7% for 40 years grows to $28,000 with simple interest — and to $149,745 with annual compounding. The extra $121,745 comes entirely from interest earned on previously accumulated interest, not from additional contributions.
How do I convert a simple interest rate to an equivalent compound rate?
They are not directly convertible to a single equivalent number because the gap between them grows over time — there is no fixed compound rate that always produces the same result as a given simple rate. The longer the period, the more compound interest exceeds simple interest at the same stated rate. Use the comparison table in this calculator to see the exact dollar difference at each year for your specific inputs.
Simple interest terminology
Principal (P)
The original amount of money borrowed or invested before any interest is applied. Simple interest is always calculated on this original amount — never on accumulated interest.
Rate (R)
The annual interest rate expressed as a percentage. Divide by 100 to convert to a decimal for calculations. A 6% rate becomes 0.06 in the SI = P × R × T formula.
Time (T)
The duration over which interest is calculated, expressed in years. Convert months by dividing by 12 (9 months = 0.75 years); convert days by dividing by 365.
Simple Interest Formula
SI = P × R × T, where R is the annual rate as a decimal and T is time in years. Total amount at end of period = P + SI. Growth is linear — the same dollar amount added every period.
Effective Annual Rate
For simple interest, the effective annual rate equals the stated rate regardless of the time period — there is no compounding effect to account for. For compound interest, the effective annual rate increases with compounding frequency and always exceeds the nominal rate.
Disclaimer: All calculators on this site are provided for informational and educational purposes only. Results are estimates based on the inputs you provide and mathematical formulas — they do not account for taxes, fees, inflation, risk, or other real-world factors that may affect financial outcomes. Past performance does not guarantee future results. Nothing on this site constitutes financial, investment, legal, or tax advice. Always consult a qualified professional before making financial decisions.
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