Simple Interest Calculator
Calculate simple interest on a principal sum using the linear I = P × r × t formula. Enter principal, annual rate, and time period in years to get total interest earned and the resulting end balance — no compounding.
Calculator Inputs
Sets the currency this calculator's numbers are denominated in. This is presentation only — it does NOT convert your numbers between currencies, it only changes how they're formatted.
The original sum the interest is calculated on.
The annual simple interest rate (e.g. 5 for 5%). Applied once per year, not compounded.
How long the interest accrues for — years and, optionally, months.
Additional months (0-11). For 18 months, enter 1 year 6 months.
- Total Simple Interest
- $2,500.00
- End Balance
- $12,500.00
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
Financial Notice: These figures are informational estimates for personal planning only. They do not constitute financial, investment, tax, or legal advice, nor do they represent a lending offer or credit approval. Provider and financial institution figures, rates, fees, and underwriting requirements may differ.
How It's Calculated
Formula
Simple interest is calculated only on the original principal — never on interest the account has already earned. That's the key distinction from compound interest: with compound interest, each period's interest is added to the balance and then itself starts earning interest, so growth accelerates over time. With simple interest, nothing compounds — the interest earned in year one, year five, and year twenty are all the same dollar amount for a fixed principal and rate. This calculator applies the formula directly: enter a principal sum, an annual interest rate, and a time period in years (fractional years are supported, e.g. 0.5 for six months), and it returns the total interest earned and the resulting end balance — principal plus interest.
Worked Examples
Standard case: $10,000 principal, 5% annual rate, 3 years
- Convert the rate to a decimal fraction: r = 5 / 100 = 0.05
- Apply the formula: I = P × r × t = 10,000 × 0.05 × 3 = $1,500.00
- End balance = P + I = 10,000 + 1,500.00 = $11,500.00
Fractional time period: $5,000 principal, 4.5% annual rate, 2.5 years
- Convert the rate to a decimal fraction: r = 4.5 / 100 = 0.045
- Apply the formula: I = P × r × t = 5,000 × 0.045 × 2.5 = $562.50
- End balance = P + I = 5,000 + 562.50 = $5,562.50
Frequently Asked Questions
How is simple interest different from compound interest?
Simple interest is calculated only on the original principal for the entire time period. Compound interest recalculates the balance at the end of every compounding period and starts earning interest on that new, larger balance — including interest already earned. Over the same rate and time period, compound interest always produces a total equal to or greater than simple interest, and strictly greater once the rate is above 0%.
Can I enter months instead of years?
The Time Period field expects years, but it accepts fractional values, so months convert directly: divide the number of months by 12. Six months is 0.5, three months is 0.25, eighteen months is 1.5, and so on.
Does this apply to loans, or only to savings?
Both. The same I = P × r × t formula applies whether you're earning interest on a deposit or owing interest on a loan — some short-term consumer loans, add-on interest loans, and certain bonds use simple interest. This calculator computes the interest amount either way; whether it's money you earn or money you owe depends on which side of the transaction you're on.
Why is my bank probably NOT using simple interest?
Most savings accounts, CDs, and credit cards compound interest — commonly daily or monthly — rather than applying it once, simply, over the whole term. Compounding grows a balance (or a debt) faster than simple interest at the same nominal rate. If your account statement quotes an APY that's higher than the stated interest rate, that gap is the compounding effect; simple interest has no such gap.
What happens if I enter a time period of 0?
The calculator returns zero interest and an end balance equal to your principal. That's a valid boundary, not an error — no time has elapsed, so no interest has accrued yet.