Simple interest calculator
Compute simple interest with I = P × r × t over a term in years, months or days (365- or 360-day basis), and see the daily and monthly interest plus how much annual compounding would add over the same period.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Inputs
Interest uses the simple-interest formula I = P × r × t, rounded to the cent; the term is converted to years using the chosen day-count basis (365 or 360).
Result
500.0010,500.001.0000 Years365 Days1.3741.67Compared with annual compounding
Whole years compound at (1 + r)^n and any part-year is pro-rated simply, so a term under one year gives the same figure either way.
10,500.00500.000.00What this tool does
- Work out the interest on a deposit or a short-term loan: 10,000 at 5% for one year gives 500 of simple interest and a total of 10,500.
- Price a short-term cash need by the day: 10,000 at 5% for 90 days is 123.29 on a 365-day basis and 125.00 on the 360-day basis banks often use.
- See what compounding would add: the same 10,000 at 5% earns 1,500 of simple interest over three years, but 1,576.25 compounded annually — 76.25 more.
- Get the daily and monthly figures fast — 1.37 a day and 41.67 a month — to reconcile products that accrue interest daily.
Example
Input
Principal 10,000, annual rate 5%, term 3 years, 365-day basis
Output
Simple interest 1,500.00; total 11,500.00; term 3 years; 1095 days; 1.37 per day; 41.67 per month; compounded total 11,576.25; compounded interest 1,576.25; compounded − simple 76.25
Simple interest is 10,000 × 5% × 3 = 1,500, while compounding gives 10,000 × 1.05³ = 11,576.25 — the extra 76.25 is interest on interest.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest always charges against the original principal (I = P × r × t) and the interest never earns interest; compounding folds each period’s interest back into the principal. The gap grows with time: 76.25 over three years on 10,000 at 5%, but more than 16,000 over thirty. Below one year both rows here match, because no compounding period has completed.
Why offer a 360-day basis?
Banking has long used a year of 360 days and months of 30, which makes the daily rate annual rate ÷ 360 and the daily interest slightly higher. On 10,000 at 5% for 90 days that is 123.29 on a 365-day basis versus 125.00 on 360. Which one applies is set by the product terms, so the tool shows both.
How are the "term in days" and "term in years" figures derived?
Years come from the unit: years stay as they are, months divide by 12, and days divide by the basis (365 or 360). Days then multiply the years back by the basis and round, which is why six months reads as 183 days rather than 182.5. Interest is computed from the years; the day count is there to help you check.
Is the interest taxed?
The tool is pure arithmetic and ignores interest tax. In mainland China, personal income tax on savings deposit interest is currently waived, while funds, bonds and other products follow their own rules — check with a professional or the tax authority for your situation.
Can it handle instalment or annuity loans?
No. This is a single-payment simple-interest model, suited to deposits, short-term borrowing and bill discounting. For instalment loans use the loan amortization tool or the mortgage calculator, which charge interest on the opening balance period by period.
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