Simple interest calculator for any unknown
Simple interest is the most basic way of charging or earning interest: a fixed percentage of the original amount for each year, with no interest on interest. This simple interest calculator works it out instantly with the classic school formula SI = P × R × T ÷ 100 and also gives the total amount (principal plus interest).
It can also run the formula backwards. Choose Principal, Rate or Time at the top, enter what you know, including the interest or the final amount, and the missing figure appears with the working shown.
It also works from date to date, accepts the Indian byaj rate as “rupees per 100 per month” (sekda), and can compound the interest. It suits homework, loans between friends or family and checking a statement. Results are estimates for learning and planning, not financial advice.
How to use the Simple Interest Calculator
- Pick what you want to find: Interest, Principal, Rate or Time.
- Choose your currency and enter the known values — principal, rate per year and time.
- Set the time unit to years, months or days, or choose Two dates and pick the start, end and day-counting method. Switch the rate unit to per 100 / month (sekda) if you were quoted a monthly rate.
- When solving for principal, rate or time, choose whether your known value is the Interest earned or the Total amount, and enter it.
- Read the answer, the worked formula, the principal-versus-interest chart and the year-by-year table, and download it as CSV if you need it.
Simple Interest Calculator features
Solves for any variable
Interest and amount, or the principal, rate or time that produces a given interest or maturity amount.
Days, months or years
Time is converted to years automatically; days use the actual/365 or banker’s 360-day basis.
Date to date and byaj
Exact days between two dates, with actual/365, actual/360 or 30-day-month counting, and sekda rates.
Simple or compound
Compound yearly, half-yearly, quarterly or monthly for the interest-from-principal mode.
Formula with your numbers
Every answer shows the exact rearranged formula used, which is perfect for checking homework.
Year-by-year table
Interest for each year, running total and amount, with partial final years shown in months.
Simple interest formula and rearrangements
The core formula is SI = P × R × T ÷ 100, where P is the principal, R the rate in percent per year and T the time in years. The total amount is A = P + SI = P × (1 + R × T ÷ 100). Rearranging gives the other unknowns:
- Principal: P = SI × 100 ÷ (R × T), or P = A ÷ (1 + R × T ÷ 100) when you know the amount.
- Rate: R = SI × 100 ÷ (P × T).
- Time: T = SI × 100 ÷ (P × R).
- Months and days: convert first — T = months ÷ 12, or T = days ÷ 365 (or ÷ 360).
Interest from date to date and the byaj (sekda) rate
Choose Two dates and the calculator counts the days (also shown as years, months and days). Actual ÷ 365 uses the real calendar. The traditional Indian byaj convention treats every month as 30 days and the year as 360, so 15 January to 15 July is exactly 180 days: ₹10,000 at 10% earns ₹500 that way, or ₹495.89 on actual days.
Informal lenders quote rupees per ₹100 per month. Multiply by 12 for the yearly rate: ₹2 per ₹100 per month is 24% a year, so ₹10,000 earns ₹200 a month; with monthly compounding a year gives ₹2,682.42. This is a custom, not an official rate: agree the method with the other party, and check local law on lending rates.
Worked examples
- ₹10,000 at 8% a year for 3 years: SI = 10,000 × 8 × 3 ÷ 100 = ₹2,400, so the amount is ₹12,400.
- What rate turns $5,000 into $600 of interest in 2 years? R = 600 × 100 ÷ (5,000 × 2) = 6% a year.
- How long does £20,000 at 5% take to earn £3,000? T = 3,000 × 100 ÷ (20,000 × 5) = 3 years.
- What principal grows to 12,000 in 2 years at 10%? P = 12,000 ÷ 1.2 = 10,000.
- 10,000 at 10% for 90 days: 246.58 on a 365-day basis, 250.00 on a 360-day basis.
Simple vs. compound interest
Over one year the two are identical with yearly compounding. After that, compound interest pulls ahead because interest earns interest. For 10,000 at 8% a year:
- Use the compound interest calculator for savings and investments that reinvest interest.
- Instalment loans use the reducing-balance method — see the EMI calculator. A “flat rate” quote costs much more than the same reducing rate.
| Years | Simple interest | Compound (yearly) | Difference |
|---|---|---|---|
| 1 | 800.00 | 800.00 | 0.00 |
| 5 | 4,000.00 | 4,693.28 | 693.28 |
| 10 | 8,000.00 | 11,589.25 | 3,589.25 |
| 20 | 16,000.00 | 36,609.57 | 20,609.57 |
Simple Interest Calculator FAQ
What is the simple interest formula?
SI = P × R × T ÷ 100, where P is the principal, R is the annual rate in percent and T is the time in years. The simple interest calculator also adds it to the principal to give the total amount.
How do I calculate simple interest for months or days?
Divide months by 12, or days by 365 (or 360). Choose the unit in the Time box and the calculator converts it.
What is sekda or “rupees per hundred per month” interest?
It is a monthly rate: ₹1 sekda means ₹1 of interest per ₹100 each month, or 12% a year. Choose “per 100 / month (sekda)” beside the rate and the tool converts it.
How do I calculate byaj between two dates?
Pick “Two dates”, enter the principal, rate and both dates, and choose actual days or 30-day months. Interest = principal × yearly rate × days ÷ 365 (or ÷ 360).
What is the difference between the 365 and 360-day basis?
Actual/365 divides the days by 365. The 360-day basis, used in some commercial lending, divides by 360 and gives slightly more interest.
How do I find the rate if I know the interest?
Use R = SI × 100 ÷ (P × T). Select Rate at the top, enter the principal, time and the interest (or the total amount) and the calculator does the rest.
Where is simple interest used in real life?
Short-term personal loans, some daily-accrual loans, short deposits, bond coupons and school exercises. Most savings products compound instead.
Is simple interest better than compound interest?
For a borrower, yes. For a saver, compound interest earns more over periods longer than a year.