Interest Calculator
Simple and compound interest
Simple vs Compound Interest
Simple interest is calculated only on the original principal amount, while compound interest is calculated on the principal plus any accumulated interest from previous periods. Over time, compound interest grows significantly faster than simple interest because you earn interest on your interest.
Simple Interest Formula
Simple Interest = Principal x Rate x Time. For example, $10,000 at 5% for 5 years produces $2,500 in simple interest, for a total of $12,500. The interest earned is the same amount each year ($500).
Compound Interest Formula
Compound Interest Total = Principal x (1 + Rate/n)^(n x Time), where n is the compounding frequency per year. The same $10,000 at 5% compounded monthly for 5 years produces approximately $2,834 in interest, for a total of $12,834. That is $334 more than simple interest, and the difference grows dramatically over longer periods.
Compounding Frequency Matters
More frequent compounding results in slightly more interest earned. Daily compounding produces more than monthly, which produces more than quarterly. However, the differences between monthly and daily compounding are relatively small. The biggest jump in returns comes from the difference between annual and monthly compounding.
Interest Calculator: practical guide
The Interest Calculator is built for people who want a fast answer without losing context. It keeps the calculation simple, shows the result clearly, and helps you understand what the number means before you use it in a real decision.
Investment and interest calculators make long-term numbers easier to compare. Small changes in time, contribution amount, rate, or compounding frequency can create large differences over many years.
What is the best way to use the Interest Calculator?
Enter the values carefully, review the units, and use the result as a reliable reference point. The Interest Calculator is most useful when you compare scenarios or repeat the calculation with consistent inputs.
Is the Interest Calculator accurate?
The calculator follows standard calculation logic, but accuracy depends on the values you enter and the assumptions behind the formula. For important finance decisions, use it as guidance and verify the result with a trusted source.
Simple interest vs compound interest — which applies to your situation?
Before calculating interest, the most important question is: which type applies? Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously accumulated interest. For most savings accounts, fixed deposits, mutual funds, and loans (which use reducing balance interest), compound interest applies. Simple interest applies to some short-term loans, treasury bills, and specific microfinance products.
Simple interest formula and examples
Simple Interest = P × R × T
where P = Principal, R = Annual rate as decimal, T = Time in years.
Total Amount = P + SI = P(1 + RT)
- $75,000 at 9% for 4 years: SI = 75,000 × 0.09 × 4 = $27,000. Total = $102,000
- $300,000 at 6.5% for 2.5 years: SI = 3,00,000 × 0.065 × 2.5 = $48,750. Total = $348,750
Compound interest formula and examples
A = P × (1 + R/n)^(n×T)
where n = number of compounding periods per year.
- $75,000 at 9%, annually compounded, 4 years: A = 75,000 × (1.09)^4 = 75,000 × 1.4116 = $105,869. CI = $30,869
- $75,000 at 9%, monthly compounded, 4 years: A = 75,000 × (1.0075)^48 = 75,000 × 1.4314 = $107,356. CI = $32,356
Monthly compounding generates $1,487 more than annual compounding on the same $75,000 investment at 9% over 4 years — the benefit of more frequent compounding.
Comparing fixed deposit options
When comparing FDs with different rates and compounding frequencies, convert to Effective Annual Rate (EAR):
EAR = (1 + Nominal Rate ÷ n)^n – 1
- FD at 7.5% annually: EAR = 7.5%
- FD at 7.3% quarterly: EAR = (1 + 0.073/4)^4 – 1 = 7.52%
- FD at 7.1% monthly: EAR = (1 + 0.071/12)^12 – 1 = 7.34%
The 7.3% quarterly FD actually yields more than the 7.5% annually compounded FD. EAR enables true apples-to-apples comparison.
Interest on loans — reducing balance method
Most loans (home, personal, car) use the reducing balance method — interest is calculated on the outstanding principal, not the original loan amount. As EMIs reduce the principal, the interest component of each subsequent EMI also decreases, even though the total EMI stays constant (amortisation).
Month 1 interest on $2,000,000 loan at 9%: 20,00,000 × (9/12/100) = $15,000. After paying EMI, outstanding principal reduces. Month 2 interest is calculated on the lower balance — slightly less than $15,000. Over 240 months, this reduction adds up to the total interest paid.
Frequently asked questions
What is the best interest rate on savings right now? Rates change frequently. Small finance banks and new-age banks often offer higher savings account rates (6–7.5%) than large PSU banks (2.7–3.5%). Senior citizens typically receive 0.25–0.5% higher FD rates. Compare on the respective banks' websites or an aggregator like BankBazaar for current rates.
How is interest on a savings account calculated? Most global savings accounts calculate interest daily on the end-of-day balance and credit it quarterly. If your balance is $50,000 and the rate is 4% p.a., daily interest = $50,000 × (4/365/100) = $5.48 per day.
Does prepaying a loan save interest? Yes — significantly. On a reducing balance loan, every rupee of principal repaid immediately reduces all future interest calculations. A lumpsum prepayment of $200,000 on a $3,000,000 home loan in year 5 can save $6–8 hundred thousand in total interest over the remaining tenure.