Interest Calculator

Romik Amreliya

Calculate simple and compound interest on your savings or investments.

Principal Amount

Annual Interest Rate (%)

Time (Years)

Compounding Frequency

What is Interest?

Interest is the cost of borrowing money or the return on invested money. When you deposit money in a savings account, the bank pays you interest. When you take a loan, you pay interest to the lender.

There are two main types of interest: simple interest and compound interest. Understanding the difference is crucial for making informed financial decisions.

Simple vs Compound Interest Formulas

Simple Interest

SI = P × r × t

SI = Simple Interest · P = Principal · r = Annual rate (decimal) · t = Time in years

Simple interest is calculated only on the original principal amount. It does not compound — meaning interest earned does not itself earn interest.

Compound Interest

A = P × (1 + r/n)^(n×t)

A = Final amount · P = Principal · r = Annual rate · n = Compounding frequency · t = Time in years

Compound interest is calculated on the principal plus any previously accumulated interest. This "interest on interest" effect causes wealth to grow exponentially over time — often called the eighth wonder of the world.

Real-World Example

Example: ₹1,00,000 at 8% for 5 years

Simple Interest: ₹1,00,000 × 0.08 × 5 = ₹40,000 → Total: ₹1,40,000

Compound Interest (monthly): ₹1,00,000 × (1 + 0.08/12)^(60) = ₹48,985 → Total: ₹1,48,985

Compound interest earns ₹8,985 more than simple interest in the same period!

Step-by-Step: Calculating Compound Interest

Step-by-Step: ₹2,00,000 at 10% compounded quarterly for 3 years

Step 1: Identify variables — P = ₹2,00,000, r = 10% = 0.10, n = 4 (quarterly), t = 3 years

Step 2: Calculate periodic rate — r/n = 0.10 / 4 = 0.025

Step 3: Calculate total periods — n × t = 4 × 3 = 12

Step 4: Apply formula — A = 2,00,000 × (1 + 0.025)^12

Step 5: A = 2,00,000 × 1.34489 = ₹2,68,978

Interest Earned: ₹2,68,978 – ₹2,00,000 = ₹68,978

Effective Annual Rate: (1 + 0.025)^4 – 1 = 10.38%

How Compounding Frequency Affects Returns

The more frequently interest is compounded, the more you earn. Here's ₹1,00,000 at 12% for 5 years:

Annually (1×): ₹1,76,234 — Interest earned: ₹76,234
Semi-Annually (2×): ₹1,79,085 — Interest earned: ₹79,085
Quarterly (4×): ₹1,80,611 — Interest earned: ₹80,611
Monthly (12×): ₹1,81,670 — Interest earned: ₹81,670
Daily (365×): ₹1,82,194 — Interest earned: ₹82,194

Monthly compounding earns ₹5,436 more than annual compounding on the same principal!

Real-Life Use Cases

Savings accounts: Banks typically compound interest daily or monthly. Use this calculator to see your actual returns versus the stated annual rate.
Fixed deposits: Compare FD offers from different banks by entering their rates and compounding frequencies to find the best effective return.
Loan cost analysis: Understand the true cost of borrowing. A loan at 12% compounded monthly actually costs 12.68% per year (effective rate).
Investment planning: Project how your savings will grow over time with different interest rates and compounding frequencies.

Interpretation of Results

When reviewing your results, pay close attention to the Compounding Frequency. Even with the same interest rate, interest that compounds monthly will grow faster than interest that compounds annually. If you are an investor, look for high compounding frequencies. If you are a borrower, simple interest or low compounding frequencies are generally more favorable. Use the Year-by-Year Growth Table below to visualize how the "interest on interest" effect accelerates your balance over time.

Frequently Asked Questions

Compounding frequency is how often interest is calculated and added to your balance. Common frequencies: annually (1×), semi-annually (2×), quarterly (4×), monthly (12×), and daily (365×). More frequent compounding results in higher returns.

For savings/investments, compound interest is better as you earn interest on your interest. For loans, simple interest is better (for borrowers) as the total cost is lower.

The effective annual rate accounts for compounding and shows the true annual return. A 12% rate compounded monthly has an EAR of about 12.68%. EAR = (1 + r/n)^n – 1.

The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by the annual interest rate. Example: at 8% interest, your money doubles in approximately 72 ÷ 8 = 9 years.

RA

Written & Reviewed by Romik Amreliya

Software Engineer & Data Analyst. Dedicated to building precise, privacy-first web calculators based on standardized financial and medical algorithms. All tools and content undergo rigorous testing against industry-standard benchmarks.

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Sources & References:

Investopedia - Financial education, formulas, and terminology definitions.
Standard banking amortization formulas for compound interest and loan schedules.
Consumer Financial Protection Bureau (CFPB) - Guidelines on credit cards, mortgages, and personal loans.
Calculations are based on universally accepted financial mathematics; actual rates may vary by institution.

Financial Disclaimer: This interest calculator is for educational purposes only. Actual interest rates, compounding methods, and returns may vary by institution. Tax implications on interest income are not included. Consult your bank or financial advisor for precise calculations.