How to Use a Compound Interest Calculator — With Real Examples

Albert Einstein allegedly called compound interest the "eighth wonder of the world". Whether he said it or not, the sentiment is accurate — compound interest is the most powerful force in personal finance, and understanding it is the foundation of any investment strategy.

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What is Compound Interest?

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. In contrast, simple interest is only calculated on the principal.

Simple interest:
₹1,00,000 at 10% for 10 years
= ₹1,00,000 + (₹1,00,000 × 10% × 10)
= ₹2,00,000

Compound interest (annually):
₹1,00,000 at 10% for 10 years
= ₹1,00,000 × (1.10)^10
= ₹2,59,374

Difference: ₹59,374 extra — just from compounding!

The Compound Interest Formula

A = P × (1 + r/n)^(n×t)

Where:
A = final amount
P = principal (initial investment)
r = annual interest rate (as decimal, e.g. 0.10 for 10%)
n = compounding frequency per year
t = time in years

Compounding Frequency Matters

₹1,00,000 at 10% for 10 years:

Annually  (n=1):  ₹2,59,374
Quarterly (n=4):  ₹2,68,506
Monthly   (n=12): ₹2,70,704
Daily     (n=365):₹2,71,791

More frequent compounding = more money. SIPs (Systematic Investment Plans) in mutual funds compound monthly, which is why they outperform lump-sum bank FDs over the long term.

SIP vs Lump Sum — Real Example

Suppose you invest ₹10,000 per month for 20 years at 12% annual return:

Monthly SIP: ₹10,000
Duration:    20 years (240 months)
Return:      12% p.a.

Total invested: ₹24,00,000
Estimated corpus: ₹99,91,478 (≈ ₹1 crore!)

vs Lump sum: ₹10,00,000 at 12% for 20 years
= ₹96,46,293

📈

Start early. ₹5,000/month at 25 years old grows to more than ₹1,00,000/month at 25 years old started at 35 — despite the same total investment amount. Time is the biggest variable.

Using the EazyTools Investment Planner

Our Investment Planner lets you model any scenario — lump sum, monthly SIP, or a combination:

Enter your initial lump sum (can be ₹0 for pure SIP)
Set your monthly contribution amount
Enter the expected annual return rate (8–12% is typical for equity mutual funds)
Set the duration in years
The chart shows year-by-year growth instantly

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