Simple interest is charged only on the original principal. Compound interest is charged on the principal plus all interest accumulated so far. That one structural difference is why two products advertising the same percentage can produce very different amounts of money.
This article puts the two side by side, quantifies the gap across time and rate, and shows which financial products in practice use which method.
Work through your own numbers with the Simple Interest Calculator and the Compound Interest Calculator.
The Two Formulas
Simple interest
Compound interest
CI = A − P
In both, P is the principal, r the annual rate as a decimal and t the time in years. Only the compound formula carries n, the compounding periods per year.
Simple interest grows linearly — the same amount is added every year. Compound interest grows exponentially, because each year's base is larger than the last.
The Gap, Year by Year
₹1,00,000 at 8 percent, compounded annually.
| Year | Simple interest total | Compound interest total | Difference |
|---|---|---|---|
| 1 | ₹1,08,000 | ₹1,08,000 | ₹0 |
| 3 | ₹1,24,000 | ₹1,25,971 | ₹1,971 |
| 5 | ₹1,40,000 | ₹1,46,933 | ₹6,933 |
| 10 | ₹1,80,000 | ₹2,15,892 | ₹35,892 |
| 20 | ₹2,60,000 | ₹4,66,096 | ₹2,06,096 |
| 30 | ₹3,40,000 | ₹10,06,266 | ₹6,66,266 |
Three observations follow directly from the table:
1. In year one the two are identical. There is no accumulated interest yet to compound.
2. The gap is modest for the first few years — under 5 percent of principal at year five.
3. After roughly a decade the compound curve pulls away sharply, and by year 30 it is nearly three times the simple result.
This is why the choice of method barely matters on a six-month deposit and matters enormously on a retirement horizon.
The Gap Across Rates
Difference between compound and simple interest on ₹1,00,000 after 10 years:
| Annual rate | Simple total | Compound total | Extra from compounding |
|---|---|---|---|
| 4 % | ₹1,40,000 | ₹1,48,024 | ₹8,024 |
| 6 % | ₹1,60,000 | ₹1,79,085 | ₹19,085 |
| 8 % | ₹1,80,000 | ₹2,15,892 | ₹35,892 |
| 10 % | ₹2,00,000 | ₹2,59,374 | ₹59,374 |
| 12 % | ₹2,20,000 | ₹3,10,585 | ₹90,585 |
The advantage of compounding scales with the rate as well as the time. At 12 percent the extra is more than eleven times what it is at 4 percent.
Worked Comparison: A Five-Year Deposit
₹5,00,000 for 5 years at 7 percent.
Same rate, same principal, same term. The difference between the best and worst outcome is ₹32,389, entirely down to the interest structure.
Which Products Use Which
Typically simple interest
- Short-term personal and consumer loans quoted as a "flat rate"
- Some car and two-wheeler loans
- Certain fixed deposits that pay interest out periodically instead of reinvesting it
- Government treasury instruments that pay a fixed coupon
- Interest on delayed payments and many statutory calculations
Typically compound interest
- Savings accounts and cumulative fixed deposits
- Recurring deposits and provident funds
- Mutual funds and equity returns, where gains are reinvested
- Home loans, education loans and most reducing-balance credit
- Credit-card revolving balances, usually compounded monthly or daily
The category matters more than the label: if interest is left in the account, it compounds; if it is paid out, it does not.
The Flat-Rate Trap
Lenders sometimes advertise a flat rate because it looks lower than the equivalent reducing-balance rate.
The problem: you repay principal every month, so your average outstanding balance over the term is roughly half the original. Yet interest is charged on the full ₹5,00,000 for all three years. The same instalment under a reducing-balance calculation corresponds to an annual rate of about 12.9 percent — nearly double the advertised figure.
Whenever a flat rate is quoted, convert it before comparing. As a rough guide, the reducing-balance equivalent is close to 1.8 to 1.9 times the flat rate for a three-year term. Check the actual instalment against the EMI Calculator rather than relying on the multiplier.
When Simple Interest Is Better
As a borrower, simple interest on the outstanding balance is preferable — no interest accrues on unpaid interest. As a saver, compound interest is preferable, and more frequent compounding is marginally better still.
The asymmetry is worth stating plainly: compounding works in favour of whoever holds the growing balance. On a savings product that is you. On a credit-card balance it is the issuer.
Assumptions and Limitations
- Constant rate throughout the term in both methods.
- No withdrawals, additional deposits, missed payments or penalties.
- Taxes and fees excluded; both reduce the effective return, and tax on interest compounds away part of the advantage shown above.
- Compound examples assume interest is retained in the account rather than paid out.
These are arithmetic comparisons for education, not product recommendations or financial advice.
Common Mistakes
- Comparing a flat rate with a reducing-balance rate at face value.
- Assuming a fixed deposit compounds when it pays interest out monthly — that is effectively simple interest unless you reinvest.
- Ignoring compounding frequency when two products quote the same nominal rate.
- Applying the simple formula to a multi-year investment where returns are reinvested.
- Forgetting that credit-card interest compounds, often monthly, on any carried balance.
Related Calculations
- Simple Interest Calculator — flat interest on a fixed principal
- Compound Interest Calculator — growth with any compounding frequency
- EMI Calculator — reducing-balance instalments
- FD Calculator — cumulative deposit maturity value
- CAGR Calculator — the compound rate behind a past return
Frequently Asked Questions
What is the main difference between simple and compound interest?
Simple interest applies only to the original principal. Compound interest applies to the principal plus all interest already earned, so the base grows every period.
Is simple interest ever higher than compound interest?
Only within the first compounding period, where they are equal. After that, compound interest is always higher for the same nominal rate.
How do I convert a flat rate to a reducing-balance rate?
There is no exact closed-form conversion. Calculate the flat-rate instalment, then solve for the rate that produces the same instalment under the EMI formula — a spreadsheet RATE function or the EMI Calculator does this directly.
Do fixed deposits use compound interest?
Cumulative deposits do, usually compounded quarterly. Non-cumulative deposits pay interest out periodically, which behaves like simple interest unless you reinvest the payouts elsewhere.
Which matters more, the rate or the compounding frequency?
The rate, by a wide margin, followed by time. Frequency is the smallest of the three effects beyond monthly compounding.
Summary
Simple interest adds a fixed amount each year; compound interest adds a growing one. The gap is negligible in year one, meaningful by year five, and decisive over decades. Check any quoted rate for its structure and compounding frequency before comparing — then verify the arithmetic with the Compound Interest Calculator.