Key takeaways
- Compounding needs time; early years look disappointing compared with later years in a smooth illustration.
- A SIP compounds each contribution for a different number of months.
- The rate field in a calculator is an assumption, not a market forecast.
- Actual yearly returns jump around; a smooth 12% line is a teaching device.
Compounding is not a product. It is a pattern: this period’s gain (or interest) becomes part of next period’s base. A fixed deposit states a rate and a compounding frequency. A market SIP does not. Using the same word for both is convenient and slightly dishonest unless you keep the difference in view.
Finaura’s older article why compounding rewards patience is a narrative about waiting. This page is the mechanics: what a calculator is doing when it compounds, and where that picture lies.
Lumpsum compounding is the school formula
A single amount left for t years at rate r compounds as A = P × (1 + r)ᵗ if r is an effective annual rate (the exact formula depends on compounding frequency). The lumpsum calculator is this idea. Double the time at the same rate and you do not double the result — you get a larger step, which is the whole visual appeal of compounding charts.
SIP compounding is a stack of small lumpsums
A monthly SIP is 12 tiny lumpsums a year, each with a different remaining life. The January contribution compounds for more months than December’s. That is why “₹10,000 a month for 20 years at 12%” is not the same as “₹24 lakh for 20 years at 12%.” The SIP calculator adds those staggered contributions. Duration still matters: the last five years of a 20-year SIP illustration usually show more rupee growth than the first five, because the base is larger.
Deposits compound on their own rules
Bank FDs and RDs use contracted rates and stated compounding (quarterly is common for FDs). PPF uses a notified rate and an annual compounding convention described on the PPF page. Do not paste a 12% SIP assumption into an FD conversation. Use the FD, RD and PPF tools when the product is a deposit or a notified scheme.
The rate assumption is the plot twist
People argue about SIPs versus lumpsums and ignore that both charts are slaves to the rate box. Lower the assumed return by two percentage points on a 20-year SIP and the illustrated corpus can drop by a third or more, depending on inputs. That is not because compounding “broke.” It is because compounding magnifies the rate you fed it — including an optimistic one.
Try it yourselfSee compounding on a monthly contributionSeparate invested amount from illustrated gains, then change the rate and the years.Markets do not draw the smooth line
A calculator that compounds at 12% every year is showing a geometric fantasy. Actual years may be +30% and −20%. Sequence matters if you are withdrawing (retirement, SWP). Sequence matters less if you are only contributing and can wait, but it still means the path will not look like the chart. Projected returns are not guaranteed.
Inflation compounds too
The same mathematics that grows a corpus grows the cost of living. A retirement plan that compounds investments at 10% and forgets to inflate expenses is telling only half the story. See how inflation changes your retirement corpus.
How to use this without fooling yourself
- Run two rates, not one.
- Read the invested-amount column.
- Match the tool to the product (SIP vs FD vs PPF).
- Keep money you need soon out of a long-horizon equity illustration.