A small change in the assumed rate of return can have a large effect on the final value of a long-term Systematic Investment Plan, or SIP. This effect becomes more visible when the investment period extends over many years.
The reason is compounding. Under a compound return model, the returns earned in one period can form part of the amount that earns returns in later periods. As the years pass, even a modest difference in the assumed return rate can therefore create a noticeable difference in the projected corpus.
To understand this effect, consider a simple example. The monthly SIP is ₹10,000 and the investment period is 15 years. Over 15 years, the investor pays a total of ₹18 lakh into the SIP. The only variable in this example is the assumed annual rate of return.
The calculation uses monthly contributions and monthly compounding. The return figures are assumptions for the purpose of illustration. They do not represent a promise or guarantee of the return that an actual mutual fund or other market-linked investment will provide.
The Basic Example
A monthly SIP of ₹10,000 means that the investor contributes ₹1.20 lakh each year, before any consideration of investment returns. Over 15 years, the total amount paid by the investor is therefore ₹18 lakh.
The final corpus can be higher than ₹18 lakh if the investment earns positive returns. The amount above the investor’s own contributions represents the effect of investment returns under the stated assumptions.
The table below shows the projected corpus under different annual return assumptions.
| Assumed annual return | Projected corpus after 15 years | Change from previous 1% |
|---|---|---|
| 8% | ₹34.83 lakh | — |
| 9% | ₹38.12 lakh | +₹3.29 lakh |
| 10% | ₹41.79 lakh | +₹3.67 lakh |
| 11% | ₹45.89 lakh | +₹4.10 lakh |
| 12% | ₹50.46 lakh | +₹4.57 lakh |
| 13% | ₹55.57 lakh | +₹5.11 lakh |
| 14% | ₹61.29 lakh | +₹5.72 lakh |
These figures show an important feature of long-term compounding. A one percentage-point rise in the assumed return does not produce the same rupee difference at every return level.
For example, the difference between the 8% and 9% cases is about ₹3.29 lakh. The difference between 11% and 12% is about ₹4.57 lakh. The difference between 13% and 14% is about ₹5.72 lakh.
This happens because the higher return rate applies not only to the original contributions but also to the returns that have already accumulated under the model.
What Happens at 12% Instead of 11%
The 11% assumption gives a projected corpus of ₹45.89 lakh after 15 years. The 12% assumption gives a projected corpus of ₹50.46 lakh.
The difference is therefore about ₹4.57 lakh.
The investor does not contribute an additional ₹4.57 lakh in the 12% case. The monthly SIP remains ₹10,000 in both cases. The total contribution also remains ₹18 lakh in both cases.
The difference comes from the return assumption.
This distinction matters when a person uses an SIP calculator for long-term financial planning. A calculator can show the effect of a particular assumed return, but it cannot establish that the assumed return will occur in the future.
For this reason, a projected corpus should generally be treated as an estimate based on stated assumptions rather than as a certain future amount.
The Effect Becomes Larger as the SIP Rises
The same return difference has a larger rupee effect when the monthly SIP is higher.
The relationship is broadly proportional because the same return assumptions apply to a larger amount of capital. If a ₹10,000 monthly SIP has a difference of about ₹4.57 lakh between the 11% and 12% cases, a ₹50,000 monthly SIP has a difference of about five times that amount under the same assumptions.
The table below shows the effect.
| Monthly SIP | Difference between 11% and 12% after 15 years |
|---|---|
| ₹10,000 | ~₹4.57 lakh |
| ₹25,000 | ~₹11.43 lakh |
| ₹50,000 | ~₹22.86 lakh |
| ₹1,00,000 | ~₹45.72 lakh |
This table does not suggest that a higher SIP will itself produce a particular return. It only shows how the same assumed return difference affects different SIP amounts when all other assumptions remain unchanged.
For example, the ₹1,00,000 monthly SIP case has a projected difference of about ₹45.72 lakh between an 11% and 12% assumption. The contribution pattern is five times the ₹20,000? No. More precisely, it is ten times the ₹10,000 example, so the projected difference is also about ten times the original ₹4.57 lakh difference.
Why One Percentage Point Matters
It can be tempting to view 11% and 12% as almost identical because the numerical gap is only one percentage point. Over a short period, that gap may not appear very large in rupee terms.
A 15-year period is different.
Each monthly contribution has a different amount of time to earn returns. Earlier contributions have more time under the compound-return model, while later contributions have less time. The return assumption affects both the contributions and the accumulated value associated with earlier contributions.
As a result, the gap between two return assumptions can widen over time.
This is one reason long-term projections require care. A small change in an assumption can produce a material change in the final number, even when the amount invested each month stays exactly the same.
The effect should not be read as evidence that a particular return rate is more likely to occur. It is simply a mathematical result of the assumptions used in the calculation.
The Difference Between Contribution and Corpus
The total contribution in the example is ₹18 lakh.
At an assumed return of 8%, the projected corpus is ₹34.83 lakh. At 14%, the projected corpus is ₹61.29 lakh.
The difference between these two projected values is substantial. Yet the investor contributes the same ₹18 lakh in both cases.
| Item | 8% assumption | 14% assumption |
|---|---|---|
| Monthly SIP | ₹10,000 | ₹10,000 |
| Investment period | 15 years | 15 years |
| Total contribution | ₹18 lakh | ₹18 lakh |
| Projected corpus | ₹34.83 lakh | ₹61.29 lakh |
The table shows why return assumptions have such a strong influence on long-term projections.
However, the difference between the two corpus values should not be treated as an expected profit difference. It is a difference between two mathematical scenarios. Actual investment results can differ from both scenarios.
Return Assumptions Are Not Guarantees
A mutual fund or other market-linked investment does not normally provide a fixed future return simply because an investor uses that return in a calculator.
Actual returns can vary from year to year. A portfolio can also have periods of positive returns, negative returns, or returns below the rate used in a financial projection.
Therefore, a 12% calculation should not be read as a statement that the investment will earn 12% each year for 15 years. It means only that the calculator uses 12% as its assumed annual return for the purpose of the illustration.
The same point applies to the 8%, 9%, 10%, 11%, 13%, and 14% cases.
The figures are useful for scenario analysis. They are not a forecast of actual market performance.
Why Multiple Scenarios Can Be More Useful
A single return assumption can create a false sense of precision.
Suppose an investor sees a projected corpus of ₹50.46 lakh based on a 12% return assumption. It may be easy to treat ₹50.46 lakh as the expected result. But the table shows that the projected value changes materially when the return assumption changes.
At 10%, the projected corpus is ₹41.79 lakh.
At 11%, it is ₹45.89 lakh.
At 12%, it is ₹50.46 lakh.
At 13%, it is ₹55.57 lakh.
At 14%, it is ₹61.29 lakh.
This range illustrates the sensitivity of a 15-year SIP to the assumed rate of return. A person who uses such projections for financial planning may therefore find it more useful to examine several scenarios rather than rely on one figure.
This does not identify which scenario will occur. It simply shows the financial effect if each stated assumption were to hold under the calculation method.
The Practical Meaning of the 1% Gap
The most direct example is the move from 11% to 12%.
With a ₹10,000 monthly SIP, the 11% assumption produces a projected corpus of ₹45.89 lakh. The 12% assumption produces ₹50.46 lakh.
The gap is approximately ₹4.57 lakh.
With a ₹25,000 monthly SIP, the same gap becomes approximately ₹11.43 lakh.
With a ₹50,000 monthly SIP, it becomes approximately ₹22.86 lakh.
With a ₹1,00,000 monthly SIP, it becomes approximately ₹45.72 lakh.
This demonstrates the combined effect of two factors: time and the amount invested.
The longer the investment period, the more scope there is for compounding to affect the result. A larger monthly contribution also increases the rupee value of the difference between return scenarios.
A Cautious Way to Use SIP Projections
An SIP projection can be useful as a planning tool if its limitations are clear.
The first point is that the monthly contribution should be realistic. A projected corpus has limited value if the investor cannot maintain the stated SIP over the full period.
The second point is that the return assumption should be treated as an assumption. It should not be described as a guaranteed return unless the relevant investment actually provides such a guarantee under its documented terms.
The third point is that inflation matters. A future corpus of ₹50.46 lakh will not have the same purchasing power as ₹50.46 lakh today. A nominal corpus figure therefore does not, by itself, show future real purchasing power.
The fourth point is that actual investment costs and taxes can affect the final amount, depending on the investment and the investor’s circumstances. A simple SIP calculator may not capture every such factor.
What the Numbers Tell Us
The central mathematical point is straightforward.
A ₹10,000 monthly SIP for 15 years requires total contributions of ₹18 lakh. Under the stated monthly-compounding assumptions, the projected corpus changes from ₹34.83 lakh at 8% to ₹61.29 lakh at 14%.
Within that range, each one percentage-point change can alter the projected corpus by several lakh rupees.
The difference between 11% and 12% is particularly useful as an example. The projected corpus rises from ₹45.89 lakh to ₹50.46 lakh, a difference of approximately ₹4.57 lakh.
The investor’s contribution does not change. The difference arises solely from the assumed return.
As the monthly SIP becomes larger, the same percentage-point difference creates a larger rupee difference. At ₹1,00,000 per month, the difference between the 11% and 12% scenarios is approximately ₹45.72 lakh over the same 15-year period.
Conclusion
A one percentage-point difference in an assumed return can have a meaningful effect on a 15-year SIP corpus. The effect becomes larger as the investment amount increases because the return assumption applies to a larger pool of contributions and accumulated returns.
For a ₹10,000 monthly SIP, the projected corpus is ₹45.89 lakh at an assumed 11% return and ₹50.46 lakh at an assumed 12% return. The difference is approximately ₹4.57 lakh.
The figures from 8% to 14% show the wider range clearly: ₹34.83 lakh at 8%, ₹38.12 lakh at 9%, ₹41.79 lakh at 10%, ₹45.89 lakh at 11%, ₹50.46 lakh at 12%, ₹55.57 lakh at 13%, and ₹61.29 lakh at 14%.
These numbers are best understood as scenario-based estimates. They show what the mathematical outcome would be under each stated assumption. They do not establish what an actual investment will earn.
For a 15-year financial goal, the key lesson is therefore not that one particular return assumption should be used. The more useful lesson is that the final corpus is sensitive to the return assumption, and even a 1% difference can become substantial over a long period.
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