A breakdown of the math behind lumpsum investment growth and what each result means.
1. The Lumpsum Growth Formula
The core equation is:
A = P × (1 + r)t
Where P is your lumpsum investment, r is the expected annual return (as a decimal), and t is the number of years. Each year, the gain is added to your balance — and the next year's gain is calculated on the new, larger balance. That's compounding. This calculator assumes annual compounding, which is the standard convention for mutual fund lumpsum projections.
2. Inflation Adjustment
Money in 10 years won't have the same purchasing power as today. Real value discounts the maturity by inflation:
Real Value = Post-Tax Maturity ÷ (1 + inflation)t
The "real return rate" is what you actually earn after inflation (Fisher equation):
Real Return = (1 + nominal) ÷ (1 + inflation) − 1
3. Tax on Gains
This calculator applies a flat 12.5% LTCG rate to the total gain. This aligns with India's long-term capital gains tax on equity and equity-oriented mutual funds:
Tax = Gain × 12.5%
For debt funds (post-April 2023) and FDs, gains are taxed at your income-tax slab rate, which is typically higher. The 12.5% used here is appropriate for equity / equity-oriented holdings held longer than 1 year.
4. Doubling Time
The exact time for your investment to double, using logarithms:
Doubling Time = ln(2) ÷ ln(1 + r)
At 12% annual return, money doubles in about 6.12 years.
Caveats & Disclaimer
Returns are illustrative. Mutual fund investments are subject to market risk; past performance does not guarantee future results. All projections assume a constant annual return over the entire tenure — real-world MF returns vary year to year, sometimes significantly. The 12.5% LTCG assumption applies to equity/equity-oriented holdings; tax treatment differs for debt funds, hybrids, and instruments held less than 1 year (where STCG rates apply). Consult a tax advisor for personalised guidance.
