Compound growth happens when investment returns stay invested and can earn additional returns of their own. For fund investors, the effect depends on the fund’s changing value, its costs and whether income stays invested or is paid out.
The math behind compounding
With a constant return, the compound-growth formula is A = P × (1 + r)^t: P is the starting amount, r the return per period, t the number of periods and A the ending amount. Each period’s return applies to the balance carried forward, including returns retained from earlier periods.
A fixed-rate illustration
In a separate illustration, $1,000 compounded annually at a constant 8% grows to about $2,159 after ten years. The displayed amounts are rounded independently; the calculation assumes no deposits, withdrawals, fees or taxes.
| Year | Opening balance ($) | Interest at 8% ($) | Closing balance ($) |
| 1 | 1,000 | 80 | 1,080 |
| 2 | 1,080 | 86 | 1,166 |
| 3 | 1,166 | 93 | 1,260 |
| 4 | 1,260 | 101 | 1,360 |
| 5 | 1,360 | 109 | 1,469 |
| 6 | 1,469 | 118 | 1,587 |
| 7 | 1,587 | 127 | 1,714 |
| 8 | 1,714 | 137 | 1,851 |
| 9 | 1,851 | 148 | 1,999 |
| 10 | 1,999 | 160 | 2,159 |
Suppose $1,000 earns a hypothetical 5% each year for 10 years, with no deposits, withdrawals, fees or taxes. If returns are compounded annually, the balance becomes about $1,628.89. With simple interest, which applies each year’s return only to the original $1,000 and pays that return out rather than reinvesting it, the total is $1,500.
| Hypothetical approach | Ending amount after 10 years | Assumption |
| Simple interest | $1,500 | 5% of the original $1,000 each year; returns paid out |
| Annual compounding | $1,628.89 | 5% each year, with returns retained and compounded |
Both examples assume the same rate and starting amount. The calculation illustrates the math, not a fund’s expected or guaranteed return.
How compounding works inside a fund
A fund’s net asset value (NAV) per share reflects the value of its assets after fund expenses. As the market value of the portfolio changes, the NAV can rise or fall. A fund therefore does not offer the same fixed-rate path as the hypothetical calculation above.
The share class determines what happens to income from the fund’s investments. An accumulating class retains income in the fund; a distributing class pays some or all of it to investors under the fund’s terms. The fund’s NAV and any cash distributions both matter when assessing a distributing class’s total return. Count a distribution once, rather than adding it again to a return figure that already includes it.
For a simple illustration, assume an investment worth $100 receives $2 in income and the prices of its underlying assets do not change. An accumulating class retains the income, leaving $102 invested. A distributing class pays the $2 to the investor, leaving $100 invested in the fund. That example isolates the effect of the distribution policy; actual fund values also move with markets and costs.
If cash distributions are not needed for spending, reinvesting them can return that money to the investment base. If an investor takes the cash, that portion is no longer in the fund to compound. The actual class terms determine whether income is retained or paid out.
Why gains and losses compound
Compounding applies to a changing balance in either direction. A 20% gain followed by a 20% loss does not return an investment to its starting value, because the loss applies to the larger, post-gain balance.
For example, a hypothetical $1,200 investment rises 20% to $1,440, then falls 20% to $1,152. It ends 4% below the starting amount. The calculation assumes no deposits, withdrawals, fees or taxes.
How fund costs affect the result
Management and depositary expenses are charged to the fund and reflected in its NAV, reducing the value available to investors. A fund may also charge subscription or redemption fees if its documents specify them. These costs reduce the return available to compound; the actual charges depend on the fund’s terms.
When reviewing a fund, check its documents for the share class’s distribution policy and for ongoing and transaction charges. The NAV change alone does not capture cash paid out by a distributing class, while a total-return calculation needs to account for those distributions.
What to check before comparing fund returns
- Share class: Does it retain income or pay distributions, and how often are payments made under its terms?
- Return measure: Is the figure a change in NAV, or does it account for distributions as well? Avoid counting a distribution twice.
- Costs: Which ongoing fund expenses and any subscription or redemption charges apply?
- Risk: The fund’s portfolio value can fall, so an assumed positive annual rate is not a promise of growth.
