The modeled balances are about $73,625, $77,641, and $88,350. The biggest account includes more money contributed, so its advantage is not all investment profit.
Starting from zero, investing $500 a month for 10 years produces about $73,625 at a steady 4% annual return in this model. Raising the return assumption to 5% brings the balance to about $77,641. Keeping 4% and contributing $600 a month instead produces about $88,350.
The larger contribution leaves the bigger account in this comparison. But most of its advantage comes from extra money deposited, not extra investment profit.
Change one input at a time
The baseline is $500 a month at 4%. The alternatives change either the return by one percentage point or the monthly contribution by $100.
All scenarios start with no initial balance and use 120 end-of-month deposits. Each month, the annual rate divided by 12 is applied to the existing balance before the new contribution is added. Taxes, fees, and inflation are excluded.
| Scenario | Contributions | Investment gain | Balance after 10 years |
|---|---|---|---|
| $500 monthly, 4% annually | $60,000 | $13,625 | $73,625 |
| $500 monthly, 5% annually | $60,000 | $17,641 | $77,641 |
| $600 monthly, 4% annually | $72,000 | $16,350 | $88,350 |
Calculated gains and balances are rounded to the nearest dollar.
The higher return works on the same contributions
Both $500 scenarios contribute $60,000. The approximately $4,016 difference in final balances is therefore entirely additional investment gain.
The higher modeled rate applies to deposits already in the account and to gains accumulated in previous months. No extra contribution is needed to create that modeled difference.
An extra $100 brings extra principal as well
Increasing contributions to $600 adds $12,000 of principal over the decade. The ending balance exceeds the baseline by about $14,725: $12,000 of additional deposits plus approximately $2,725 of growth on them.
Notice that the $500-at-5% scenario actually has more investment gain than $600 at 4%. The latter still has the larger balance because more money was deposited. Account size and investment profit are different comparisons.
A deposit decision and a return assumption are different
An extra contribution must come from money available in the household budget. A higher return is an assumption in a model; it is not an extra percentage point an investor can simply order from the market.
Actual investments rise and fall. Changing the sequence of returns or missing deposits can change the result. This comparison isolates two inputs at the stated amounts and horizon. It does not establish that increasing contributions always beats a higher return over every period.
Reproduce both sides
Open the $500 monthly scenario to compare 4% and 5% on the same screen. The $600 monthly scenario is a separate calculation at 4%.
When changing the monthly amount, compare total contributions alongside the final balance. That separates money you supplied from money the investment earned.