Compounding is growth on top of earlier growth
If $100 grows by 5%, it becomes $105. If the next year's 5% applies to the new $105 balance, the second year's growth is $5.25 rather than $5. The extra quarter is tiny, but repeating the process for decades is what makes compounding powerful.
Time can matter more than a dramatic starting amount
Using Terra's effective-annual return convention, an illustrative $50 monthly contribution earning 7% for 47 years grows to about $203,795. Waiting ten years and contributing the same $50 for 37 years grows to about $99,251. These are illustrations, not forecasts; actual returns vary and can be negative.
Change one thing: the starting age
Open Terra's Compound Interest Calculator. Keep the contribution and return assumption the same, but compare 47 years with 37 years. Then lower the assumed return and see whether the lesson about time still holds.
Try the Compound Interest Calculator →Compounding does not mean smooth returns
Investment returns do not arrive as a guaranteed percentage every year. Markets rise and fall. A long-term return assumption in a calculator is a modeling shortcut that helps you understand the relationship between contribution, time and return; it is not a promise.
The Rule of 72 is a shortcut, not a forecast
Dividing 72 by an assumed annual growth rate gives a rough estimate of how many years it could take money to double. At 6%, the shortcut is about 12 years. It is useful for intuition, but real investment returns are uneven.
Can starting 10 years earlier beat contributing twice as much later?
In one Terra illustration, a person who contributes $100 a month for 45 years at a modeled 7% effective annual return finishes with about $353,766. Someone who waits ten years, then contributes $200 a month for 35 years under the same return assumption finishes with about $342,283. The later saver contributes much more of their own money, yet the earlier dollars had ten extra years to compound.
This is not a promise that 7% will occur. It is a controlled example that changes contribution timing while holding the modeled return convention constant. Try 5%, 6% and 8% in the calculator and watch how sensitive the gap is to the assumption.
Common mistake
Turning a long-run return assumption into an expectation for every year. Markets can fall sharply, stay flat, or deliver returns in an uneven sequence. The smooth percentage exists to explore a relationship, not to predict a path.
What could change the answer?
The return earned, fees, taxes, missed contributions, contribution timing and how long the money remains invested all matter. Starting early is powerful, but increasing savings later still matters enormously.
Quick check
Why does starting earlier help in a compound-growth model?
Primary references
No login. No quiz answers or personal financial information are stored with your progress.