How Compound Interest Works
Understand how starting balance, contributions, rate, and time combine in a compound-growth estimate.
$10,000 plus $250 monthly for 10 years at 7%
This is a mathematical projection using a constant 7% annual rate compounded monthly. It is not a prediction or guaranteed return.
The four inputs
A compound-growth estimate needs a starting balance, recurring contribution, assumed return, and time horizon. Increasing any one can raise the result, but time has a special effect because it creates more periods for prior growth to compound.
- Starting balance begins compounding immediately.
- Regular contributions add new principal over time.
- The assumed rate controls the modeled pace of growth.
- The time horizon determines how many compounding periods occur.
Contributions versus growth
Separate money deposited from modeled growth. In the example, $10,000 starts the account and $30,000 is added over 120 months. The remaining projected balance comes from the constant-rate assumption.
- Contributed: $40,000.
- Modeled growth: about $23,368.
- Projected total: about $63,368.
Use projections as a range
Real returns change over time and fees, taxes, and inflation can reduce usable value. Run a lower, middle, and higher rate rather than relying on one outcome. The calculator is best used to compare scenarios, not promise a future balance.
Frequently asked questions
Is 7% guaranteed?
No. It is an illustrative constant assumption. Actual returns can be lower, higher, or negative in individual periods.
Does the estimate include fees or taxes?
No. Subtract expected fees or use a lower net return assumption when those costs apply.