Written by FreeToolCalc Team
Formulas based on standard financial/medical equations. Last updated: March 2026.
Why compound interest matters
Compound interest is one of the biggest drivers of long-term wealth building. The longer your money stays invested, the more growth can build on previous growth.
Compound interest formula
A = P(1 + r/n)^(nt)
P = principal, r = annual rate, n = compounding periods, t = years
Example growth scenarios
| Starting Amount | Monthly Contribution | Rate | 20 Years |
|---|---|---|---|
| $5,000 | $0 | 7% | $19,348 |
| $5,000 | $200 | 7% | $114,589 |
| $10,000 | $500 | 8% | $319,813 |
What has the biggest impact on growth
- Time: Starting earlier gives compounding more room to work.
- Rate of return: Small differences add up over decades.
- Contributions: Consistency can matter as much as the starting balance.
- Fees: High fees can quietly eat away at long-term growth.