What is compound interest?
Compound interest is interest calculated on the initial principal plus all previously earned interest. Unlike simple interest, which only applies to the principal, compound interest allows your money to grow exponentially over time.
A worked example
Take $10,000 invested with $500 added every month, at a 7% annual rate, over 20 years.
Total invested
$130,000
$10,000 plus $500/month
Interest earned
$415,678
At a constant 7%
Final value
$545,678
After 20 years
Growth
320%
$3.20 for every $1.00 invested
The compound interest formula
A = P(1 + r/n)^(nt)
| Symbol | Meaning |
|---|---|
| A | Final amount |
| P | Principal |
| r | Interest rate |
| n | Compounding frequency |
| t | Time (years) |
Example calculation
Calculate $10,000 invested at 7% annual interest, compounded annually for 10 years:
- A = $10,000(1 + 0.07/1)^(1 x 10)
- A = $10,000(1.07)^10
- A = $10,000 x 1.967
- A = $19,672
Your $10,000 would grow to $19,672 - nearly double in 10 years.
Compounding frequency impact
How often interest is compounded affects your final return. More frequent compounding leads to higher returns, but the difference diminishes quickly at higher frequencies.
| Compounding | Frequency per year | 10-year value* | Difference |
|---|---|---|---|
| Annually | 1 | $19,672 | Base |
| Semi-annually | 2 | $19,799 | +$127 |
| Quarterly | 4 | $19,867 | +$195 |
| Monthly | 12 | $19,904 | +$232 |
| Daily | 365 | $19,930 | +$258 |
*Based on $10,000 principal at 7% annual rate for 10 years.
Key variables that affect compound growth
Time (most important)
The longer you invest, the more powerful compounding becomes. Starting early is the most effective way to build wealth.
- $100/month for 40 years: $265,000
- $200/month for 20 years: $105,000
Interest rate
Higher returns accelerate growth significantly. Even a 1-2% difference compounds to substantial amounts over time.
- 6% for 30 years: $100K grows to $574K
- 8% for 30 years: $100K grows to $1.01M
Principal amount
Starting with more money provides a larger base for compounding, but regular contributions often matter more than the initial amount.
- $10K start: final $167K
- $50K start: final $359K
Regular contributions
Consistent monthly additions can dramatically increase your final amount through dollar cost averaging and continuous compounding.
- No contributions: $197K
- $500/month: $1.48M
How to use compound interest calculators
Set realistic parameters
Use conservative estimates for returns (6-8% for stock market averages) rather than optimistic projections. The historical S&P 500 average is about 10% before inflation.
Include regular contributions
Most wealth building comes from consistent contributions rather than the initial investment. Include monthly or annual additions to see realistic projections.
Consider inflation
Look for calculators that adjust for inflation so you understand purchasing power. A dollar today will not buy the same amount in 20 years.
Test different scenarios
Try various combinations of contribution amounts, time periods and return rates to understand how each variable affects your outcome.
Tips for maximising compound growth
Start early
Start investing as early as possible - time is the variable you cannot buy back.
Contribute consistently
Contribute regularly, in good markets and bad.
Reinvest everything
Reinvest all dividends and returns rather than taking them as cash.
Keep costs low
Choose low-cost index funds so fees do not eat your compounding.
Avoid early withdrawals
Every withdrawal removes the base that future growth would have built on.
Increase with income
Raise contributions as your income grows.
Use tax-advantaged accounts
Shelter growth where you can - see tax-efficient investing.
Stay patient
Avoid market timing and let the arithmetic work.