Savings growth
Compound Interest Calculator Guide
Compound interest is growth on growth. Instead of earning returns only on the money you put in, you can also earn returns on previous interest or investment gains. The effect starts small, then becomes more noticeable as time and contributions stack up.
Open the compound interest calculator to estimate a future balance from a starting amount, monthly contribution, annual return, and number of years.
How Compound Interest Works
If you start with $5,000, add $250 per month, and estimate a 6% annual return for 10 years, the ending value is about $50,067. Your deposits total $35,000, and the remaining amount is estimated growth.
Future value = starting balance growth + contribution growth. The exact result depends on the compounding frequency and timing of contributions.
Inputs That Matter Most
| Input | Effect | Practical note |
|---|---|---|
| Starting balance | Gives compounding a head start. | A larger initial deposit can reduce the required monthly contribution. |
| Monthly contribution | Adds new money regularly. | Consistency often matters more than perfect timing. |
| Annual return | Changes the growth rate. | Use conservative assumptions for planning. |
| Time | Lets growth repeat. | Longer timelines make compounding more powerful. |
Savings vs Investing
Compound interest can describe bank interest, but people also use the phrase for investment growth. A savings account may have lower but steadier returns. Investments can have higher long-term potential, but values can fall and returns are not guaranteed.
For emergency funds, taxes, or a purchase in the next year or two, stability may matter more than chasing a high return. For longer goals, return assumptions should still be tested against bad years and lower-than-expected growth.
Common Mistakes
- Using an optimistic return rate and treating the result as guaranteed.
- Forgetting inflation, fees, taxes, or account charges.
- Comparing APY and APR as if they mean the same thing.
- Stopping contributions too early because the first few years look slow.
FAQ
Simple interest is calculated only on the original principal. Compound interest is calculated on principal plus previous interest or gains.
The homepage calculator uses a simple annual return assumption and compounds monthly. If you have an APY, the result may differ slightly from a bank's exact calculation.
For investments, yes. A negative return assumption can estimate falling values, although the real path will vary.