Compound Interest Calculator
Enter a starting amount, interest rate and time to see how compounding grows your money — add a monthly contribution to supercharge it.
Reviewed by the ToolNestr Editorial Team — July 2026
How compound interest is calculated
Compound interest uses the formula A = P(1 + r/n)^(nt), where A is the future value, P is the principal, r is the annual interest rate (decimal), n is the number of compounding periods per year, and t is the number of years. When you add monthly contributions, the formula extends to A = P(1 + r/n)^(nt) + PMT × ((1 + r/12)^(12t) - 1) / (r/12), where PMT is the monthly contribution.
The compounding frequency matters — more frequent compounding (daily vs yearly) means interest is calculated and added to your balance more often, resulting in slightly faster growth. The calculator handles all common frequencies automatically.
Worked example
You invest $10,000 at a 5% annual interest rate for 20 years, compounded monthly.
About compound interest
Albert Einstein reportedly called compound interest "the eighth wonder of the world." The earlier you start and the longer you leave money invested, the more powerful it becomes, because you earn interest on your interest. This calculator shows that growth clearly, with or without regular monthly deposits. Pair it with our Inflation Calculator to see whether your future balance will keep up with rising prices.
How to use it
- Enter your starting amount and the annual interest rate.
- Choose how often interest compounds (yearly, quarterly, monthly or daily).
- Add an optional monthly contribution to see the impact of regular saving.
- See your future value, total contributed and total interest earned.
When to use this tool
Use this calculator when you're planning long-term savings goals like retirement, a child's college fund, or a major purchase several years away. It is especially powerful for comparing outcomes of starting to save now versus waiting a few years — the difference in total growth can be eye-opening. It also helps you decide between lump-sum investing and dollar-cost averaging with regular deposits.
Tips for best results
- Use a conservative interest rate (5–7%) for long-term stock-market projections and a lower one (2–4%) for savings accounts or CDs.
- Start with whatever you can — even small regular contributions add up dramatically over 20+ years.
- Remember that inflation reduces real returns; subtract your expected inflation rate from the interest rate for a more realistic picture.
What different interest rates mean
The annual interest rate you use dramatically changes your results. Here is how different rates typically correspond to different investment types.
Long-Term Saver
Plan your retirement nest egg by seeing how monthly contributions compound over decades. Small changes today make a massive difference later.
Young Investor
See the massive advantage of starting early. A small amount invested in your 20s can outgrow much larger contributions started later in life.
Parent
Project college fund growth with regular contributions. See how compounding helps a 529 plan or education savings account grow over 18 years.
Pre-Retiree
Estimate how your existing savings will grow in the remaining years before retirement and decide if you need to increase contributions.
| Starting amount | Monthly contribution | After 10 years | After 20 years | After 30 years |
|---|---|---|---|---|
| $1,000 | $100 | $18,870 | $56,650 | $135,210 |
| $5,000 | $200 | $41,680 | $125,340 | $299,410 |
| $10,000 | $500 | $103,410 | $311,220 | $744,180 |
| $25,000 | $1,000 | $224,110 | $670,420 | $1,601,960 |
| $50,000 | $1,500 | $383,480 | $1,136,840 | $2,702,360 |
| $100,000 | $2,000 | $569,340 | $1,661,290 | $3,914,640 |
How to use the compound interest calculator
Enter your starting amount
Type your initial deposit or current savings balance and choose your currency.
Set rate and frequency
Enter your expected annual interest rate and choose how often interest compounds — yearly, quarterly, monthly, or daily.
Add contributions and time
Include an optional monthly contribution and the number of years to see your future value, total invested, and total interest earned.
Tips for maximizing compound growth
Start as early as possible
Time is the most powerful ingredient in compounding. Investing even a small amount in your 20s can outperform much larger contributions started in your 40s. Every year you delay costs you exponential growth.
Be consistent with contributions
Regular monthly investing through market ups and downs (dollar-cost averaging) removes the risk of mistiming the market and builds a powerful savings habit.
Reinvest all earnings
To maximize compounding, always reinvest dividends, interest, and capital gains. Taking distributions breaks the compounding cycle and slows your growth significantly.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Over time this "interest on interest" can grow your savings dramatically.
What does compounding frequency mean?
It's how often interest is added — yearly, monthly or daily. More frequent compounding grows money slightly faster.
Can I include regular deposits?
Yes. Add a monthly contribution and the calculator includes those deposits and the interest they earn.
How does starting early make a difference?
Time is the most powerful factor in compounding. Investing $5,000 once at age 25 could grow more by retirement than investing $5,000 every year starting at age 35 — that's the magic of time in the market.
What happens if I double my monthly contribution?
Doubling your contribution more than doubles your ending balance because the extra deposits also earn compound interest. The calculator shows exactly how additional contributions accelerate your growth.
Do taxes affect compound interest?
In tax-deferred accounts like a 401(k) or IRA, your money compounds without being reduced by taxes each year, which can significantly boost long-term growth. In taxable accounts, you pay tax on interest each year, slowing the compounding effect.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes money to double: divide 72 by the annual interest rate. At 7%, your money doubles roughly every 10.3 years (72 ÷ 7 ≈ 10.3).
How does inflation affect compound interest?
Inflation reduces your real return. If your investment earns 7% but inflation is 3%, your real return is about 4%. Use the Inflation Calculator alongside this one to see whether your future balance maintains its purchasing power.
Should I invest a lump sum or use dollar-cost averaging?
Historically, investing a lump sum all at once tends to outperform dollar-cost averaging about two-thirds of the time because markets generally rise over time. However, DCA can reduce the emotional stress of investing a large amount right before a market drop.
What's the difference between simple and compound interest?
Simple interest is earned only on the original principal each period. Compound interest is earned on both the principal and previously earned interest. Over long periods, compounding produces dramatically larger returns — that's why it's called "interest on interest."