Compound Interest Calculator
See the future value of a starting balance plus regular monthly contributions, compounded monthly at any rate of return.
How to use this compound interest calculator
- 1Enter your current savings or investment balance as the starting amount.
- 2Enter the amount you will contribute every month going forward.
- 3Enter the expected annual return. Use 7% for a diversified stock portfolio (historical average after inflation is ~4%).
- 4Set the number of years you will invest.
- 5The future value shows what your money grows to. Notice how growth earned can dwarf your contributions over long periods.
How it's calculated
FV = P(1+i)^n + PMT × ((1+i)^n − 1) ÷ i. Monthly compounding: i = annual rate ÷ 12, n = years × 12.
About the Compound Interest Calculator
Albert Einstein reportedly called compound interest the eighth wonder of the world — and while the attribution may be apocryphal, the mathematics behind the statement is not. Compound interest is the mechanism by which patient, consistent investing transforms modest contributions into substantial wealth.
The core insight of compounding is deceptively simple: you earn returns not just on what you put in, but on all the returns you have previously earned. In the early years, this feels slow — your $500/month contributions growing at 7% show modest differences from simple growth in years one through five. But in years 20 through 30, the compounding effect becomes dominant. Of a final balance of $600,000 after 30 years of $500/month contributions, over half the total ($420,000) came from investment growth — not from your $180,000 in contributions.
The most powerful application of compound interest understanding is the cost of waiting. Every year you delay investing, you lose not just one year of returns, but all the compounding that year's returns would have generated in every subsequent year. A 25-year-old who invests $5,000 once and never adds to it has $107,000 at 65 (at 7%). A 35-year-old who invests the same $5,000 once has only $54,000 at 65. Ten years of delay cost $53,000 from a single $5,000 investment.
For practical application: maximize tax-advantaged accounts first (401k up to employer match, then max IRA, then back to 401k, then taxable brokerage). The tax treatment multiplies the effect of compounding — a 7% return in a Roth IRA is a true 7% because future withdrawals are tax-free, while a 7% return in a taxable account might yield only 5.6% after capital gains taxes.
Frequently asked questions
What is compound interest and how does it differ from simple interest?
Simple interest is calculated only on the original principal — $10,000 at 7% simple interest for 20 years earns $14,000 in interest (7% × $10,000 × 20 years), for a total of $24,000. Compound interest earns interest on previously earned interest. The same $10,000 at 7% compounded annually for 20 years grows to $38,697 — more than $14,000 additional compared to simple interest. The difference grows exponentially with time, which is why time in the market is the most powerful investment variable. Monthly compounding grows slightly faster than annual compounding because interest is added to principal 12 times per year rather than once.
What annual return rate should I use for realistic projections?
The US stock market (S&P 500) has returned approximately 10% per year nominally (before inflation) and 7% per year in real terms (after inflation) over the long run, based on data going back to 1926. For a diversified portfolio of stocks and bonds, 6–8% nominal or 3–5% real is a common planning assumption. For high-yield savings accounts or CDs, use the current rate (4–5% in 2024). For broad stock market index funds with a 10+ year horizon, 7% real return is a reasonable central estimate. Be conservative for planning — if you plan at 5% and earn 8%, you end up better off than planned. If you plan at 10% and earn 6%, you fall short.
How does the frequency of compounding affect growth?
More frequent compounding produces slightly more growth. $10,000 at 7% for 30 years compounds to: $76,123 (annual), $79,057 (monthly), $79,432 (daily). The difference between annual and monthly compounding is meaningful ($2,934 over 30 years) but not transformative. The frequency of your contributions matters far more — monthly contributions from the start of each month (beginning-of-period) grow slightly more than end-of-period contributions because each payment has an extra month of growth.
What is the Rule of 72?
The Rule of 72 is a simple mental shortcut to estimate how long it takes money to double. Divide 72 by your annual return rate to get the approximate doubling time in years. At 7% return, money doubles in approximately 72 ÷ 7 = 10.3 years. At 10%, it doubles in 7.2 years. At 4%, it takes 18 years. This rule works for any compound growth rate — it also applies to debt (at 18% APR, credit card debt doubles in 4 years if unpaid) and inflation (at 3% inflation, prices double in 24 years). The Rule of 72 reveals why small differences in return rates have massive long-term consequences.
Is it better to start investing with a large lump sum or regular monthly contributions?
Research consistently shows that lump-sum investing beats dollar-cost averaging (regular contributions) about two-thirds of the time in historical market data, because markets go up more often than they go down and lump sums get more time invested. However, most people do not have a lump sum — they have regular income. For regular earners, monthly contributions are the only practical approach, and they work extremely well over time due to compounding. The most important decision is simply to start — waiting one year to begin investing at 30 instead of 29 costs far more in lost compounding than any difference between strategies.
Why does starting early matter so much?
Starting 10 years earlier can result in a larger final balance even with fewer total contributions. Example: Investor A contributes $500/month from age 25–35 (10 years, $60,000 total), then stops. Investor B contributes $500/month from age 35–65 (30 years, $180,000 total). At age 65 with 7% returns, Investor A has approximately $602,000 and Investor B has approximately $567,000 — despite contributing $120,000 less. Investor A's early contributions had 40 years to compound; B's had at most 30. This is the power of compound interest — time, not amount, is the dominant variable.