See how your money grows with compound interest. Project your future value with an initial investment, monthly contributions, and an expected rate of return — plus a year-by-year growth chart.
An investment calculator projects how much your money will grow over time using compound interest — the process where your returns earn their own returns. You enter an initial amount, optional monthly contributions, an expected annual return, and a time frame, and it calculates your future value. For example, $10,000 plus $500/month at a 7% annual return grows to about $691,000 over 30 years, of which roughly $501,000 is pure investment growth.
Enter your numbers below. The chart and breakdown update instantly.
| Year | Contributed | Interest | Balance |
|---|
Project your wealth in under two minutes:
Compound interest is interest earned on both your original money and on the interest you've already earned. It's often called "interest on interest," and it's the single most important concept in building long-term wealth.
Albert Einstein is often (probably apocryphally) credited with calling compound interest "the eighth wonder of the world." Whether he said it or not, the math is undeniable: given enough time, compounding turns modest, regular savings into substantial wealth.
The core formula for a lump-sum investment is:
When you add regular monthly contributions, each deposit also compounds from the moment it's invested. The future value of a series of contributions uses:
The calculator above combines both: it grows your initial principal and your stream of contributions, then adds them together. You don't need to do this by hand — but understanding it shows you why both your starting amount and your monthly habit matter.
Step 1: Grow the initial $10,000
Step 2: Grow the monthly contributions
Step 3: Add them together
Step 4: Separate contributions from growth
Future value: ~$691,150 · You contributed $190,000 · Compound growth added $501,150 — more than 2.6× what you put in
The difference between simple and compound interest is dramatic over time. Simple interest only ever pays on your original principal; compound interest pays on the growing balance.
Starting with $10,000 at 7%, simple interest gives you $31,000 after 30 years. Compound interest gives you over $76,000 — more than double — from the exact same starting amount and rate. The only difference is that compound interest let your earnings earn their own earnings.
The Rule of 72 is a quick way to estimate how long it takes your money to double. Just divide 72 by your annual return rate:
| Annual Return | Years to Double |
|---|---|
| 2% | 36 years |
| 4% | 18 years |
| 6% | 12 years |
| 7% | ~10.3 years |
| 9% | 8 years |
| 12% | 6 years |
This simple trick helps you sanity-check investments instantly. At a 7% return, your money doubles roughly every decade — so a 30-year horizon means your initial sum doubles about three times (2× → 4× → 8×), even before counting contributions.
Here's the most important lesson most calculators won't emphasize: time matters more than the amount you invest.
Because compound growth accelerates over time, the years you invest early are worth far more than later years. Waiting even a few years to start can cost you a fortune in lost growth. Consider two investors who both contribute $300/month at 7%:
| Investor | Starts At | Invests Until 65 | Total at 65 |
|---|---|---|---|
| Early Erin | Age 25 | 40 years ($144k in) | ~$719,000 |
| Late Liam | Age 35 | 30 years ($108k in) | ~$340,000 |
Erin invested only $36,000 more than Liam (ten extra years of $300/month), but ended up with roughly $379,000 more. Those ten early years did more work than all of Liam's contributions combined. This is why the best time to start investing is as early as possible.
Don't wait for the "perfect" time or a bigger paycheck to start investing. Starting small today beats starting big later. Even modest contributions, given enough time, compound into substantial sums. Time in the market beats timing the market.
The calculator lets you choose any return rate, but using a realistic one matters. Here are rough historical long-term averages (before inflation) for context — not guarantees:
| Investment Type | Historical Long-Term Average | Risk Level |
|---|---|---|
| US stocks (S&P 500) | ~10% (before inflation) | Higher |
| Balanced portfolio (60/40) | ~7-8% | Moderate |
| Bonds | ~4-5% | Lower |
| High-yield savings | ~2-4% | Very low |
| Cash / checking | ~0-1% | Lowest (loses to inflation) |
Past performance does not guarantee future results. Markets are volatile — a 10% average means some years are up 25% and others are down 20%. Don't assume smooth, guaranteed returns. For long-term planning, many advisors use a conservative 6-7% to avoid over-optimistic projections.
A dollar today buys more than a dollar in 30 years. Inflation — the gradual rise in prices — quietly erodes the purchasing power of your money over time. Historically, inflation in the US has averaged around 2-3% per year.
This means a "nominal" return of 9% is really only about 6-7% in real (inflation-adjusted) terms. When the calculator shows a large future value, remember that the future dollars won't buy as much as today's dollars. To plan in today's money, use a lower "real" return — for example, enter 5% instead of 8% to roughly account for inflation.
For a rough inflation-adjusted projection, subtract your expected inflation rate (say 3%) from your nominal return before entering it. So a 9% market return becomes about 6% "real." This gives you a future value in today's purchasing power — a more honest picture of what your money will actually be worth.
Compound interest is calculated with the formula A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is the number of years. Unlike simple interest, compound interest earns interest on previously earned interest, which is why investments grow faster over time. When you add regular contributions, each deposit also begins compounding.
Historically, the US stock market (S&P 500) has returned roughly 10% per year on average before inflation, or about 7% after inflation, over the long term. However, returns vary widely year to year and past performance doesn't guarantee future results. Conservative portfolios with more bonds typically return less (4-6%), while individual years can range from large gains to significant losses. A 6-7% assumption is common for long-term planning.
A one-time $10,000 investment at a 7% annual return would grow to about $38,697 in 20 years with no additional contributions, thanks to compound interest. If you also added $200 per month, it would grow to roughly $137,000. The exact amount depends on your rate of return and how often interest compounds. Use the calculator above to model your own scenario.
The Rule of 72 is a quick way to estimate how long it takes an investment to double. Divide 72 by your annual rate of return to get the approximate number of years. For example, at a 7% return, your money doubles in about 72 ÷ 7 = 10.3 years. At 9%, it doubles in 8 years. It's an approximation, but remarkably accurate for typical rates.
Historically, investing a lump sum immediately has outperformed spreading it out about two-thirds of the time, because markets tend to rise over time and earlier money compounds longer. However, investing monthly (dollar-cost averaging) reduces the risk of investing everything right before a downturn and is more practical for most people who earn money gradually. The best approach is usually to invest consistently as you earn.
Simple interest is calculated only on the original principal, so you earn the same amount each period. Compound interest is calculated on the principal plus all previously earned interest, so your earnings accelerate over time. For example, $10,000 at 7% over 30 years grows to $31,000 with simple interest but over $76,000 with compound interest — more than double, from the same rate.
For long-term planning, yes — inflation erodes purchasing power over time. A simple approach is to subtract your expected inflation rate (around 2-3%) from your nominal return to get a "real" return. For instance, enter 5% instead of 8% to roughly express your future value in today's dollars. This gives a more realistic sense of what your money will actually buy.
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This calculator provides estimates for educational purposes only and is not financial, investment, or tax advice. Investment returns are not guaranteed, and you can lose money investing. Past performance does not predict future results. Consult a licensed financial advisor before making investment decisions.