Investment Calculator

An investment calculator adds two pieces: what your starting amount grows to, P × (1 + r)ⁿ, and what your regular contributions grow to, C × ((1 + r)ⁿ − 1) ÷ r. With $10,000 to start, $500 a month and an assumed 6% yearly return for 20 years, those pieces are $33,102.04 and $231,020.45, so the balance could reach $264,122.49.

How to use the investment calculator

1. Pick what you want to find: the future value, the contribution needed for a goal, the time to reach a goal, or the return a goal would need.
2. Type your starting amount, your contribution and the number of years. Choose monthly or yearly contributions and whether you add money at the start or the end of each period.
3. Enter a return you want to test. It is an assumption, not a promise. Try a cautious rate and a hopeful one to see the range.
4. Optional: add a yearly raise for your contributions, an inflation rate to see the result in today’s dollars, and your yearly fees.
5. Read the answer, then open Show steps for the working, the table of other returns to compare a cautious and a hopeful rate, and the Year-by-year table for each year’s deposits and growth.

Investment growth formulas

Every projection on this page comes from two standard formulas. The first is the future value of a lump sum:

FV = P × (1 + r)ⁿ

The second is the future value of an annuity, a series of equal deposits made at the end of each period:

FV = C × ((1 + r)ⁿ − 1) ÷ r

• P is the starting amount.
• C is the contribution made each period.
• r is the return per period as a decimal. With monthly deposits and monthly compounding, 6% a year becomes 0.06 ÷ 12 = 0.005 a month.
• n is the number of periods: years × 12 for monthly deposits.

If you deposit at the start of each period instead (an annuity due), every deposit earns one extra period of growth, so multiply the annuity result by (1 + r). When the compounding frequency differs from how often you contribute, the calculator first converts the yearly rate to a rate per contribution period: q = (1 + annual rate ÷ m)^{m ÷ p} − 1, where m is compounding periods per year and p is contributions per year.

Worked example: $500 a month for 20 years

Start with $10,000, add $500 at the end of every month for 20 years, and assume 6% a year compounded monthly.

1. Monthly rate: r = 0.06 ÷ 12 = 0.005. Number of months: n = 20 × 12 = 240.
2. Growth factor: (1.005)²⁴⁰ ≈ 3.310204.
3. Starting amount: 10,000 × 3.310204 ≈ $33,102.04.
4. Contributions: 500 × (3.310204 − 1) ÷ 0.005 = 500 × 462.0409 ≈ $231,020.45.
5. Ending balance: $33,102.04 + $231,020.45 = $264,122.49.

You put in $10,000 + 240 × $500 = $130,000, so growth supplies $134,122.49, a bit more than half the balance. Making each deposit at the start of the month instead multiplies the contribution part by 1.005, which gives $232,175.55 and an ending balance of $265,277.59.

Worked example: how much to invest each month

To find the contribution for a goal, solve the annuity formula for C: C = goal × r ÷ ((1 + r)ⁿ − 1). For $1,000,000 in 30 years at an assumed 7% compounded monthly, r = 0.07 ÷ 12 ≈ 0.0058333 and n = 360. The formula gives about $819.69, so you would round up to $819.70 a month. Over 40 years the same goal needs about $380.98 a month. If you already have savings, first subtract what they will grow to from the goal, then divide what is left by the annuity factor. The Contribution tab above does this for you.

The effect of starting early

The SEC’s investor education site puts it simply: the earlier you start investing, the more powerful the impact of compounding becomes. Here is $300 a month at an assumed 6% a year, compounded monthly, until age 65:

Waiting ten years cuts the deposits by a quarter but the ending balance by about half. Another view: someone who invests $300 a month only from 25 to 34 ($36,000 in total) and then stops would have about $296,092.71 at 65, almost the same as someone who invests from 35 to 64 and puts in three times as much. When the result shows a cost of waiting, it is this same comparison run with your numbers.

Does compounding frequency matter?

More frequent compounding adds a little, but much less than the rate or the time. Here is $10,000 left alone for 20 years at 6% a year:

For a single deposit with no contributions, our compound interest calculator shows this in more detail.

Rule of 72

To estimate how long money takes to double, divide 72 by the yearly return in percent. The SEC gives this example: at 9%, an investment doubles in value about every 8 years. The shortcut is closest for rates near 8%:

Inflation and today’s dollars

A balance 20 years from now will not buy what the same number buys today. To convert, divide by (1 + inflation)^years. With 3% inflation, $264,122.49 ÷ 1.03²⁰ ≈ $146,238.22 in today’s dollars. Use an inflation rate you consider reasonable; the calculator does not assume one for you.

How fees change investment growth

Fund and advisory fees are usually a yearly percent of your balance, so they shrink every year’s growth. The SEC’s Investor.gov shows $100,000 growing at 4% a year for 20 years: with a 0.25% yearly fee it ends at about $208,000, with 0.50% at about $198,000, and with 1% at about $179,000. Type your fees in the Yearly fees box to see the balance after fees and how much the fees cost you.

Why investment returns are not guaranteed

The formulas assume the same return every single year. Real investments do not work that way. The SEC notes that all investments involve some degree of risk, that securities held at a broker, including mutual funds, are not insured against loss in value, and that large company stocks as a group have lost money on average about one out of every three years. A steady 6% in a projection could arrive as +20% one year and −15% the next, and the order of those years changes the ending balance.

So treat any rate you type as a scenario. The result shows the same plan at 2 points below and above your rate, and the Return needed tab shows what your plan depends on. Taxes also reduce real results and are not included here. Results from this page are estimates, not investment advice.

Common mistakes with investment projections

• Using the yearly rate per month. Plugging 0.06 instead of 0.005 into the monthly formula turns $500 a month for 20 years into billions of dollars.
• Counting years as n. With monthly deposits, 20 years is n = 240.
• Forgetting the starting amount. The annuity formula covers only the regular deposits; add the lump sum separately.
• Calling all of the balance profit. Subtract what you put in to see the growth.
• Ignoring inflation. A large future number can be worth far less in today’s dollars.

Sources

Checked in November 2026.

• SEC Investor.gov: What is Risk?
• SEC Investor.gov: Introduction to Investing
• SEC Investor.gov: What is compound interest?
• SEC Investor.gov: Compound Interest Calculator
• SEC Investor.gov: Understanding Fees

Investment growth practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How much will $500 a month be worth in 20 years?

At an assumed 6% a year compounded monthly, $500 deposited at the end of each month grows to about $231,020.45 after 20 years. You put in $120,000, so about $111,020.45 is growth. A different return gives a different answer, so test several rates.

How much do I need to invest monthly to reach $1 million?

Starting from zero at an assumed 7% compounded monthly, you would need about $819.70 a month for 30 years or about $380.98 a month for 40 years. Use the Contribution tab to include savings you already have.

What rate of return should I use?

There is no fixed answer, because returns are not guaranteed and change from year to year. Compare a cautious and a hopeful rate in the table of other returns, and use the Return needed tab to see what rate your plan depends on.

How does the rule of 72 work?

Divide 72 by the yearly return in percent to estimate the years it takes money to double. At 6%, 72 ÷ 6 = 12 years; the exact figure with yearly compounding is about 11.9 years.

Is it better to invest at the start or the end of the month?

In the math, deposits at the start of each period earn one extra period of growth. In the 20-year example, that adds about $1,155 to the balance. The difference is small next to the effect of the rate and the number of years.

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