Skip to content
Omnikalc

Compound interest calculator with monthly contributions

See how an initial deposit, regular monthly contributions and an annual return grow over time, and what the result is worth after inflation.

Updated on
Your inputs
$

Value out of range

$ /mo

Value out of range

%

Value out of range

years

Value out of range

%

Value out of range

Results

Future balance

$196,665

Interest earned

$114,665

Snowball effect Your interest now exceeds everything you put in: time is doing most of the work.
Total contributions
$82,000
Share of interest in the final balance
58.3%
Future balance in today's dollars
$120,019
Equivalent monthly rate
0.583%

Future balance by investment period

Total contributionsInterest earnedFuture balanceFuture balance in today's dollars
5 years current$28,000$7,654$35,654$31,513
10 years current$46,000$26,022$72,022$56,264
15 years current$64,000$59,578$123,578$85,326
20 years current$82,000$114,665$196,665$120,019
25 years current$100,000$200,276$300,276$161,966
30 years current$118,000$329,156$447,156$213,178
40 years current$154,000$796,558$950,558$354,017

Why compound interest matters

Compound interest means the interest you earn is added to your balance and starts earning interest itself. In the first few years the effect looks small. Then the curve bends upward: after a couple of decades, the interest you have accumulated can exceed everything you contributed.

This calculator brings together the three drivers of long-term savings: your starting balance, your monthly contributions and time. It also converts the final balance into today’s dollars, so you can see real purchasing power rather than a headline number.

The formula

The annual rate is first turned into an equivalent monthly rate based on the compounding frequency:

r = (1 + rate ÷ f)^(f ÷ 12) − 1, where f is the number of compounding periods per year.

The future balance then adds the growth of your initial deposit and the growth of every monthly contribution:

Future balance = P × (1 + r)^n + C × ((1 + r)^n − 1) ÷ r

where P is the initial deposit, C the monthly contribution (made at the end of each month) and n the number of months. The inflation-adjusted value divides the result by (1 + inflation)^years.

Worked example

You invest $10,000, add $300 a month for 20 years and earn 7% a year compounded monthly.

  1. Monthly rate: 7% ÷ 12 = 0.583% per month.
  2. Number of contributions: 20 × 12 = 240.
  3. The initial deposit grows to 10,000 × 1.00583²⁴⁰ ≈ $40,387.
  4. The contributions grow to 300 × (1.00583²⁴⁰ − 1) ÷ 0.00583 ≈ $156,278.
  5. Future balance: ≈ $196,665, for $82,000 contributed, so $114,665 of interest.

With 2.5% inflation, that balance has the purchasing power of about $120,019 today.

Time is the biggest lever

Years Contributed Interest Future balance In today’s dollars
5 $28,000 $7,654 $35,654 $31,513
10 $46,000 $26,022 $72,022 $56,264
20 $82,000 $114,665 $196,665 $120,019
30 $118,000 $329,156 $447,156 $213,178
40 $154,000 $796,558 $950,558 $354,017

Going from 30 to 40 years adds $36,000 of contributions but nearly half a million dollars of interest. Starting early beats contributing a lot later.

Choosing a realistic rate

Enter a return after fees, and ideally after tax. High-yield savings accounts and CDs are safe but pay modest rates. Bond funds pay a little more. A diversified stock index fund has historically returned more over long periods, but it can drop sharply in any given year, so do not use a high rate for a short-term goal. Tax-advantaged accounts such as a 401(k), IRA, Roth IRA or a UK ISA let the full return compound without annual tax drag.

Key takeaways

  • Time matters more than the rate, and the rate matters more than the compounding frequency.
  • Regular contributions, even small ones, feed the snowball every month.
  • Plan long-term goals in today’s dollars so inflation does not catch you out.

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is paid only on your original deposit. Compound interest is paid on the deposit plus all interest already earned. Over 20 years at 7%, $10,000 grows to $24,000 with simple interest but to about $40,400 with monthly compounding.

How long does it take to double my money?

Use the rule of 72: divide 72 by the annual rate. At 7% money doubles in about 10 years, at 4% in about 18 years. The exact figure is ln(2) divided by ln(1 + rate).

Does compounding frequency make a big difference?

Less than most people think. At 7%, annual compounding gives exactly 7% a year while daily compounding gives about 7.25%. The rate itself and the number of years matter far more than the frequency.

Are taxes included?

No, the calculator shows pre-tax growth. Inside a 401(k), IRA, Roth IRA or a UK ISA, growth is tax-deferred or tax-free. In a taxable account, interest and dividends are taxed every year, so enter an after-tax rate for a more realistic projection.

Why show the balance in today's dollars?

Because a dollar in 20 years will buy less than it does today. With 2.5% inflation, prices rise by about 64% over 20 years. Adjusting for inflation shows the real purchasing power you are building, which is what matters for retirement or any long-term goal.

Was this calculator helpful?

Thanks for your feedback!

Results are estimates provided for information purposes. They do not replace professional advice.

Related calculators