Compound Interest Calculator

Project how an investment grows from starting principal, monthly contributions, annual return, compounding frequency, and time.

$
%
yrs
$

Compound Growth Checks

Main driver
Time
Longer horizons usually matter more than small rate changes.
Controllable input
Contribution
Monthly deposits are often easier to change than market returns.
Read first
Interest earned
This separates growth from money you personally added.

Your Compound Interest Results

Calculated
Future value
$0
Projected ending balance
Interest earned
$0
Growth beyond contributions
Total contributed
$0
Principal plus monthly deposits
Growth multiple
0x
Future value divided by contributions

Balanced Projection

The default scenario uses a long time horizon, steady contributions, and a moderate annual return.

YearBalanceContributedInterest

How Compound Interest Is Calculated

Compound interest means each period's growth is added back to the balance, so future growth is earned on both your original principal and prior interest. This calculator also includes monthly contributions because most long-term investing plans grow through regular deposits, not only a one-time starting amount.

Future principal = P x (1 + r / n)^(n x t)
Future monthly deposits = PMT x (((1 + r / 12)^(12 x t) - 1) / (r / 12))

In the formula, P is starting principal, r is the annual return as a decimal, n is compounding periods per year, t is years, and PMT is the monthly contribution made at the end of each month.

Worked Example

With $10,000 invested, a 7% annual return, monthly compounding, and $250 added every month for 20 years, the ending value is much larger than the $70,000 personally contributed. That gap is the compound growth created by time and reinvested returns.

Compounding Frequency

Annual, quarterly, monthly, and daily compounding change how often interest is credited. Frequency matters, but time horizon, return assumption, and contribution amount usually have a larger practical effect. Use frequency to match the account or product you are modeling.

Planning assumption: monthly contributions are treated as end-of-month deposits. The calculator does not subtract taxes, fees, inflation, or investment volatility.

Limitations and Financial Context

This is an educational planning calculator, not financial advice. Actual investment returns vary, and taxes, fees, withdrawal timing, inflation, and market losses can materially change real outcomes. For major retirement, college savings, or debt decisions, compare conservative and optimistic scenarios and consider professional advice.

Frequently Asked Questions

What is compound interest?
Compound interest is growth earned on both the original amount and prior interest. Over long periods, reinvesting returns can make the balance grow faster than simple interest.
Does monthly contribution timing matter?
Yes. This calculator assumes deposits are made at the end of each month. Deposits made at the beginning of each month would have slightly more time to grow.
Should I use nominal or real returns?
Use nominal returns for account-balance projections and real, inflation-adjusted returns when comparing future purchasing power.
Why does compounding frequency have a small effect?
When the annual return is the same, moving from monthly to daily compounding is usually less important than adding more time, increasing contributions, or changing the return assumption.

Related finance calculators

Savings Calculator Retirement Calculator Future Value Calculator CAGR Calculator Index Fund Cost Calculator