Compound Interest Calculator
Calculate how an initial investment plus monthly contributions grows over time at a given interest rate and compounding frequency.
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Compound interest projects how an initial investment plus monthly contributions grows over time as returns generate their own returns. It's calculated as A = P(1 + r/n)^nt, plus the future value of every monthly contribution compounded at the same rate.
Estimated Future Balance
$300,851
Total Interest Profit
$170,851
Total Contributed
$130,000
Your Inputs
Strategy Tip
Increasing your monthly contribution by just $100 could add an extra $52,093 to your final balance over this timeframe.
Growth Trajectory
Wealth Accumulation Model
Marginal Impact Analysis
| Extra Monthly | New Future Balance | Additional Balance vs. Current Plan |
|---|---|---|
| +$100 | $352,943 | +$52,093 |
| +$250 | $431,082 | +$130,232 |
| +$500 | $561,314 | +$260,463 |
The Mathematics of Wealth: Compound Interest
Compound interest is often called the "eighth wonder of the world" because of its ability to turn modest, consistent savings into significant wealth over time. Unlike simple interest, which only pays on the principal, compounding pays interest on the interest already earned.
The Exponential Growth Formula
How to Use This Calculator
Initial Investment is the lump sum you're starting with today (enter $0 if you're starting from scratch). Monthly Contribution is what you plan to add every month. Rate is the annual return you expect, before taxes and fees. Horizon is how many years you'll leave the money invested. Compounding Frequency controls how often interest is credited to the balance (12 for monthly is the standard assumption for most investment accounts).
The Impact of Contributions
While the formula above describes a single lump sum, most real-world investors add money every month. Mathematically, that turns the investment into an annuity: each monthly contribution compounds for a different length of time, so the total future value is the sum of the initial principal's growth plus the future value of every individual deposit.
Why Starting Early Beats Contributing More Later
Here's a concrete comparison, using this calculator's own math (7% annual return, monthly compounding, no initial principal). Saver A invests $500 a month starting today and keeps going for 30 years, contributing $180,000 in total and ending with about $609,985. Saver B waits 10 years, then contributes twice as much ($1,000 a month for the remaining 20 years), putting in $240,000 in total ($60,000 more than Saver A). Saver B still ends up with only about $520,927, roughly 15% less than Saver A, despite contributing more money overall. The difference is entirely due to time: Saver A's early dollars had 20-30 years to compound, while Saver B's had at most 20.
Assumptions and Limitations
This calculator assumes a constant annual return and constant monthly contribution. Real investment returns vary year to year and rarely follow a smooth curve. It does not account for taxes on interest or capital gains, account fees, inflation, or contribution limits on tax-advantaged accounts like a 401(k) or IRA. Treat the results as a planning estimate, not a guarantee. For a plan that accounts for your tax situation, risk tolerance, and other goals, consider talking to a licensed financial advisor or tax professional.
Common Inquiries
?What is the formula for compound interest with monthly contributions?
The formula is a combination of two parts: the future value of the initial principal and the future value of a series of monthly contributions (an annuity). For principal: A = P(1+r/n)^nt. For contributions: FV = PMT * (((1+r/n)^nt - 1) / (r/n)).
?What is the difference between simple and compound interest?
Simple interest is calculated only on the principal amount of a loan or deposit. Compound interest is calculated on the principal and also on the accumulated interest of previous periods, leading to exponential growth.
?What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it will take for your money to double at a fixed annual interest rate. Simply divide 72 by the annual interest rate (e.g., 72 / 7% = approx 10.3 years).
?Does compounding frequency matter?
Yes. The more frequently interest is compounded (daily vs. annually), the faster your wealth grows. However, for most long-term investments like index funds, monthly or annual compounding is the standard projection model.
About this calculator. Results are estimates for education and planning, based on the inputs you enter and the published formula described above. Everything runs in your browser; nothing you type is sent to us or stored. It is not financial, tax, legal or medical advice. Read the disclaimer and our methodology.
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