Compound Growth Calculator
See how an initial investment plus steady monthly contributions grow over time — and how much of your ending balance is money you put in versus money the market made for you. Everything runs in your browser; nothing is uploaded.
A simplified approximation for planning intuition, not financial advice. Assumes a constant annual return every year, which real markets do not provide.
| Year | Contributed | Growth | Balance |
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How compound growth works
Compound growth is the reason a modest, consistent investing habit can turn into a substantial nest egg. When your money earns a return, that return is added to your balance — and in the next period, you earn a return on the larger balance, including on the gains you already made. Growth builds on growth. Given enough time, the curve stops looking like a gentle slope and starts to bend sharply upward.
The engine behind it is simple. If you invest an amount and it earns an annual return, after one year you have your original amount plus that year's gain. Leave it invested and the second year's gain is calculated on that new, higher total. Repeat this over decades and the effect is dramatic: the majority of a long-term portfolio's final value is typically growth, not the contributions themselves. This calculator makes that split explicit so you can see exactly how much of your ending balance you contributed versus how much the market compounded on your behalf.
Three levers drive the outcome, and it helps to understand how each one behaves:
- Time is the most powerful input. Because compounding is exponential, the last ten years of a long horizon usually add far more dollars than the first ten. Starting earlier beats contributing more later.
- Rate of return compounds too. A seemingly small difference — say 6% versus 8% — produces a large gap over 30 years. A broad stock-market index has historically returned roughly 7% per year after inflation, which is why 7% is a common planning default.
- Contributions keep feeding the machine. Regular monthly investing, sometimes called dollar-cost averaging, adds new principal that then has its own runway to compound.
This tool uses an effective monthly rate derived from your annual return, so contributions made partway through a year still earn a fair share of that year's growth. With contributions turned off, the math reduces to standard annual compounding — for example, $10,000 growing at 7% for 10 years reaches about $19,671. That is the plain power of leaving money invested and letting it work.
Tips for getting the most from compounding
The biggest mistakes investors make are starting late, interrupting their contributions, and reacting to short-term market swings by selling. Compounding rewards patience and consistency. Automate your monthly contribution so it happens without a decision, keep costs low because fees compound against you the same way returns compound for you, and give the plan enough time to let the exponential part of the curve do its work.
Frequently asked questions
What return rate should I assume?
A diversified stock portfolio has historically returned around 7% per year after inflation, or roughly 10% before inflation. Many planners use 6–7% as a conservative long-term assumption. Bonds and cash return less. Because future returns are uncertain, it is wise to test a range rather than rely on a single optimistic figure.
Does it matter whether returns compound monthly or annually?
For long horizons the difference is small. This calculator applies an effective monthly rate that compounds to exactly your stated annual return over twelve months, which keeps contributions fair without overstating the total.
Is this adjusted for inflation?
The result is in nominal dollars. If you enter a real (after-inflation) return such as 5%, the ending balance is expressed in today's purchasing power instead. Planomy's full app models inflation explicitly across every year of your plan.
Are taxes included?
No — this is a pre-tax growth estimate. In a tax-advantaged account like a 401(k) or IRA, growth compounds untaxed until withdrawal. In a taxable brokerage account, dividends and realized gains are taxed along the way, which slightly reduces compounding. Planomy accounts for account types and taxes in its projections — see the tax-aware withdrawal calculator.
Want the full picture?
Planomy projects your entire plan — taxes, retirement accounts, Monte Carlo, and plan-vs-actual tracking — not just one number. It's free, private, and runs right in your browser.