Investment Growth Calculator
Ten thousand dollars. Five hundred a month. Twenty years. Eight percent. The answer is $343,778, and only $130,000 of it is money you actually put in. That gap is the reason this page exists. This investment growth calculator compounds a starting balance plus a steady monthly deposit at any return rate you choose, then shows the final balance split into deposits and growth. Change the rate by two points and watch six figures appear or vanish. It's the cheapest way to get a feel for compounding before your real money does it for you.
How this investment growth calculator works
It applies one month of growth to your balance, adds your deposit, and repeats for every month of your horizon. The result is a final balance plus the split that actually matters: how much you put in versus how much growth added for free.
Two engines drive the number. Your starting amount compounds the longest, so it punches above its weight. Your monthly deposits arrive later and grow less each, but there are a lot of them, and they keep the balance climbing even when markets are flat. Notice how the chart bends upward near the end. That's not optimism, it's arithmetic: growth is calculated on a bigger base every single month, so the curve steepens as the years roll on. The last five years of the default plan add more than the first ten combined.
What it assumes: one steady average return, converted to a monthly rate, with deposits landing at the end of each month. What it doesn't model: the bumpy ride in between, taxes, fees, or inflation. A 1% annual fee sounds like nothing, yet over 30 years it can eat a real slice of the final balance, so plenty of people run their numbers one point lower than they expect. Sensible.
Formula
In plain language: P is your starting amount. PMT is your monthly deposit. r is your annual return divided by 12, and n is the total number of months. The first term compounds your lump sum for the whole horizon. The second term adds up every deposit, each one grown for exactly the months it was in the account. First deposit: 240 months of growth. Last deposit: barely any. The formula respects that, which is why early deposits matter more than late ones of the same size.
Example
One worked example, step by step. Start with $5,000, add $200 a month, earn 8% a year, and let it run 30 years.
The monthly rate: 0.08 ÷ 12 ≈ 0.006667. Months: 30 × 12 = 360. The growth factor: (1.006667)360 ≈ 10.94. So the lump sum becomes $5,000 × 10.94 ≈ $54,679. The annuity factor: (10.94 − 1) ÷ 0.006667 ≈ 1490.4, which makes your deposits worth $200 × 1490.4 ≈ $298,072. Total: $352,751.
You invested $5,000 + $200 × 360 = $77,000. Growth supplied the other $275,751, about 3.6 times what you paid in. If the split surprises you, good. That's the feeling this calculator is here to teach.
How to read your results
Start with the split between deposits and growth, not the headline number. In the default example, growth ($213,778) is nearly double your deposits ($130,000). That ratio tells you whether your money is doing the heavy lifting or you are. The longer the horizon, the more the split tilts toward growth. Watch for the crossover: growth passes what you put in around year 15 for the default inputs, when the balance hits roughly $206,000 on $100,000 deposited. Everything after that is the portfolio paying you.
Next, poke the return rate. Move it from 8% to 6% and the final balance drops hard. Move it to 10% and it leaps. That sensitivity is exactly why picking the return rate is the part people get wrong most. Our guide on the return-rate mistake that wrecks Coast FIRE math shows how to choose one you can defend. A quick sanity tool: the rule of 72. Divide 72 by your rate and you get the years to double. At 8%, money doubles every 9 years, so 20 years is a little more than two doublings, which is why the curve looks the way it does.
Finally, test the deposit lever. Doubling the monthly deposit to $1,000 over the same 20 years at 8% gives $638,288, with $250,000 invested and $388,288 from growth. More money in, more money out. No surprises there, but now you know the exact size of the trade-off before you cut it from your budget.
Scenario comparison
Same starting point every time: $10,000 invested once, $500 a month, 20 years. The only thing that changes is the return rate.
| Scenario | Annual return | Total invested | Final balance | Growth |
|---|---|---|---|---|
| Cautious (bonds-heavy) | 5% | $130,000 | $232,643 | $102,643 |
| Middle (balanced) | 8% | $130,000 | $343,778 | $213,778 |
| Aggressive (stocks-heavy) | 10% | $130,000 | $452,965 | $322,965 |
Two percentage points, one way or the other, moves the final balance by about $110,000. Nothing else on this page has that kind of leverage. Not the deposit size, not the starting amount. The return assumption dominates everything, which is why it deserves more thought than the other inputs combined. Presets above load all three so you can feel the difference.
Frequently asked questions
What is a realistic return rate for investing?
Is this the same as a savings account calculator?
Is it better to invest a lump sum or contribute monthly?
Does this calculator include taxes and fees?
Why does growth accelerate so much later on?
How is this different from the retirement calculator?
One warning before you trust any projection: the return you type in must be a real return. How Coast FIRE accounts for inflation shows what goes wrong when nominal and real returns get mixed up.
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Sources & methodology
The math here is the standard future-value formula: a lump sum compounded over n periods, plus the future value of a monthly annuity. Growth compounds monthly, and each deposit is assumed to land at the end of the month. Every calculation runs locally in your browser, which means no numbers are sent to a server or stored anywhere. Read our compound growth calculation methodology page for the full set of assumptions in plain English.