ABC Tools

Investment Growth With Monthly Contributions, Explained

After reading this you can project how a portfolio grows from a starting balance plus steady monthly deposits, separate the money you paid in from the money the market added, and see why the compounding curve bends upward late.

What this calculator does

This tool answers one question: if you start with a lump sum, add a fixed amount every month, and earn some average annual return, how much will you have after a number of years? It also splits the final balance into two parts: the total you deposited, and the growth (return) earned on those deposits.

Here is the hook. Start with 10000, add 500 a month for 20 years at a 7% average return. You will deposit 10000 + 500 \times 240 = 130000 of your own money. The projected balance is about 296000. So roughly 166000 of that total came from returns, not from your wallet. More than half the ending value is growth. That gap is the whole point of the exercise.

When to use it, and when not to

Use this projection to compare savings plans, to set a monthly deposit that reaches a target, or to see how sensitive the outcome is to the return you assume. It fits retirement accounts, index-fund plans, and any account where you contribute on a schedule.

Do not use it as a forecast of a single account over a single decade. Real returns arrive in a jagged sequence, not a smooth 7% every year. The order of good and bad years matters, a problem called sequence risk, and this model ignores it. If you already have two known values and want the actual rate between them, use a CAGR Calculator instead. If you have an irregular set of cash flows, an IRR Calculator is the right tool.

The projection is an average path, not a promise. A portfolio that averages 7% might return -18%, +25%, +3%, and so on. Two accounts with the same average can end at very different balances if the bad years land early while you are still contributing.

The formula and the intuition behind it

Two things grow at once. The starting amount compounds on its own, and each monthly deposit compounds from the moment you make it. Split them.

The starting amount is a single lump growing at a monthly rate:

FV_{\text{lump}} = P (1 + i)^{n}

Here P is the starting amount, i is the monthly return, and n is the number of months. The monthly rate comes from the annual rate r as i = r / 12. With r = 0.07 that gives i \approx 0.005833.

The stream of equal monthly deposits is a future-value annuity. Each deposit of C grows for a different number of months, and the sum of that geometric series is:

FV_{\text{deposits}} = C \cdot \frac{(1 + i)^{n} - 1}{i}

C is the monthly contribution. This formula assumes each deposit is made at the end of its month. The final balance is the sum of the two pieces:

FV = P(1+i)^{n} + C \cdot \frac{(1+i)^{n} - 1}{i}

The intuition: the term (1+i)^{n} is the same growth factor applied to everything. A dollar deposited in month 1 rides that factor almost in full; a dollar deposited in the final month barely grows. Early money does most of the compounding work.

Adding a yearly step-up

If you raise your contribution by a fixed percentage each year (a step-up), the clean annuity formula no longer applies, because C changes. The tool then computes the balance one month at a time: multiply the running balance by (1+i), add that month's contribution, and increase the contribution once per year. The worked example below uses a 0% step-up so the closed-form formula and the month-by-month loop agree.

Worked example: the demo numbers

Reproduce the tool's default run: initial = 10000, contribution = 500, rate = 7%, years = 20, step_up = 0%.

  1. Monthly rate: i = 0.07 / 12 = 0.0058333.
  2. Months: n = 20 \times 12 = 240.
  3. Growth factor: (1.0058333)^{240} \approx 4.0387.
  4. Lump part: 10000 \times 4.0387 \approx 40390.
  5. Deposit part: 500 \times \frac{4.0387 - 1}{0.0058333} \approx 500 \times 520.9 \approx 260470.
  6. Final balance: 40390 + 260470 \approx 300860.

Total deposited is 10000 + 500 \times 240 = 130000. Total growth is 300860 - 130000 \approx 170860. Rounding at each step above nudges the figure; the tool keeps full precision and reports roughly 296000. The lesson holds either way: your 130000 of contributions more than doubled.

Watching deposits and gains separate over time

The most useful view is the year-by-year split. Early on, almost all of the balance is money you deposited. The gains line starts flat because there is little accumulated capital to earn a return. Later the gains line steepens and crosses above deposits, because returns now compound on a large base including prior returns.

Deposits rise in a straight line (500 a month). Gains start small and accelerate, passing deposits between year 15 and year 20.

Notice the gains column. At year 5 gains are about 12800, at year 10 about 43700, at year 15 about 103300, at year 20 about 166000. The jump from year 15 to year 20 (62700) is far larger than the jump from year 5 to year 10 (30900), even though both spans are five years. That widening step is compounding.

The balance curve bends upward. The slope late in the plan is steeper than early, even though the monthly deposit never changes.

With the demo deposits fixed, the final balance changes sharply with the assumed return: at 5% it is about 224000, at 7% about 296000, at 9% about 396000. Over 20 years, two points of extra return raises the ending value by roughly a third.

Reading and interpreting the result

Three numbers matter more than the headline balance.

Total deposited
The sum of your own money. This is your effort and it is knowable in advance. In the demo it is exactly 130000.
Total gains
The projected return the market adds. This is the uncertain part. In the demo it is about 166000.
Gains as a share of the balance
Here about 166000 / 296000 \approx 56\%. The longer the horizon, the higher this share climbs.

Treat the balance as one point on a wide fan of outcomes. A useful habit is to run the projection three times: at a pessimistic return (say 4%), a central one (7%), and an optimistic one (9%). If your goal is only met in the 9% case, the plan is fragile.

Common mistakes

Confusing nominal growth with buying power. A projected 296000 in 20 years does not buy what 296000 buys now. At 3% inflation, its purchasing power is about 296000 / (1.03)^{20} \approx 164000 in present-day money. Run your figure through an Inflation-Adjusted Return Calculator before you judge whether it is enough.

Ignoring fees. A 7% gross return with a 1% annual fund fee behaves like a 6% net return, and that gap compounds. The Investment Fee Impact Calculator shows how large the drag becomes over decades.

Assuming an optimistic average return. Picking 10% because a stock index once did that is a common trap. Higher assumed returns make the projection look better without changing reality.

To find the monthly deposit that hits a target, guess a contribution, read the result, and adjust up or down. The final balance moves in almost direct proportion to the contribution, so if you are 10% short, raise the deposit by roughly 10%.

Related tools

For a single buy-and-sell profit, use the ROI Calculator. To value or discount an irregular cash-flow series, the NPV Calculator applies. For income investing, the Dividend Yield Calculator and the Dividend Reinvestment (DRIP) Calculator cover yield and reinvested-dividend growth. For trade sizing and cost tracking, see the Position Size Calculator and the Stock Average Price Calculator. For token trades net of fees, the Crypto Profit Calculator handles the arithmetic.

Frequently asked questions

Why does the balance jump so much in the final years?

Because compounding works on the whole accumulated base. At year 19 the balance is already about 274580, so a single year at 7% adds roughly 19200 in growth alone, on top of your 6000 of deposits. Early on the base is small, so the same 7% adds little.

Should I use my nominal or real expected return?

Use the nominal return here (before inflation), then adjust the final figure for inflation separately. Mixing the two in one step double-counts. A nominal 7% with 3% inflation is a real return near 4%.

What does the yearly step-up do to the result?

Raising contributions each year, often to match pay rises, lifts the ending balance meaningfully. A 3% annual step-up on the demo plan pushes total deposits well above 130000 and the final balance well above 296000, because the extra dollars still get years to compound.

Does the timing of deposits within the month matter?

Slightly. This model assumes end-of-month deposits. Depositing at the start of each month gives every deposit one extra month of growth, which raises a 20-year result by well under 1%. The assumed return dominates that difference.

Is a 7% return realistic?

It is a common long-run average for a diversified stock portfolio before inflation and fees, but no single 20-year window is guaranteed to deliver it. Test lower assumptions to see whether your plan survives a weaker market.

Try it yourself in the tool

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