Compound interest is usually explained with a single deposit left alone for decades. Real life looks different: you start with something, then add money every month. This guide shows how to calculate growth with regular contributions, walks through a full example, and points out the assumptions that matter most.
Simple vs compound interest in one paragraph
With simple interest, you earn interest only on the original amount. With compound interest, the interest you earned also starts earning interest. Over short periods the difference is small. Over decades it's the main source of growth.
The formulas
For a single lump sum:
FV = P × (1 + r/n)^(n × t)
- P = starting amount
- r = annual interest rate (as a decimal)
- n = compounding periods per year (12 for monthly)
- t = years
For a fixed contribution added at the end of every period (an "ordinary annuity"):
FV = PMT × [((1 + i)^N − 1) ÷ i]
- PMT = contribution per period
- i = rate per period (r/n)
- N = total number of periods
With both a starting balance and contributions, add the two results together.
Worked example: $5,000 start, $300 a month, 7% for 20 years
Assume interest compounds monthly and you add $300 at the end of every month.
- Monthly rate i = 7% ÷ 12 ≈ 0.5833%
- N = 20 × 12 = 240 months
- Lump sum part: 5,000 × (1.005833)^240 ≈ $20,194
- Contribution part: 300 × [((1.005833)^240 − 1) ÷ 0.005833] ≈ $156,278
- Total ≈ $176,472
You contributed $5,000 + 240 × $300 = $77,000. The other $99,472 is interest, more than you put in yourself.
Year by year
| Year | Total contributed | Total interest | Balance |
|---|---|---|---|
| 1 | $8,600 | $479 | $9,079 |
| 5 | $23,000 | $5,566 | $28,566 |
| 10 | $41,000 | $20,974 | $61,974 |
| 15 | $59,000 | $50,333 | $109,333 |
| 20 | $77,000 | $99,472 | $176,472 |
Look at the interest column. In the first five years, interest adds about $5.6k. Between years 15 and 20 it adds about $49k, almost nine times as much, on contributions that are the same size every year. That's compounding: the balance itself does more and more of the work.
You can reproduce the full table and chart with the free compound interest calculator.
Time vs contribution size
Keeping 7%:
| Scenario | Contributed | Ending balance |
|---|---|---|
| $300/month for 20 years (no starting amount) | $72,000 | $156,278 |
| $600/month for 20 years | $144,000 | $312,556 |
| $300/month for 30 years | $108,000 | $365,991 |
Doubling the contribution doubles the result. Adding ten more years at the original contribution produces even more, while you put in $36,000 less than the $600 scenario. Starting earlier is the cheapest lever most people have.
Rate assumptions matter (be conservative)
The same $5,000 + $300/month for 20 years:
- at 7% → about $176,472
- at 5% → about $136,873
Two percentage points change the outcome by about $40,000. Nobody knows future returns, so run several scenarios. If you want the result in today's money, use a return after inflation (a "real" return), or adjust the result with the inflation calculator.
Does compounding frequency matter?
Less than people think. $10,000 at 6% for 10 years:
- compounded yearly → $17,908
- compounded monthly → $18,194
- compounded daily → $18,220
The jump from yearly to monthly is noticeable. Monthly to daily barely matters. Focus on the rate, the contributions and the time horizon.
Contributions at the start vs end of the month
The formula above assumes deposits at the end of each month. If you invest at the start, each deposit gets one extra month of growth. The result is the end-of-month value multiplied by (1 + i), about 0.58% more at 7%. This is a small effect, but it explains why two calculators can disagree slightly.
A quick way to estimate: the rule of 72
For a rough mental check, divide 72 by the annual return to estimate how many years it takes money to double. At 7%, 72 ÷ 7 ≈ 10.3 years. The exact figure at 7% compounded yearly is about 10.2 years, so the shortcut is close. At 4% it's about 18 years, at 10% about 7. It only applies to a lump sum without contributions, but it's handy for sanity-checking calculator results.
What the formula leaves out
- Returns are not smooth. Investments that average 7% might return +20% one year and −15% the next. The order of returns matters a lot when you are adding or withdrawing money.
- Fees. A 1% annual fee reduces a 7% return to roughly 6%, which over 20 years is the difference between the two scenarios above.
- Taxes. Interest and gains may be taxed each year, which slows compounding unless the money is in a tax-advantaged account.
- Inflation. $176k in 20 years will buy less than $176k today.
Key takeaways
- Growth with monthly contributions = lump-sum growth + annuity growth.
- In long horizons, interest can exceed what you contributed yourself.
- Starting earlier usually beats contributing more later.
- Use conservative, after-inflation returns and compare scenarios.
Turning this into a plan
- Pick a realistic return (or two or three to compare).
- Decide on a monthly contribution you can sustain.
- Run the numbers, then increase the contribution whenever your income rises.
- Track the actual balance against the plan once a year.
If your goal is financial independence, the next step is working out your target number. See the 4% rule and your FIRE number or try the FIRE calculator. To keep track of real holdings and allocation across accounts as the balance grows, the Portfolio Tracker & Rebalancer template does it in Excel or Google Sheets.
This article is educational and not financial advice. Example returns are assumptions, not predictions.