Monte Carlo Stock Investment Simulator (VND)
A compound-interest calculator gives you one number from one fixed rate of return. Real equity investing never behaves that smoothly — some years are up 30%, others down 20% — so "how much will 100 million VND become in 20 years?" has no single answer, only a range of outcomes with different probabilities. This tool draws that range using Monte Carlo simulation.
Instead of assuming your portfolio grows in a straight line, it generates a thousand random market paths (1,000 runs by default). Each month it applies a random "shock" around the expected return you enter, scaled by the volatility you set. It then sorts the thousand ending balances and reports three landmarks: the 10th percentile (downside), the 50th percentile (median, the typical outcome) and the 90th percentile (upside). With the default inputs — 100,000,000 VND starting capital, 5,000,000 VND/month, 20 years, an illustrative 8%/year return and 15% volatility — the median lands near 3.130.806.690 VND, but the worst 10% of paths finish below 1.734.890.899 VND while the best 10% clear 5.713.419.795 VND.
The key intuition: higher volatility widens the range, stretching both ends — the upside gets better but the downside gets worse too. That is why this page always shows a "probability gap" (the ratio of the 90th to the 10th percentile) rather than a single flattering figure. One caveat throughout: the 8%/year return and 15% volatility are illustrative assumptions chosen so the math is easy to follow — they are not forecasts for the VN-Index or any specific stock. Enter numbers that reflect your own expectations and risk tolerance.
How Monte Carlo simulation works
Geometric Brownian Motion (GBM)
Each simulated path advances in monthly steps using the geometric Brownian motion equation:
P(t+1) = P(t) × e^((μ − σ²/2)·Δt + σ·√Δt·Z) + C
| Symbol | Meaning |
|---|---|
| P(t) | Portfolio balance at time t |
| μ | Expected annual return as a decimal (8% = 0.08) |
| σ | Annual volatility as a decimal (15% = 0.15) |
| Δt | Time step = 1/12 (one month) |
| Z | A standard-normal random draw (that month's market shock) |
| C | The monthly contribution |
The (μ − σ²/2) term is the drift adjusted for the fact that returns are log-normal — which is exactly why high volatility drags the median below the headline return. The contribution C is added at the end of each month.
Why a thousand runs?
A single path is meaningless — it is just one roll of the dice. Running 1,000 paths and sorting the ending balances lets the tool read off percentiles: the 10th percentile means "90% of scenarios did better than this", the 90th means "only 10% did better". More runs make the percentiles steadier but slower to compute. The tool uses a pseudo-random generator seeded from the inputs themselves, so identical inputs always reproduce the identical chart — no reshuffling on reload.
Reading the three numbers
- 50th percentile (median): the typical outcome — half the scenarios do better, half worse. Use it for your base plan.
- 10th percentile (downside): the stress test. If this still meets your goal, the plan is robust.
- 90th percentile (upside): the potential reward — do not treat it as a given.
What the model leaves out
The model assumes log-normally distributed returns with constant μ and σ over the whole horizon. It does not model fat-tail risk, black-swan events, taxes, trading fees or correlations between assets. Past performance does not guarantee future results — treat this as an educational tool for building intuition about risk and ranges, not a promise of returns.
Worked example: 100M VND + 5M/month for 20 years
Illustrative assumptions: 8%/year expected return, 15%/year volatility, 1,000 runs. Total you actually pay in over 20 years = 100,000,000 + 5,000,000 × 12 × 20 = 1.300.000.000 VND.
| Milestone | Downside (P10) | Median (P50) | Upside (P90) |
|---|---|---|---|
| Year 1 | 145.421.469 | 168.204.700 | 197.931.050 |
| Year 5 | 373.472.934 | 495.065.444 | 655.594.682 |
| Year 10 | 705.985.154 | 1.073.309.935 | 1.663.267.512 |
| Year 15 | 1.191.719.699 | 1.917.038.137 | 3.239.051.562 |
| Year 20 | 1.734.890.899 | 3.130.806.690 | 5.713.419.795 |
After 20 years the median reaches about 3.130.806.690 VND — against the 1.300.000.000 VND you actually paid in, that is 1.830.806.690 VND of growth. But do not read only the median: the downside (P10) is just 1.734.890.899 VND while the upside (P90) is 5.713.419.795 VND. The probability gap (P90 / P10) is 3.3x — the good case delivers roughly that many times the bad case. That spread is the price of holding equities instead of a term deposit: higher expected return, far more uncertainty.
How volatility reshapes the range
Keeping the same plan (100M + 5M/month, 20 years, 8%/year) and changing only the volatility assumption:
| Volatility assumption | Downside (P10) | Median (P50) | Upside (P90) | Gap (P90 / P10) |
|---|---|---|---|---|
| 10% (lower risk) | 2.107.427.525 | 3.192.912.949 | 4.839.817.552 | 2.3x |
| 15% (base) | 1.734.890.899 | 3.130.806.690 | 5.713.419.795 | 3.3x |
| 30% (higher risk) | 705.377.528 | 2.033.114.026 | 7.399.977.405 | 10.5x |
Notice how, as volatility climbs from 10% to 30%, the upside balloons but the downside shrinks and the P90/P10 gap widens from 2.3x to 10.5x. Volatility is not free — it buys extra upside by accepting extra risk of loss. If you are aiming at a specific target, pair FiMo's compound-interest calculator for the base case with this tool to stress-test "what if the market is poor?".
Frequently asked questions
What is Monte Carlo simulation in investing?
Monte Carlo runs thousands of random scenarios instead of assuming one fixed return. Each month the tool applies a random "shock" around the expected return you enter, scaled by your volatility input. After 1,000 runs it sorts the ending balances and reports three percentiles: downside (P10), median (P50) and upside (P90). The result is a probabilistic range of outcomes, which is far more realistic for equities than a single number.
How much will 100M VND plus 5M/month grow to in 20 years?
There is no single figure — it depends on the market. Under an illustrative 8%/year, 15% volatility assumption: the median (P50) is about 3.130.806.690 VND, the downside (P10) about 1.734.890.899 VND, and the upside (P90) about 5.713.419.795 VND. You pay in 1.300.000.000 VND in total. These are assumptions, not a forecast — enter your own expectations.
What do the 10th, 50th and 90th percentiles mean?
After sorting 1,000 outcomes low to high: the 50th percentile (median) is the middle result — half the scenarios beat it, half fall short, so use it for your base plan. The 10th percentile means 90% of scenarios did better than this — the downside, for stress-testing. The 90th percentile means only 10% did better — the upside, which you should not count on.
How does volatility affect the outcome?
Higher volatility widens the range. For the same 100M + 5M/month plan over 20 years at 8%/year: at 10% volatility the P90/P10 gap is only 2.3x; at 30% volatility it widens to 10.5x — the upside balloons but the downside gets much worse. Volatility buys extra upside by accepting extra risk of loss; it is never free.
Can this simulator predict the VN-Index?
No. The 8%/year return and 15% volatility on this page are illustrative assumptions chosen to make the math easy to follow, not forecasts of the VN-Index or any stock. The model assumes log-normal returns with constant parameters and ignores fat-tail risk, black-swan events, taxes and fees. Use it to build intuition about risk, not to predict prices.
Why do repeated runs give the same result?
Because the tool uses a pseudo-random generator seeded from your inputs. Identical inputs always reproduce the identical range, so the chart never reshuffles on reload and you can verify the numbers. Only changing the starting amount, contribution, years, return, volatility or the number of runs will change the output.
How is this different from a compound-interest calculator?
A compound-interest calculator gives one number from one fixed, smooth rate — fine for a term deposit. This tool acknowledges that equities fluctuate, so it returns a probabilistic range instead of a point estimate. Use compound interest for the base case, then use this simulator to see "what if the market is poor?" and "how wide is the spread?".