Financial Goal Calculator
Work out how much to save each month to hit a savings goal at your chosen confidence level — or test whether what you're already saving is actually enough, run against thousands of simulated market paths.
Solved by testing contribution amounts against 5,000 randomized paths until the smallest one clears your confidence level — the same engine as the Monte Carlo simulator, run in reverse. Reproducible seed: 1. A normal-returns model understates rare crashes — treat the required contribution as a well-reasoned estimate, not a guarantee. Educational only, not financial advice.
One goal, two very different answers
Say you have $10K saved, want $500K in 20 years, and expect a 7% return with 15% volatility. A simple calculator gives you one number based on the average return. Ask instead for an 80% chance of actually getting there across thousands of simulated paths, and the required contribution here is $1K/mo. Push the bar to a 95% chance and it jumps to $2K/mo — the tool is compensating for the unlucky paths, not just the average one.
That gap is the entire value of running this as a simulation instead of a single formula: the "right" contribution depends entirely on how much risk of falling short you are willing to accept, and a straight-line calculator can't show you that trade-off at all.
How it works
Pick a mode. How much do I need to save? binary-searches for the smallest monthly contribution that reaches your goal at the confidence level you choose. Test my own contribution instead runs a contribution you already have in mind through thousands of paths and reports the probability it gets you there. Both run on the same engine as the Monte Carlo simulator: each simulated year draws a random return from your mean and volatility, compounds the balance, and adds that year's contribution.
Honest limitation: this model draws returns from a normal distribution, which understates the odds of rare crashes. Treat the required contribution as a well-reasoned estimate built on your own assumptions, not a promise. Educational only, not financial advice.
Keep going
Already retired and drawing down instead of saving up? Use the retirement withdrawal calculator. For the straight-line version without randomness, the compound interest calculator.
Common questions
How does this differ from a simple savings calculator?
A simple calculator assumes the same fixed return every year and gives you one number. This tool runs your assumptions through thousands of randomized market paths and reports a required contribution at a chosen confidence level — for example, the monthly amount that gets you to your goal in 8 out of 10 simulated futures, not just the single average one.
What confidence level should I use?
There is no universal answer — it is a trade-off between how much you can actually save and how sure you want to be. 80% is a reasonable middle ground for a goal with some flexibility (like a house down payment); 95% is more conservative and will ask for a noticeably higher monthly contribution for the same goal.
Why does the required contribution change so much with volatility?
Higher volatility widens the whole range of outcomes, including the bad ones. To keep the SAME confidence level with a wider range of bad outcomes, the tool has to solve for a higher contribution — it is compensating for the paths where markets disappoint, not just the average path.
Can I use this for a house down payment or a shorter goal?
Yes — set the years to your actual timeline. For short horizons (under 5 years), consider using a much lower expected return and volatility than a long-run stock allocation; a goal you need soon is usually better held somewhere more stable than the market.