How the simulators work
Every MoneySims tool is a plain function: inputs in, numbers out, no black box. This page explains the mechanics behind that — how the math runs, how randomness is handled, and where the numbers come from.
Pure, deterministic math
The core of every simulator is a pure TypeScript function: given the same inputs, it always returns the same output. There's no server, no database, no hidden state — the calculation you see is the entire calculation. That also means every tool runs entirely in your browser; your inputs never leave your machine.
Reproducible randomness
Some tools — the Monte Carlo retirement simulator, DCA vs lump sum — need randomness to model uncertain markets. Ordinary randomness would make a shared link useless, since reloading it would produce a different result every time. Instead, these tools use a seeded PRNG (mulberry32): the random sequence is generated from a seed value that's part of the scenario. Share a link with a given seed and inputs, and anyone who opens it gets the exact same simulated paths, run for run.
What this means in practice
A "random" Monte Carlo run isn't actually random once it's in the URL — it's a deterministic function of the seed. Change the seed and you get a new set of simulated paths; keep it the same and you get the identical run back, forever.Stated distributional assumptions
Every stochastic tool states, on its own page, exactly what distribution it's drawing from — e.g. annual returns drawn from a normal distribution with a given mean and standard deviation, rather than resampled historical data. These are simplifications of how markets actually behave (real returns aren't normally distributed, and years aren't independent of each other). The assumptions box on each tool spells out exactly what's baked in so you can judge how much to trust the shape of the output, not just the headline number.
Tax figures
Tools that touch the tax code — take-home pay, capital gains, Roth conversions, tax loss harvesting — use 2025 federal figures (brackets, standard deduction, FICA wage base, capital gains thresholds), and say so directly on the page. State taxes, AMT, phase-outs, and most credits are out of scope unless a tool explicitly says otherwise. These are approximations of the federal calculation, not a substitute for a tax preparer or software that files your actual return.
Methodology box + table view, on every tool
Every tool ships two things beyond the chart: an assumptions box listing the model's mechanics in plain language, and a table view exposing the underlying year-by-year (or trial-by-trial) numbers behind the visualization. If you don't trust a chart, open the table and check the arithmetic yourself.