run the numbers

Don't take money advice. Run the simulation.

MoneySims is a set of free, interactive money simulators — compound interest, retirement Monte Carlo, options payoffs, tax brackets, debt payoff — built with open assumptions and no hand-waving. They're the same engines that power our YouTube channels: every video that cites a number links back to the exact scenario used here, so you can rerun it, change the inputs, and see the result yourself instead of taking our word for it.

Retirement & Compounding

Compounding curves, retirement survival odds, and how long your money actually lasts.

Compound Interest

Watch a starting balance and monthly contributions compound over decades — principal, contributions, and growth split apart.

Project your 401(k) with salary growth, employer match, and contribution limits.

Monte Carlo Retirement

Run thousands of simulated retirements against random market sequences and see your plan's survival rate.

Money Longevity

A nest egg, a withdrawal rate, and a return assumption — see the exact year the money runs out.

Compare a lump sum against an annuity's guaranteed payments, with payout period and rate.

Trading Math

Payoff diagrams, position sizing, and backtested strategy math. Education only — no picks, no signals.

Payoff diagrams for calls, puts, and covered calls at expiration — breakevens, max gain, max loss.

Position Size / Kelly

Risk-based position sizing plus the Kelly criterion — see what over-betting does to a simulated account.

DCA vs Lump Sum

Dollar-cost averaging against lump-sum investing across simulated market paths.

Tax Math

Brackets, capital gains, Roth conversions, and take-home pay — visualized.

Gross salary to net paycheck: federal brackets, FICA, and 401(k)/HSA pre-tax effects, visualized as a flow.

Short-term vs long-term capital gains tax on a sale, stacked on top of your ordinary income.

Roth Conversion

Convert now and pay the tax, or leave it deferred? Side-by-side lifetime after-tax outcomes.

Tax Loss Harvesting

What harvesting a loss actually saves — offsets, the $3,000 ordinary-income cap, and carryforward.

Debt & Payoff

What interest really costs, and the fastest mathematical way out.

Credit Card Payoff

Minimum payments vs fixed payments — the years and interest between them, animated.

Snowball vs Avalanche

Stack multiple debts and compare snowball (smallest first) against avalanche (highest APR first).

Mortgage Payment

Full monthly cost of a home — principal & interest, property tax, insurance, HOA, and PMI — with the income the 28% rule says you'd need.

Mortgage Payoff

Extra principal payments against a mortgage: payoff date moved up, interest saved.