Money, Checked · the arithmetic, shown

Most money advice skips the part where you check it.

These calculators publish the model, state every assumption, and say so when the answer is too close to call. Change the inputs and watch the answer move.

Roth or traditional 401(k)?

There is no single crossover salary. Across the assumptions that matter it moves between $53,000 and $228,000 — see where yours lands.

Rent or buy?

Break-even is 6.7 years at today's rate. At 8% it never arrives inside thirty years. Put your own numbers in.

How these are built

Every figure is computed by a model, never typed in by hand.
Each model exists twice — a Python reference and a JavaScript port — and a test fails the build if they ever disagree.
Where a result sits inside the noise floor of its own assumptions, the calculator says "too close to call" rather than naming a winner.
Tax year 2026 figures throughout.