Thinking

The calculator is an instrument of an argument. This is the argument.

• The risk nobody models

  Running out of money gets all the attention. Dying with a decade of unspent work in the account gets none, and it happens far more often.

• Where 4% actually came from

  One start year in a century, one bond index, and a retiree who never once looks up from the plan. Useful as a floor. Ruinous as a budget.

• Guardrails, and what they cost

  Guyton-Klinger converts 'you might run out' into 'you might have to spend less'. That is a much better trade — and it is not free.

• How the numbers are made

  Every data source, every assumption, and every place this model is wrong. Written so you can check it rather than trust it.