What FIRE is trying to measure
FIRE stands for financial independence, retire early. The core question is not how much income you earn today, but how much annual spending a portfolio may need to support once paid work becomes optional. That is why most FIRE planning starts with spending and only then works backward into a target portfolio.
This calculator keeps that structure. It uses annual spending and a withdrawal rate to size the target, then checks how long current assets plus annual savings may take to catch that target if inflation and investment returns continue at the selected rates.