How the 4% rule calculator works
Where 4% comes from
The "Trinity study" lineage: withdraw 4% of the starting balance, adjust for inflation annually, and historical US portfolios survived 30 years in nearly every sequence. It is a statement about history, not a guarantee — which is why the useful frame is a range (3–5%) tuned to horizon, flexibility, and portfolio mix.
Why the first years dominate
Withdrawal plans die by sequence: heavy withdrawals during early bear markets permanently drain shares that would have recovered. This is why 3.25–3.5% feels right for 40+ year retirements, why a cash buffer of 1–2 years of spending helps psychologically and mathematically, and why flexible spending (cutting 10% in bad years) adds more safety than any extra percentage point of return.
Layering income sources
Social Security, pensions, and rental income each reduce what the portfolio must carry — the calculator's total-income line shows the layering. A $1.2M egg paying $4,000/month plus $1,800 Social Security supports a materially different life than the egg alone. Claiming age, COLA features, and taxation belong in that conversation too.