Why today's expenses are the wrong number
Spending 3,000 a month today does not mean needing 3,000 a month at retirement. At 6% inflation over 28 years, that same lifestyle costs about 15,300 a month. Planning around today's figure understates the requirement by a factor of five.
This calculator inflates your current expenses forward to your retirement date first, then works out the corpus required to sustain that inflated level for the whole of retirement.
The role of real returns
During retirement your corpus keeps earning while you draw from it, but inflation keeps raising what you need to draw. What matters is the real return: roughly your investment return minus inflation. Earning 8% with 6% inflation gives a real return near 1.9%, which is what genuinely sustains withdrawals.
Small changes to that gap move the required corpus enormously. Raising your assumed return by two points can cut the target substantially, which is exactly why conservative assumptions matter more here than anywhere else in personal finance.
What this deliberately leaves out
The figure excludes any state pension, employer pension or rental income you expect, so subtract the corpus equivalent of those before judging the gap. It also assumes constant real spending, whereas real retirements often cost more early on for travel and more later for healthcare.