Retiring at 40 means funding maybe forty-five years of life from a working window of around twenty — your money has to last longer than you spent earning it. The uncomfortable part most "retire at 40" articles skip is that the investment return matters far less than your savings rate: at 40 you're usually looking at putting away half your income or more, and no clever fund selection rescues a savings rate that isn't there.
This calculator shows whether the number actually holds rather than whether it sounds nice. Put in your age, savings and contributions, set an expected return and an inflation figure (it adjusts the projection for you), and watch whether the pot survives to a normal lifespan. The pre-filled example is a 28-year-old aiming for 40 — change it to your own and see the gap. The deeper market stress-testing (the "Survival Test") is a paid extra, but the core projection is free. Just try it, and if 40 looks out of reach on today's savings rate, the tool tells you plainly instead of pretending.