What a coast number is
A coast number is the amount you'd need invested once, today, so that growth alone reaches a target by retirement, with no more deposits. It's the compound growth formula run backward: coast number = target ÷ (1 + r)^(years to retirement).
The ladder from the video
For $1,000,000 by 65 at a hypothetical 7% a year, compounded once a year: at 25 (40 years) the coast number is $66,780.38. At 30 (35 years) it's $93,662.94, at 35 (30 years) $131,367.12, and at 40 (25 years) $184,249.18. Check the first one: $66,780.38 × 1.07^40 = $1,000,000.00.
Why waiting costs so much
Starting at 40 instead of 25 takes about 2.76 times as much up front, because the money loses 15 years of compounding. At 7%, money doubles roughly every ten years (1.07^10 ≈ 1.97), so each decade of waiting roughly doubles the lump sum needed.
What this page leaves out
The rate is hypothetical and held constant; real returns jump around and can be negative, and past returns don't predict future returns. There are no fees, taxes or further deposits. And there's no inflation: a future $1 million will buy less than $1 million does today. Treat the result as an illustration of the math, not a plan.
Sources
Last reviewed: Oct 9, 2026