At typical market returns, starting to invest 10 years earlier roughly doubles your final balance.
1.07¹⁰ ≈ 1.97. The last doubling of any compound curve is the biggest — and you only get it by starting early.
It assumes the extra ten years are at the same return, which is the assumption most likely to fail: a decade beginning in 2000 returned close to nothing in real terms. And it says nothing about whether you could have afforded to invest then — starting earlier with less can lose to starting later with more.
At 7% the rule says 2× and the exact answer is 1.967× — 1.7% high. It holds to within 10% from 6.2% to 8.3%, and drifts outside that.
Doubling needs 7.2% a year for ten years, which is why the rule feels true — it is quoting long-run equity returns without saying so. At a cautious 4% ten years buys you half again, not double; at 11% it buys nearly triple.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
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