The rule of 70: why small growth rates are a big deal

Growth rates sound boring and small — 2% here, 3% there — which is exactly why most people underestimate them catastrophically. A single percentage point seems trivial in a year and turns out to be life-changing over a generation, because growth compounds: each year's growth builds on the last. The rule of 70 is a simple mental shortcut that makes this vivid, and once you internalize it, you'll never again dismiss the difference between 2% and 3% as a rounding error. It's one of the most useful pieces of arithmetic in all of personal finance.

The rule itself

To estimate how long something takes to double, divide 70 by its annual growth rate. Growing at 2% a year? It doubles in about 70 ÷ 2 = 35 years. At 7%? About 70 ÷ 7 = 10 years. That's it. The rule works for anything that compounds: an economy, an investment, a population, or — running in reverse — the erosion of purchasing power from inflation. The magic it reveals is how sensitive the doubling time is to the rate: doubling the growth rate more than halves the time, because you're compounding faster on a faster-growing base.

Why one point matters so much

The gap between 2% and 3% growth looks like nothing — but at 2%, an economy (or a nest egg) doubles in about 35 years; at 3%, it doubles in about 23. Over a long lifetime, the 3% economy doubles roughly three times while the 2% economy doubles barely twice, ending up dramatically richer. This is why economists obsess over seemingly tiny differences in growth rates, and why a country growing 1 point faster than its neighbor pulls far ahead over decades. The same arithmetic governs your savings: a portfolio earning 7% versus 5% doesn't end up 40% bigger — over 35 years it ends up more than double, because the faster rate compounds through more doublings.

The rule works in reverse for inflation

The same math shows how inflation destroys purchasing power. At 3% inflation, prices double — and your dollar's value halves — in about 23 years. At 7% inflation (as in bad periods), your money loses half its purchasing power in just a decade. This is why even 'modest' inflation is a serious long-term force, and why cash sitting at a yield below inflation is quietly halving in real value on a predictable schedule. Run the rule on inflation and you understand instantly why letting money sit idle is not 'safe' — it's a slow, compounding loss on a timer you can calculate.

The bottom line

The rule of 70 — divide 70 by a growth rate to find its doubling time — is the antidote to underestimating small percentages. It reveals why one extra point of growth compounds into enormous differences over a lifetime, why economists fight over decimals, and why even modest inflation halves your money on a predictable schedule. Keep it in your head as a gut check: it turns the boring, dismissible percentages of economic and financial life into the powerful, compounding forces they actually are — and makes the case for growing your money, and not letting it sit idle, impossible to ignore.