Finance 2026-05-03 3 min read By Konvertibly Team

Rule of 72: The Mental Maths Shortcut Every Investor Should Know

Use the Rule of 72 to instantly estimate how long money takes to double at any interest rate — and apply it to debt, inflation, and investment decisions.

The Rule of 72: Instant Doubling-Time Mental Maths

The Rule of 72 is one of the most useful mental shortcuts in personal finance. Divide 72 by an annual interest rate and you get the approximate number of years it takes for money to double. No calculator required.

The Formula

Years to double ≈ 72 ÷ annual rate (%)

6% rate → 72 ÷ 6 = 12 years
9% rate → 72 ÷ 9 = 8 years
12% rate → 72 ÷ 12 = 6 years

Doubling Time Reference Table

1% (savings account): 72 years
3% (inflation): 24 years — purchasing power halves
4% (index bond): 18 years
7% (equity market average): ~10 years
10% (growth stocks target): 7.2 years
18% (credit card APR): 4 years — debt doubles in 4 years if unpaid
36% (payday loan): 2 years

Applying It to Debt

The Rule of 72 is equally powerful for debt. A £3,000 credit card balance at 20% APR with no payments doubles to £6,000 in 72÷20 = 3.6 years. This is why minimum payments on high-interest debt are financially catastrophic.

Reverse: Finding the Rate Needed to Double in X Years

Required rate = 72 ÷ desired years
Want to double in 6 years → need 72 ÷ 6 = 12% per year

Calculate exact doubling time: Free Rule of 72 Calculator