Rule of 72 Calculator
Estimate how long it takes to double your money at a given interest rate using the Rule of 72.
Last updated: April 2026 · Source: Bank of England
At 7%, your money doubles in
10.3 years
Exact: 10.24 years
To double in 10 years, you need
7.2%
annual return
The Rule of 72:
Divide 72 by your annual return to estimate how long it takes to double your money. Simple, surprisingly accurate for rates between 2-20%.
Quick reference:
Disclaimer
This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.
How It Works
The Rule of 72 is a mental arithmetic shortcut for estimating how long it takes an investment to double at a given compound annual growth rate. The formula is: Years to Double = 72 / Annual Rate (as a whole number). At 6% growth, money doubles in approximately 72 / 6 = 12 years. At 8%, it doubles in 72 / 8 = 9 years. The rule is most accurate for rates between 2% and 15%.
The mathematical basis is the natural logarithm: the exact doubling time is ln(2) / ln(1 + r), which equals 0.693 / ln(1 + r). For small rates, ln(1 + r) approximates to r, giving 0.693 / r, or roughly 69.3 / r%. The number 72 is used instead of 69.3 because it has more factors (divisible by 2, 3, 4, 6, 8, 9, 12) making mental division easier, and the slight overestimate partially compensates for the approximation error.
The rule also works in reverse: if you want to double your money in a specific number of years, divide 72 by the years to get the required rate. To double in 10 years, you need 72 / 10 = 7.2% annual growth. The rule applies equally to inflation erosion: at 3% inflation, the cost of living doubles every 72 / 3 = 24 years, meaning your £1 buys only 50p worth of today's goods.
Doubling time at various rates using Rule of 72
- Cash savings at 4.5%: 72 / 4.5 = 16 years to double
- Equity investment at 7%: 72 / 7 = 10.3 years to double
- High-growth fund at 10%: 72 / 10 = 7.2 years to double
- Inflation at 3%: purchasing power halves every 72 / 3 = 24 years
- Exact doubling time at 7% (using ln(2)/ln(1.07)): 10.24 years — Rule of 72 estimate of 10.3 is very close
Source: Bank of England
Frequently Asked Questions
- What does the Rule of 72 Calculator do?
- Estimate how long it takes to double your money at a given interest rate using the Rule of 72. All calculations are performed in your browser using official UK rates and thresholds.
- Is this calculator suitable for financial decisions?
- This calculator provides estimates for guidance only. Investment returns are not guaranteed and your capital is at risk. Consider seeking independent financial advice before making investment decisions.
- Are ISA contributions tax-free?
- Yes. The annual ISA allowance for 2025/26 is £20,000. Any interest, dividends or capital gains within an ISA are completely tax-free.