Savings and investing guide
The Rule of 72: Estimate How Long Savings May Take to Double
The Rule of 72 gives a rough estimate of doubling time: divide 72 by an annual percentage rate. At a constant 6% annual return, 72 ÷ 6 suggests about 12 years. It is a mental shortcut, not a forecast; actual savings rates, investment returns, taxes, fees, and deposits can change the result.
How the shortcut works
Use a percentage number rather than a decimal: 72 ÷ annual rate in percent ≈ years to double. At 4%, the shortcut estimates 18 years; at 8%, it estimates 9 years. It assumes growth compounds and the rate remains constant. A negative or zero rate does not produce a meaningful doubling estimate.
A $5,000 example
At a hypothetical 6% annual rate compounded once per year, the Rule of 72 estimates about 12 years for $5,000 to reach $10,000 without additional deposits. The compound-interest formula gives a more precise result for that assumption: solve 2 = (1.06)^t, which yields about 11.9 years. Neither figure predicts what a real account will earn.
Why the estimate is approximate
The shortcut approximates the logarithmic relationship between a growth rate and time. It is generally closer for ordinary positive rates than for extremely high rates, and the precise answer depends on how compounding is defined. Monthly compounding, deposits, withdrawals, changing rates, and fees require a more complete calculation. Do not treat a historical investment return as a guaranteed future rate. It also ignores inflation: doubling the number of dollars does not mean the dollars buy twice as much after prices change.
Compare doubling with a full projection
Use the shortcut to sanity-check a scenario, then enter the starting amount, rate, time, and any recurring contributions in a compound-interest calculator. For example, deposits of $100 each month mean the account can reach $10,000 sooner than a $5,000 lump sum alone, but the exact date depends on deposit timing and the modeled rate.
When it is useful
The rule can make percentage growth easier to discuss, such as comparing two hypothetical constant rates. It does not show volatility, the chance of loss, purchasing power after inflation, tax treatment, account access, or whether an investment is suitable. For a savings decision, check current account disclosures and consider the need for the money and its risk.
Keep the question precise
“How long to double?” is different from “How much will I have by a date?” If you need a deadline or recurring contribution estimate, model the actual starting balance and deposits. If the rate changes, run multiple scenarios or calculate periods separately rather than applying one shortcut to the whole horizon. For instance, a $5,000 balance growing at an assumed 3% takes much longer to double than at 9%; the shortcut shows sensitivity but does not say either rate is available or appropriate.
| Annual rate | 72 ÷ rate | Approximate years to double |
|---|---|---|
| 4% | 72 ÷ 4 | 18 |
| 6% | 72 ÷ 6 | 12 |
| 8% | 72 ÷ 8 | 9 |
Frequently asked questions
Is the Rule of 72 exact?
No. It is a shortcut for a constant positive compounding rate. Use the compound-interest formula for a more precise result under stated assumptions.
Does it include monthly deposits?
No. It estimates doubling from a lump sum. Model recurring deposits separately.
Sources and further reading
Try the calculator
Use the Compound Interest Calculator to explore your own numbers with the assumptions shown in the tool.
Related guides
Educational content only; not individualized financial, tax, or legal advice.