Quick estimates
Rule of 72 and Cost of Delay (A Quick Way to See Why Waiting Matters)
The Rule of 72 offers a fast way to estimate doubling time. Waiting uses up part of that window. For precise numbers, compare start-now vs start-later in the Cost of Delay Calculator.
Published: December 22, 2025 · Updated: December 22, 2025 · By FinToolSuite Editorial
Test timing in the calculator
Rule of 72 is quick; the calculator is exact and compares start dates.
Disclaimer
Educational purposes only; not financial advice. Rule of 72 is a rough approximation. Real returns vary and investments can go down as well as up. Fees, taxes, inflation, and rules vary by country/provider.
Quick answer
- Rule of 72: 72 ÷ rate(%) ≈ years to double (rough).
- Waiting uses up part of that doubling window.
- Use the calculator for precise start-now vs start-later comparisons.
The Rule of 72
Rule of 72 is a quick way to estimate doubling time: divide 72 by your rate (%) to get approximate years to double. It works best for moderate, steady rates and ignores fees, taxes, and inflation.
Tiny Rule of 72 table (approximate)
| Rate (illustrative) | Years to double (≈ 72 ÷ rate) |
|---|---|
| 3% | ~24 years |
| 5% | ~14.4 years |
| 7% | ~10.3 years |
Approximate only—actual results will differ.
Connecting Rule of 72 to cost of delay
If doubling takes ~14 years at 5%, waiting one year is like giving up about one year of that doubling window. Longer waits consume more of the compounding timeline in the model.
Comparison example (illustrative)
Lump sum: £10,000, assumed 5%, 15-year horizon. Comparing start now vs start in 12 months. Rule of 72 hints at ~14 years to double at 5%; the calculator shows the exact estimated end values and the gap.
| Scenario | Start | Estimated end value (calculator) |
|---|---|---|
| Start now | Today | [calculator output] |
| Start in 12 months | +12 months | [calculator output] |
Plug these inputs into the Cost of Delay Calculator to see the exact projected values and the opportunity cost of waiting.
Try these inputs in the calculatorWhy the calculator is better than Rule of 72
- Exact compounding math with your inputs.
- Direct comparison of start-now vs start-later.
- Save scenarios and test low/base/high assumptions.
- Handles your horizons and amounts without mental math.
FAQ
What is the Rule of 72?
A quick shortcut: 72 divided by the rate (%) ≈ years to double. It’s an approximation.
Does Rule of 72 work for any rate?
It’s roughest at very low or very high rates. It’s mainly a mid-range shortcut, not an exact formula.
How does it relate to cost of delay?
If doubling takes time, waiting removes part of that window. The calculator shows the precise gap for your inputs.
How much does a 1-year delay matter?
Rule of 72 gives intuition; the calculator shows the exact estimated difference for your horizon and rate.
What assumptions should I use?
Test low/base/high scenarios. Keep the same assumptions across start dates for fair comparisons.
Do fees, taxes, or inflation affect Rule of 72?
Rule of 72 ignores them. You can approximate by using a lower “net” rate when testing in the calculator.
Are calculator results guaranteed?
No. Both the shortcut and the calculator are based on assumptions. Real outcomes can differ and can be negative.
Why might the calculator differ from Rule of 72?
The calculator uses exact compounding and your specific inputs, while Rule of 72 is a rough mid-range shortcut.
Next steps
Run three scenarios: start now, start in 1 year, start in 5 years. Compare the chart gap and the end values to see the modeled cost of waiting.