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math.finance-rule-of-72 Calculator
Applies the Rule of 72 (years to double ≈ 72/r%) and variants (Rules of 69 and 70) to estimate investment doubling time. The Rule of 72 is accurate within 2% for rates between 4% and 12% — an excellent mental arithmetic shortcut.
Inputs
R
Reference formula or conversion factor shown for context.
P
Original borrowed or invested amount before interest. Interest accrues on the outstanding principal balance.
Results
doubling time (rule 72)
The computed duration or time value.
exact doubling time
The computed duration or time value.
value after doubling
The computed numeric or monetary value.
future value
What your investment will be worth at the end of the period. The power of compounding: $10,000 at 8% for 30 years grows to about $100,000.