Use the rule of 72: divide 72 by the annual return in percent. At 8 percent a year, money doubles in about 9 years; at 6 percent, about 12 years. It is an approximation for compound growth, and a free compound interest calculator gives the exact year-by-year figures in your browser.
Free compound interest calculator. See how savings grow with regular contributions, compounding frequency, rate and time, plus interest earned instantly.
Open Compound Interest Calculator → Free toolFree simple interest calculator. Compute interest and final balance from principal, annual rate and time using the classic I = P x r x t formula.
Open Simple Interest Calculator →Compound growth doubles when (1 + rate) raised to the number of years reaches 2. Solving that with logarithms gives roughly 69.3 / rate, and 72 is used instead because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12. For the single-digit rates typical of savings and index investing, the rule lands within a few months of the exact answer.
The same rule estimates how fast inflation halves your buying power: at 3 percent inflation, prices double, and cash halves in real terms, in about 24 years. It is a quick way to sanity-check any growth or decay rate before running precise numbers.
The rule assumes a lump sum and a steady rate. If you are adding monthly contributions, compounding at different frequencies, or comparing scenarios, a compound interest calculator shows the exact balance curve, total contributions and total interest, all computed locally.
No, it is an approximation of the logarithmic formula. It is closest for rates roughly between 4 and 12 percent, which covers most realistic cases.
A similar shortcut is the rule of 114: divide 114 by the rate for an approximate tripling time.
No. With regular contributions the balance grows faster than the rule suggests, use the compound interest calculator for that case.