Td = ln(2) / r ≈ 70 / (growth rate %)
Doubling time is the period needed for a quantity undergoing exponential growth to double. Td = ln(2)/r, where r is the per-period growth rate. The Rule of 70 approximation: Td ≈ 70/(%growth rate). Examples: world population at 1.1%/yr → doubles in ~64 years; bacteria at 3%/min → doubles in ~23 minutes; investment at 7%/yr → doubles in ~10 years. Doubling time applies to: population ecology, cell biology, epidemiology (disease spread), economics (GDP growth), and pharmacology (drug accumulation). Shorter doubling times mean faster growth. Cancer tumor doubling time is a key prognostic indicator.