Td = Δt × ln(2) / ln(N₂/N₁)
Doubling time (Td) measures how long it takes for a cell population to double. Td = Δt × ln(2) / ln(N₂/N₁). Typical values: E. coli 20 min, yeast 90 min, HeLa cells ~24 hours, primary fibroblasts 36-48 hours, stem cells 24-36 hours. Doubling time is essential for: planning experiments (when will cells be confluent?), comparing growth conditions, assessing drug cytotoxicity, and monitoring cell line health. A sudden increase in Td may indicate contamination, senescence, or media problems. The population doubling level (PDL) tracks cumulative doublings — primary cells senesce at PDL 50-70 (Hayflick limit). Cancer cells bypass this limit (immortalization).