Cell doubling time (td) is calculated from two cell counts taken at different times using the formula: td = t × ln(2) / ln(N / N0), where N0 is the initial cell count, N is the final cell count, and t is the elapsed time in hours. For example, if you seed 1 × 105 cells and count 8 × 105 cells after 72 hours, td = 72 × 0.693 / ln(8) = 24 hours. Both counts must be during exponential (log-phase) growth for accurate results.
Population doubling level (PDL) tracks the cumulative number of times a cell population has doubled since first culture. Per-passage PDL = 3.322 × log10(Nharvested / Nseeded). Cumulative PDL is the running sum across all passages. Unlike passage number, PDL accounts for different split ratios and is the standard metric for tracking cell age in primary cells, which senesce at a defined PDL (the Hayflick limit, typically 50-70 for human diploid fibroblasts).
For practical purposes, doubling time and generation time are the same: the time for a population to double. Generation time is more common in microbiology (the time for one cell division cycle), while doubling time is preferred in mammalian cell culture. Both relate to specific growth rate by td = ln(2) / μ. Typical values: E. coli 20 minutes, yeast 90 minutes, CHO cells 20-24 hours, MSCs 30-50 hours.
Cells are in exponential phase when they are dividing at a constant rate. On a semi-log plot (log cell count vs time), exponential growth appears as a straight line. Practically, cells should be subconfluent (below 80% confluency for adherent cells, or below the density where growth begins to slow for suspension cells). The doubling time formula assumes exponential growth, so both your initial and final counts should be taken within the log phase. Taking one count during lag phase or stationary phase will give an artificially long doubling time.
HEK293 cells double in 20-24 hours in DMEM + 10% FBS at 37 °C. CHO-K1 cells also double in 20-24 hours. Other common cell lines: HeLa 20-24 hours, Vero 22-26 hours, Jurkat 24-28 hours, MDCK 18-22 hours, iPSCs 18-24 hours, MSCs 30-50 hours. If your measured doubling time is significantly longer than the reference value, check for mycoplasma contamination, suboptimal seeding density, or media/supplement degradation.
Specific growth rate (μ, in h-1) and doubling time (td, in hours) are inversely related: μ = ln(2) / td = 0.693 / td. You can also calculate μ directly from two cell counts: μ = ln(N/N0) / t. The specific growth rate is more useful in bioprocess kinetic models because it appears directly in the exponential growth equation N(t) = N0 × eμt and in Monod kinetics μ = μmax × S / (Ks + S).