Use this OD600 calculator to convert OD600 readings to CFU/mL, cells/mL, and dry cell weight (DCW). E. coli preset uses 8 × 108 cells/mL and 0.36 g/L per OD600; yeast, Pichia, Bacillus, and C. glutamicum presets included. Paste 2+ live readings into the induction timing helper to predict when the culture will hit your IPTG, arabinose, methanol, or heat-shift induction OD target.
Adjust these for your specific strain and conditions. Presets load typical literature values.
| Organism | Cells/mL per OD | DCW (g/L) per OD | Linear Range |
|---|---|---|---|
| E. coli (BL21, K-12) | 8 x 108 | 0.36 | OD 0.04 – 0.40 |
| S. cerevisiae | 3 x 107 | 0.50 | OD 0.05 – 0.50 |
| P. pastoris | 5 x 107 | 0.55 | OD 0.05 – 0.50 |
| B. subtilis | 5 x 108 | 0.30 | OD 0.04 – 0.40 |
| CHO / HEK293 | 1 x 106 | — | Use VCD instead |
| C. glutamicum | 3 x 108 | 0.25 | OD 0.05 – 0.50 |
Paste 2 or more live OD600 readings with timestamps. The tool computes the current specific growth rate (μ) and projects when the culture will reach your induction target. Ideal for planning IPTG, arabinose, methanol, or tetracycline addition.
Optical density at 600 nm (OD600) is the standard method for estimating microbial cell density in liquid culture. It measures light scattering (turbidity), not true absorbance. The relationship between OD600 and cell number follows Beer-Lambert law only at low densities (typically OD < 0.4-0.7), where single scattering dominates.
At higher densities, multiple scattering causes significant nonlinearity — an OD of 2.0 may represent 5x more cells than an OD of 1.0, not 2x. For accurate measurements above the linear range, always dilute samples to OD 0.1-0.4 and multiply by the dilution factor.
Conversion factors between OD600 and cells/mL or dry cell weight vary significantly by organism, strain, growth phase, media composition, and spectrophotometer model. The values in this calculator are typical literature values — for precise work, generate your own calibration curve by plating serial dilutions (CFU) or drying pellets (DCW) at known OD values.
Multiply the diluted-corrected OD600 by your organism's cells-per-OD factor. For E. coli, OD600 = 1.0 ≈ 8 × 108 CFU/mL; for S. cerevisiae, OD600 = 1.0 ≈ 3 × 107 CFU/mL; for P. pastoris, OD600 = 1.0 ≈ 5 × 107 CFU/mL; for B. subtilis, OD600 = 1.0 ≈ 5 × 108 CFU/mL. So OD 0.4 of E. coli reads as 0.4 × 8 × 108 = 3.2 × 108 CFU/mL. Keep readings below OD 0.7 (linear range) and multiply by your dilution factor for samples diluted above that. For a strain-specific number, plate serial dilutions at three OD points and fit CFU vs OD600 by linear regression.
For E. coli K-12 and BL21 strains in defined or rich media, 1 OD600 ≈ 0.30–0.40 g/L dry cell weight, with 0.36 g/L per OD600 the most commonly cited textbook value (Glazyrina et al. 2010; Sezonov et al. 2007). Reported numbers vary because dry cell weight is sensitive to cell size, growth phase, salt content of the wash buffer, and drying protocol. The 0.3 g/L per OD600 figure used in many high-cell-density fed-batch references applies to stationary-phase washed pellets dried at 105 °C to constant weight. For exponential-phase BL21 in TB or LB, 0.36 g/L per OD600 is a closer fit. Calibrate by sampling 3–5 OD points across a growth curve and drying triplicate pellets.
For E. coli in standard conditions, OD600 of 1.0 corresponds to approximately 8 × 108 CFU/mL (0.8 billion cells per mL). This varies by strain, media, growth phase, and spectrophotometer. Always calibrate your own OD-to-CFU correlation for precise work.
Beer-Lambert law is only linear up to about OD 0.4–0.7 for most spectrophotometers. Above this range, multiple scattering causes underestimation of true cell density. Dilute samples to OD 0.1–0.4 and multiply by the dilution factor for accurate results.
The conversion factor is organism-specific. For E. coli, 1 OD600 ≈ 0.36–0.39 g/L dry cell weight (DCW). For S. cerevisiae, 1 OD600 ≈ 0.30–0.60 g/L DCW. For P. pastoris, 1 OD600 ≈ 0.55 g/L DCW. For B. subtilis, 1 OD600 ≈ 0.30 g/L DCW. These factors should be calibrated for your specific strain and conditions by drying cell pellets and plotting OD vs DCW.
Standard practice is to induce E. coli at mid-log phase, OD600 0.4–0.8. From two or more live OD readings, you can compute the current specific growth rate (μ) and project the time to hit your target induction OD. For example, if μ = 0.7 h−1 and current OD is 0.25, induction target OD 0.6 will be reached in ln(0.6/0.25)/0.7 = 1.25 hours. Use this to plan IPTG addition timing. See our IPTG induction optimisation guide for protocol detail.
For inoculation, target a starting OD600 of 0.02–0.05 from an overnight starter; this gives roughly 4–6 hours to reach mid-log (OD600 0.4–0.6) at 37 °C in LB or TB. For induction, OD600 0.4–0.8 is the standard window for T7/lac, tac, and pBAD systems — early enough that nutrients and ribosomes are abundant, late enough for adequate biomass. Below OD 0.2, per-cell yield is high but volumetric yield is poor. Above OD 1.5, acetate overflow in E. coli BL21 starts to inhibit expression. Use the induction timing helper above to plan IPTG addition from live readings.
Different spectrophotometers have different bandwidths, detector sensitivities, and path lengths. A benchtop UV-Vis, a plate reader, and a portable densitometer will all give different OD values for the same sample. Always use the same instrument for comparable results, and calibrate your own OD-to-cell conversion.
OD600 is not recommended for mammalian cells. They are much larger than bacteria (10-15 um vs 1-2 um), settle quickly, and the OD-to-cell correlation is poor. Use a hemocytometer, automated cell counter (Vi-CELL, Countess), or trypan blue exclusion for mammalian cell density. Our Cell Counting Calculator is designed for this.