# OD600 to CFU/mL Converter (Calibration Curve Slope) Convert OD600 to CFU/mL via a calibration slope with organism presets, blank correction, dilution handling, and linearity warnings. > Canonical page: https://elysiatools.com/en/tools/bacterial-growth-od - **Category:** Health - **Keywords:** od600, cfu calculator, od to cfu, cell density, growth curve, spectrophotometer, calibration curve, bacterial growth, e coli, cfu/ml, microbiology, lab calculator ## Overview Convert a spectrophotometer OD600 reading into estimated CFU/mL using a calibration slope. Subtract the medium blank, apply an organism preset (E. coli, B. subtilis, S. cerevisiae) or your own CFU-per-OD factor, then scale by the fold-dilution used before the measurement. Optionally enter culture volume to get total CFU. The calculator flags net OD values above about 0.6 as outside the typical linear range. ## Inputs - **Measured OD600** (number): Raw absorbance reading of the (possibly diluted) sample at 600 nm. - **Blank OD600 (medium only)** (number): Absorbance of sterile medium; subtracted from the reading. - **Organism calibration preset** (select) - **Custom factor (CFU/mL per OD600)** (number): Your own calibration slope (CFU/mL at OD600 = 1); used when the preset is Custom. - **Sample dilution before reading (fold)** (number): Fold-dilution of the sample before reading (1 = undiluted). - **Culture volume (mL, optional)** (number): Leave empty to skip the total-CFU calculation. ## When to use - You have an OD600 reading and need a rapid CFU/mL estimate for mid-log cultures of E. coli, B. subtilis, or S. cerevisiae. - You diluted a dense sample before measuring and must back-calculate the original cell density. - You want a linearity warning plus an optional total-CFU figure from culture volume, without running a plate count. - You have a lab-specific calibration slope and need to apply it instead of a built-in preset. ## How it works - Enter the raw OD600 of the (possibly diluted) sample and, if available, the blank OD of sterile medium; the tool subtracts the blank to obtain net OD. - Choose an organism preset (E. coli 1×10⁹, B. subtilis 5×10⁸, or S. cerevisiae 1×10⁷ CFU per OD unit) or supply a custom CFU/mL-per-OD600 factor. - Provide the fold-dilution used before the reading (1 = undiluted). CFU/mL = net OD × factor × dilution. - Optionally add culture volume in mL to compute total CFU. If net OD exceeds ~0.6, a warning reminds you to dilute further and remeasure. ## Use cases - Estimate E. coli density from an undiluted OD600 0.45 reading after blank correction for a growth-curve time point. - Back-calculate original CFU/mL of a 10× diluted B. subtilis culture whose OD600 is 0.85, including total CFU in a 25 mL flask. - Apply a lab-calibrated custom slope to convert OD600 of yeast or an unlisted bacterium into CFU/mL. ## Frequently asked questions ### How is CFU/mL calculated from OD600? CFU/mL = (measured OD600 − blank OD) × calibration factor × sample dilution. The factor is the CFU/mL expected at OD600 = 1 for the chosen organism or your custom slope. ### What do the organism presets mean? They are order-of-magnitude slopes: E. coli ≈ 1×10⁹, B. subtilis ≈ 5×10⁸, and S. cerevisiae ≈ 1×10⁷ CFU per OD unit. Real values depend on strain, medium, and spectrophotometer path length; calibrate with plate counts when accuracy matters. ### When should I use a custom factor? Select Custom calibration factor and enter the CFU/mL you measured at OD600 = 1 from your own standard curve. The presets are only approximate. ### Why does the tool warn about net OD above 0.6? OD600 is typically linear only up to about 0.2–0.6. Higher readings underestimate density; dilute the sample, remeasure, and enter the new dilution fold. ### What does culture volume do? If you enter volume in mL, the tool also reports total CFU in that culture (CFU/mL × volume). Leave it empty to skip the total. ## Related tools - [A-a Gradient (Alveolar-Arterial O₂)](https://elysiatools.com/en/tools/a-a-gradient-calculator): Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. 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