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Bacterial Growth: When Does My Culture Reach OD600 0.6?

Estimate time to an induction OD600 from starting OD, doubling time and lag. Compare Gompertz and exponential growth, size the starter, and check measurement dilution.

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Published 28. October 2026 · version 1

#Gompertz model #induction timing #inoculum dilution #OD600

Plan when to induce or harvest a culture without repeatedly guessing how long the next increase in OD600 will take. This bacterial growth time calculator uses the OD after inoculation, a measured doubling time, and a lag estimate to forecast the time to your chosen target. The default target is OD600 0.6; use the value required by your own experiment.

Enter the starting OD of the mixed culture, rather than the OD of the undiluted starter. All OD values should be blank-corrected and reported on the same instrument and measurement scale. If you diluted an aliquot before measuring it, multiply its corrected reading by that dilution factor first. A high-density starter may need a substantial measurement dilution.

Choose a growth model. Modified Gompertz gives a smooth transition through lag, rapid growth and a plateau. It needs an estimated plateau from a previous growth curve. Its doubling-time input represents the fastest logarithmic growth rate, and its lag is defined by a tangent to the log-growth curve. Lag plus exponential is useful for a culture expected to remain in log phase: it adds a period of no growth to the time required for the calculated number of doublings. The two lag definitions are different, so the displayed exponential comparison is a planning comparison, not an uncertainty interval.

Read the time-to-target result as elapsed time from inoculation. Press play to watch the flask cloud over and follow the predicted OD at each moment. If other inputs shorten the forecast, the displayed time stops at the new end. The liquid level stays fixed; changing cloudiness is an illustration, not a calibrated optical simulation. The growth chart shows the selected model and the target line. If the initial OD already meets the target, the waiting time is zero.

The starter-volume calculation answers a separate preparation question. Enter the starter OD and the desired final volume including the starter. The tool balances OD times volume to calculate how much starter and fresh medium to combine. Use comparable, corrected OD values for that balance.

This OD600 calculator also estimates cell concentration from an editable calibration factor. Enter 8 for 8 × 10⁸ cells/mL per OD600; the tool applies the scale. The E. coli default is only a rule of thumb; cell size, strain, medium and instrument geometry can change the relationship. The suggested sample dilution brings the predicted reading below your chosen linear-range limit. A threefold dilution means one part culture plus two parts diluent. It does not mean dilute the growing flask.

Use the forecast to decide when to take the next measurement and when to induce culture according to your protocol. Confirm the actual OD before acting. Changes in temperature, aeration, nutrients or cell physiology can move the crossing time. This tool does not fit parameters from measurements, model oxygen transfer, predict viability, or choose an induction reagent or dose.

How it is calculated

The maximum logarithmic growth rate is $\mu=\ln(2)/t_d$ . In the lag-plus-exponential model, $OD(t)=OD_0\exp[\mu\max(0,t-\lambda)]$ , so the crossing time is $\lambda+t_d\log_2(OD_*/OD_0)$ for a higher target.

Modified Gompertz: Zwietering et al. (1990), Table 2 and Eq. 11, express logarithmic growth as

G(t)=A\exp\{-\exp[1+(\mu e/A)(\lambda-t)]\}.

Here $OD(t)=B\exp[G(t)]$ and $OD_\infty=B\exp A$ . The published lower asymptote $B$ is not generally the OD measured at time zero. This tool reparameterizes that same curve to enforce the measured starting OD exactly: solve

A[1-\exp\{-\exp(1+\mu e\lambda/A)\}]=\ln(OD_\infty/OD_0)

by 45 bracketed bisections, then set $\ln B=\ln OD_\infty-A$ . The target time follows from the analytical inverse:

t_* = \lambda+\frac{A}{\mu e}\left[1-\ln\left(-\ln\frac{\ln OD_*-\ln B}{A}\right)\right].

The initial-condition reparameterization and inverse are derived here. Both models return zero when the target is already reached.

Starter volume follows $V_s=V_f OD_0/OD_s$ ; fresh medium is $V_f-V_s$ . The calibration field uses units of 10⁸ cells/mL per OD600 (enter 8 for 8 × 10⁸); convert it to cells/mL before multiplying by $OD_*$ . Sample dilution is $D=\max[1,\lceil OD_*/OD_{linear}\rceil]$ .

Assumptions

The culture is homogeneous, grows monotonically, and has the same effective OD-to-biomass relationship over the forecast interval. No death phase is modeled.
Doubling time, lag and Gompertz plateau are estimates for the actual strain, medium, temperature and aeration. Defaults are illustrative planning inputs, not a measured strain-specific dataset.
Gompertz lag is the intercept of the steepest log-OD tangent with the lower log-OD asymptote. It is not a period of exactly zero growth. The exponential option instead uses a hard lag.
The final culture volume includes the starter. Fresh medium has zero OD after blank correction; mixing and sampling losses are neglected.
OD is dimensionless. Cell numbers are counts; calibration has inverse-volume units. Cell estimates are approximate and are not colony-forming units.

Where it applies

Use calibrated, blank-corrected and dilution-corrected OD values on a common measurement scale. OD600 0.6 is not a universal induction point or a universal linearity limit. Stevenson et al. (2016) discuss a low-density regime around OD600 0.2 and show why calibration depends on cell geometry and the instrument. The editable 0.2 reading limit is conservative guidance, not a device specification. Dilute an aliquot into the validated range, subtract its blank, and restore the dilution factor.

The Implen manual's E. coli conversion (8 × 10⁸ cells/mL per OD600) is a rule of thumb; replace it with your measured factor. Its approximate 0.6 linear range is instrument-specific and does not override a lower validated limit. The attached Stevenson source is the legal 2016 preprint; the reference points to the final Scientific Reports paper.

A Gompertz target at or above the plateau has no finite crossing and is blocked. Near-plateau forecasts are highly sensitive to the fit. Exponential forecasts apply only while growth remains exponential. Neither model predicts effects of changing temperature, oxygen limitation, induction burden or instrument saturation. Confirm the actual OD before induction. The animation is illustrative.

References

Zwietering MH et al. (1990). Modeling of the Bacterial Growth Curve. Applied and Environmental Microbiology 56:1875–1881, Table 2 and Eq. 11. https://doi.org/10.1128/aem.56.6.1875-1881.1990
Stevenson K et al. (2016). General calibration of microbial growth in microplate readers. Scientific Reports 6:38828. https://doi.org/10.1038/srep38828 . Attached legal preprint: https://doi.org/10.1101/061861
Implen. OD600 DiluPhotometer User Manual v1.3, p.9 (instrument-dependent linearity and E. coli rule of thumb). https://www.implen.de/wp-content/uploads/downloads/OD600-user-manual.pdf
iGEM (2018). InterLab calibration protocol: conversion of OD600 to colony forming units, Starting Sample Preparation worked dilution. https://www.protocols.io/view/calibration-protocol-conversion-of-od600-to-colony-36wgq9e1xlk5/v1
Nor Amdan. UCL doctoral thesis, printed p.116: worked OD growth-rate and doubling-time example. https://discovery.ucl.ac.uk/10050800/7/Nor%20Amdan_10050800_thesis_redacted_revised.pdf

Inputs

Input	Symbol	Unit	Default
Growth model Gompertz uses a fitted plateau and tangent-defined lag. The exponential option holds OD constant during lag, then applies the doubling time.	`growth_model`		Modified Gompertz (smooth lag and plateau)
Starting OD600 after inoculation Blank-corrected, dilution-corrected OD of the mixed culture at time zero, on the same calibrated scale as the target.	`OD_start`		0.05
Target OD600 Use the induction or harvest OD in your protocol. OD600 0.6 is an example, not a universal induction point.	`OD_goal`		0.6
Doubling time Measured under your strain, medium, temperature and aeration. For Gompertz this sets the maximum slope of ln(OD), μ = ln(2)/doubling time.	`t_d`	min	30
Lag time For Gompertz: intercept of the steepest ln(OD) tangent with its lower asymptote. For exponential: a period of no growth. These definitions differ.	`lag`	min	45
Gompertz plateau OD600 Fitted biological plateau on the same OD scale. Use corrected measurements, not detector saturation. It must exceed both starting and target OD.	`OD_max`		4
Final culture volume (including starter) The starter is part of this volume. Fresh medium = final volume minus starter volume.	`V_final`	mL	100
Starter culture OD600 Blank-corrected OD multiplied by the measurement dilution factor; use a comparable calibrated OD scale.	`OD_starter`		2
Cell calibration per OD600 (×10⁸ cells/mL) Enter 8 for 8 × 10⁸ cells/mL at OD600 = 1. This E. coli example is an Implen rule of thumb. Replace it with your own calibration; it is not a CFU conversion.	`k_cells`	1e8/mL	8
Upper OD600 for a linear sample reading Use your validated instrument limit. The conservative 0.2 example comes from Stevenson et al.; it is not universal. Dilute aliquots before reading, then correct for dilution.	`OD_linear`		0.2
Requested elapsed time ▶ Play the forecast from inoculation. If growth inputs shorten the forecast, the displayed time is bounded to the new end. Flask cloudiness is illustrative; the displayed OD is calculated.	`t_view`	min	150

Results

With the default inputs above, the calc gives:

Result	Value
Time to target from inoculation	155.5 min
Lag + exponential comparison	152.5 min
Starter volume	2.5 mL
Fresh medium volume	97.5 mL
Estimated cell concentration at target	480,000,000 1/mL
Estimated total cells at target	48,000,000,000
Sample dilution at target (fold)	3
Expected diluted sample OD600	0.2
Time remaining at animated moment	5.5 min

Checked against sources

Each case below was calculated by this calc and agrees with the value from its source (within the tolerance where one is shown).

Implen: OD600 1 corresponds to about 8 × 10⁸ cells/mL

Source: Implen OD600 DiluPhotometer user manual v1.3, p.9: the E. coli rule of thumb is a calibration example, not a universal conversion or viable count.

Inputs: Target OD600 = 1; the others as above.

Result	Source	This calc
Estimated cell concentration at target	800,000,000 1/mL	800,000,000 1/mL

iGEM: 82 µL starter in a final 1 mL

Source: iGEM 2018 InterLab CFU calibration, Starting Sample Preparation worked example: (1.224)(x)=(0.1)(1000 µL), x=82 µL, medium=918 µL. https://www.protocols.io/view/calibration-protocol-conversion-of-od600-to-colony-36wgq9e1xlk5/v1 . Published volumes are rounded; the exact starter is 81.699 µL.

Inputs: Starting OD600 after inoculation = 0.1; Starter culture OD600 = 1.224; Final culture volume (including starter) = 1 mL; the others as above.

Result	Source	This calc
Starter volume	0.082 ± 5 × 10⁻⁴ mL	0.0817 mL
Fresh medium volume	0.918 mL	0.9183 mL

Published OD600 0.3 → 3.3 in 2 h

Source: Nor Amdan thesis (UCL), printed p.116, worked growth-rate and doubling-time calculation: OD600 0.3 at 1 h and 3.3 at 3 h; doubling time rounded to 34.7 min. https://discovery.ucl.ac.uk/10050800/7/Nor%20Amdan_10050800_thesis_redacted_revised.pdf . Using the rounded doubling time gives about 120.04 min; dense OD values require calibration/dilution.

Inputs: Growth model = Lag + exponential (log-phase estimate); Starting OD600 after inoculation = 0.3; Target OD600 = 3.3; Doubling time = 34.7 min; Lag time = 0 min; the others as above.

Result	Source	This calc
Time to target from inoculation	120 ± 0.2 min	120 min

Bacterial Growth: When Does My Culture Reach OD600 0.6?