OD600 (optical density at 600 nm) is a measure of light absorbance by a cell suspension. It is the most common proxy for cell density in microbiology because cells scatter light proportionally to their concentration at low densities.

How accurate is this conversion?

The conversion factors are empirical averages from published literature. Actual cell counts vary with strain, growth phase, media, and spectrophotometer path length. For precise counts, combine OD readings with direct counting methods such as flow cytometry or a hemocytometer.

Why does E. coli OD600 = 1 equal roughly 8 x 10^8 cells/mL?

This commonly cited value (0.8–1.0 x 10^9 cells/mL at OD600 = 1) comes from calibration studies using 1 cm path-length cuvettes. Many modern plate readers use shorter path lengths, which shifts the value — always calibrate for your instrument.

What is the linear range of OD600?

Beer-Lambert law applies linearly up to roughly OD600 = 0.3–0.6 for most bench spectrophotometers. Above this range, readings underestimate true cell density. Always dilute high-density samples and multiply the measured OD by the dilution factor.

Do I need to account for dilution?

Yes. If you diluted your sample before measuring, enter the dilution factor (e.g. 10 for a 1:10 dilution). The calculator multiplies the computed cell density by this factor to give you the concentration in the original undiluted culture.

Can I use this for yeast cultures?

Yes. S. cerevisiae (budding yeast) is included. Note that yeast cells are about 5–10 times larger than E. coli, so the cells/mL value at the same OD600 is considerably lower.

OD600 to Cell Density Calculator

Name: OD600 to Cell Density Calculator
Availability: InStock
Author: Nham Vu

Enter an OD600 absorbance value, select an organism, and instantly get the estimated cell density in cells per mL.

Measurement Inputs

OD₆₀₀ Reading

Enter the absorbance value from your spectrophotometer (linear range: 0 – 0.6).

Organism

Dilution Factor (1 = undiluted)

If you measured a 1:10 dilution, enter 10. Multiply measured OD by this factor before calculating, or enter the raw measured OD and let the calculator handle it.

Enter an OD₆₀₀ value and click Calculate to see results.

Reference — Conversion Factors

Organism	Cells/mL per OD₆₀₀ unit
E. coli	8.0 × 10⁸
S. cerevisiae	3.0 × 10⁷
B. subtilis	1.0 × 10⁹
P. pastoris	5.0 × 10⁷

Factors are empirical averages at 1 cm path length. Calibrate against direct counts for your specific strain and instrument.

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