OD600 GUIDE
What OD600 measures and how to take a reading
What optical density at 600 nm represents, how spectrophotometer settings affect the reading, and practical steps for reproducible OD600 measurements.
OD600 is a turbidity measurement. The instrument sends light at 600 nm through your sample and reports how much of it fails to reach the detector — as “optical density,” a logarithmic ratio of incident to transmitted light. Denser cultures scatter more light, so the reading rises as biomass increases. The number is a proxy, not a cell count, and understanding what sits behind it is the difference between a reproducible measurement and a vague one.
Why 600 nm
Six hundred nanometres sits in a region where culture media absorb little and common pigments absorb weakly, so most of the signal comes from cells scattering the beam rather than from the medium’s own chemistry. Nothing special happens to cells at exactly 600 nm — it is a practical convention chosen to keep the medium out of the way. If your organism is highly pigmented or your medium is strongly coloured, a blank in the same cuvette becomes essential, and some protocols shift the wavelength to reduce interference.
Absorbance, scattering, and the Beer–Lambert connection
The instrument reports “absorbance,” but at 600 nm almost all of the attenuation is scattering, not molecular absorption. Over a limited range, OD rises roughly proportionally with cell concentration — a Beer–Lambert-like linear region. At higher densities, multiple scattering (a photon scattered more than once before escaping) compresses the response, and the line bends over. This is the physical reason OD600 instruments lose linearity somewhere around 0.8–1.0 for a standard 1 cm path, and why readings outside the linear range must be diluted rather than trusted.
Taking a usable reading
- Blank with the uninoculated medium. The blank establishes the zero point for that medium in that cuvette. Re-blank whenever the medium, cuvette set, or instrument changes — a blank from yesterday’s medium batch is not today’s zero.
- Stay in the linear range. Dilute dense cultures into fresh medium and multiply the corrected OD by the dilution factor. The OD600 calculator includes a dilution correction field; enter the factor and the result refers to the undiluted culture.
- Keep the path length consistent. A 1 cm cuvette is the reference. Microplate readers use shorter effective path lengths, so plate readings are not directly comparable to cuvette readings without correction — and different plate readers are not always comparable to each other.
- Mix before reading. Settled cells give low, drifting readings that recover as the sample is handled. Mix the tube, wipe the cuvette free of fingerprints and droplets, and read promptly.
- Read replicates. Duplicate or triplicate readings catch bubbles, pipetting errors, and dirty cuvettes cheaply. A single drifting decimal place at low OD is usually a bubble.
Microplates vs cuvettes
Cuvettes give a fixed 1 cm path and straightforward blanking; plates give throughput. In a plate, the effective path length depends on the volume in the well and the reader’s optics, so an OD600 of 0.5 in a plate is a different number from 0.5 in a cuvette. If you must work in plates, calibrate in plates — build your cells/mL calibration on the same format you will use for real samples, and keep volume per well constant.
Common reading errors at a glance
| Mistake | What you see | Fix |
|---|---|---|
| Blank is a different medium batch | Consistent offset across all readings | Re-blank with the exact medium used |
| Bubble in the cuvette or well | One replicate far above its siblings | Flick or pipette the bubble out, re-read |
| Fingerprints on the cuvette | Elevated, erratic absorbance | Wipe with lint-free tissue along the light path |
| Sample read after settling | Reading climbs as the tube sits | Mix immediately before every read |
| Dilution forgotten in the result | Values 10× or 100× off expectations | Enter the dilution factor in the calculator’s correction field |
| Reading past the linear range | Compressed differences at high OD | Dilute into fresh medium and re-blank |
What OD600 does not tell you
It does not distinguish live from dead cells, dividing from dormant cells, or large from small cells — all of them scatter light. A culture of small healthy cells at 10⁸ CFU/mL can read lower than a stressed culture of large cells at the same CFU/mL. That is why an OD600 value converts to cells/mL only through a calibration measured for your organism and instrument, and to CFU/mL only through a separately validated pairing. See building an OD600 to cells/mL calibration for the procedure, and why OD600 to CFU/mL needs validation for the viability gap.
Using the reading
Enter the measured OD and blank into the OD600 calculator to estimate cells/mL with your factor, linear curve, or log model. During growth experiments, repeated readings trace a growth curve; interpreting OD600 growth curves covers how to read lag, log, and stationary phases from them, and when OD alone stops telling the whole story.
Frequently asked questions
What does OD600 actually measure?
Optical density at 600 nm is a turbidity measurement: the instrument reports how much light at 600 nm is scattered or absorbed as it passes through a culture. Denser cultures scatter more light, so the reading rises with biomass — but it counts live cells, dead cells, and debris alike.
Why is 600 nm the standard wavelength?
Six hundred nanometres sits where common culture media absorb little and cellular pigments absorb weakly, so most of the signal comes from cells scattering light rather than from the medium chemistry. It is a practical convention, not a special property of cells.
What OD600 range is reliable?
Standard 1 cm spectrophotometers are usually reliable from about 0.05 to 0.8–1.0 OD. Above that, multiple scattering makes the response non-linear and readings compress. Dilute dense cultures into fresh medium and multiply the corrected OD by the dilution factor — the OD600 calculator has a dilution correction field for exactly this.
More od600 guides
- OD600 and CFU/mL: why the relationship needs validationWhy OD600 cannot be converted to CFU/mL with a universal constant, what shifts the ratio, and how to build a paired calibration for your own system.
- OD600 during bacterial growth: growth curves and log phaseHow to read lag, exponential, and stationary phases from an OD600 growth curve, calculate doubling time, and know when OD stops tracking viability.
- OD600 to cells/mL: build and use a calibrationHow to build a calibration from paired OD600 and cell-count data, choose between a factor, linear fit, or log model, and apply it with the OD600 calculator.