Optical Density at 600 nm: What It Measures

Optical density at 600 nm (OD600) is a spectrophotometric measurement of how much light at a 600-nanometer wavelength is absorbed by a liquid sample, used mainly to estimate the concentration of cells in a culture. You place a sample in a spectrophotometer, which shines 600 nm light through it; the instrument reports the OD, which rises as the culture becomes cloudier with more cells. This page explains how OD600 works, how to measure it correctly, and how to convert it to cell counts.

Formula

The raw OD600 reading from the spectrophotometer is already a logarithmic measure of transmittance:

OD = -log10(T)

where T is the fraction of light that passes through the sample (transmittance). For example, if 10% of the light passes through, T = 0.10 and OD = 1.0. To correct for a dilution or a different cuvette path length, use:

Corrected OD = Raw OD × Dilution Factor

and

Absorbance per cm = (Raw OD × Dilution Factor) / Path Length (cm)

Variables: Raw OD600 is the number your instrument displays (unitless), Dilution Factor is how many times the sample was diluted (e.g., 10 for a 1:10 dilution), and Path Length is the distance the light travels through the sample, typically 1 cm for standard cuvettes or 0.2–0.5 cm for some plate readers.

Worked Example

Using the defaults from the calculator: a culture gives a raw OD600 of 0.5, you diluted it 1:1 (no dilution), and the cuvette path length is 1 cm.

1. Corrected OD = 0.5 × 1 = 0.5
2. Absorbance per cm = (0.5 × 1) / 1 = 0.5 per cm

So the corrected OD600 is 0.5 and the absorbance per cm is 0.5. If you had diluted the sample 1:10 (dilution factor 10), the corrected OD would be 5.0, meaning the undiluted culture is actually much denser than the raw reading suggests.

Why 600 nm?

The 600 nm wavelength is chosen because it is in the visible red region where most bacterial culture media and cellular components absorb very little light. Pigments like phenol red (common in growth media) absorb strongly at lower wavelengths, but at 600 nm they are nearly transparent. This means the measured absorbance comes almost entirely from light scattering by the cells themselves, not from the medium. The scattering is proportional to the number of cells, so OD600 correlates well with cell density across a wide range. For yeast and mammalian cells, other wavelengths (e.g., 660 nm) are sometimes used, but 600 nm is the standard for E. coli and many other bacteria.

How to Measure OD600 Correctly

Getting a reliable OD600 reading requires attention to a few practical steps:

• Blank the spectrophotometer with fresh, sterile culture medium (not water) to subtract background absorbance.
• Keep the OD reading in the linear range, typically 0.1–0.8 for most instruments. Above that, scattering causes the relationship to become nonlinear.
• If the culture is above the linear range, dilute it with the same medium and multiply by the dilution factor.
• Use the same cuvette type and path length for all readings; switching between 1 cm and 0.5 cm cuvettes changes the values.
• Mix the sample gently before measuring to ensure an even cell suspension.
• Record the exact time of measurement if you are plotting a growth curve, because cell density changes quickly during exponential growth.

Common Problems and Misconceptions

A frequent mistake is assuming OD600 gives an exact cell count. It does not—it gives a relative measure of turbidity. The relationship between OD and cell number varies with species, strain, cell size, and even the specific spectrophotometer model. For example, an OD600 of 1.0 corresponds to roughly 8×10^8 cells/mL for E. coli, but only about 1×10^7 cells/mL for larger yeast cells. Another misconception is that OD600 is a linear measure; it is logarithmic, so an OD of 2.0 is not twice as dense as 1.0—it is ten times more opaque (only 1% of light passes through). Also, cells that clump or form biofilms scatter light unpredictably, giving readings that are too low or too high. Finally, some plate readers use a shorter path length, so readings from a plate reader are not directly comparable to those from a 1 cm cuvette without a correction factor.

How to Convert OD600 to Cell Count

To turn OD600 into an actual cell concentration, you need a calibration curve specific to your organism and instrument:

1. Grow a culture to a known OD600 (e.g., 0.5).
2. Prepare serial dilutions (e.g., 1:2, 1:4, 1:8) and measure the OD600 of each.
3. Plate each dilution on solid medium and count colony-forming units (CFU) after incubation.
4. Plot CFU/mL against OD600 and fit a line through the linear range.
5. Use that line to convert future OD600 readings to estimated cell counts.

This calibration must be repeated if you change the organism, the growth medium, or the spectrophotometer. For quick estimates, many published tables give approximate conversions for common lab strains, but they are only rough guides.

Limits of OD600

OD600 is a fast, non-destructive method, but it has limits. It cannot distinguish live from dead cells—both scatter light. It also fails for very dense cultures where multiple scattering events cause the reading to plateau or even decrease. Some media components, such as insoluble particles or precipitated salts, add turbidity that inflates the reading. Additionally, cells that change size during growth (e.g., filamentous bacteria) do not have a constant scattering cross-section, so the OD-to-count relationship shifts over time. For these reasons, OD600 is best used for relative comparisons (e.g., growth curves, induction timing) rather than absolute cell numbers unless you have a validated calibration.

Alternatives to OD600

When OD600 is not suitable, other methods give more accurate cell counts:

• Plate counting (CFU/mL) – counts only viable cells, but takes 24–48 hours.
• Hemocytometer or automated cell counter – counts all cells (live and dead) under a microscope.
• Flow cytometry – can count cells and assess viability with fluorescent dyes.
• Dry weight measurement – filters and dries a known volume of culture; gives biomass, not cell number.
• Quantitative PCR – measures DNA copy number, useful for slow-growing or clumping organisms.

Each alternative has trade-offs in speed, cost, and the type of information it provides. OD600 remains the standard for real-time, non-destructive monitoring of microbial growth in research and bioprocessing.

Frequently Asked Questions

What does OD600 stand for? OD600 stands for optical density measured at a wavelength of 600 nanometers. It is a unitless number that indicates how much light is scattered and absorbed by particles in a liquid sample.

What is a good OD600 for E. coli? For most E. coli strains, an OD600 of 0.4–0.8 is ideal for mid-log phase growth, where cells are healthy and dividing rapidly. At OD600 above 1.0, the culture enters stationary phase and the reading becomes less reliable.

Can I use OD600 to measure mammalian cells? Yes, but it is less common because mammalian cells are larger and more fragile. OD600 can be used for suspension cultures, but other methods like trypan blue exclusion or automated counters are usually preferred because they give viability information.

Why is my OD600 reading negative? A negative OD can occur if the blank (reference) sample is more turbid than the sample, or if the cuvette has fingerprints or scratches. Re-blank with fresh medium and clean the cuvette thoroughly.

How do I convert OD600 to CFU/mL? You cannot convert directly without a standard curve. For a rough estimate, many labs use 1 OD600 ≈ 8×10^8 CFU/mL for E. coli, but this varies by strain and instrument. Always calibrate for accurate results.

What is the difference between OD and absorbance? Technically, absorbance measures light absorbed by molecules, while OD measures light lost due to both absorption and scattering. For turbid cell cultures, OD is the correct term because scattering dominates. In practice, the two are used interchangeably in many lab protocols.

Related

• Compression Ratio Calculator: How to Use It
• CST to EST Converter Widget: How It Works
• Calculate Square Feet Area: Formula & Examples

in c | Culture OD600 | - | 0.5 | 0
in d | Dilution factor | - | 1 | 1
in p | Path length | cm | 1 | 0.1
out odc | Corrected OD600 | - | c * d | 2
out abs | Absorbance per cm | 1/cm | (c * d) / p | 2