What you are given
- OD600 (measured): 0.6
- Cells/mL per OD: 8e8
Formula: cells/mL = OD x factor
Cell culture
How do you convert OD600 to cells per mL?
cells/mL = OD600 x the strain factor, roughly 8 x 10^8 for E. coli. The factor is instrument and strain specific, so a number from a paper is an estimate, not a constant.
Worked example
Every number below is computed by the calculator, not typed in
The working
- Multiply OD by the strain factor:
cells/mL = OD600 · factor - Substitute:
cells/mL = 0.6 · 800000000=480000000 cells/mL
That culture is about 4.8 × 10⁸ cells/mL.
What each symbol means
- n: Cell density (cells/mL)
- OD₆₀₀: Optical density at 600 nm
- f: Strain factor, cells/mL per OD (cells/mL)
A printable one-page version is on the formula sheet.
Where this usually goes wrong
- Reading above about OD 0.8 without diluting, where the relationship stops being linear.
- Reusing a factor across strains or spectrophotometers.
- Blanking against water rather than against the medium.
Try it yourself
Two problems, with the working revealed
Problem 1 OD600 / cell density
Convert the OD reading, or work out the back-dilution.
- Mode: Dilute to target OD
- Current OD (dilute mode): 2
- Target OD (dilute mode): 0.5
- Final volume (dilute mode): 10 mL
Show the answer and the working
Mix 2.5 mL of culture with 7.5 mL of fresh medium.
- Solve for culture volume:
V₁ = (OD₂ · V₂) / OD₁ - Substitute:
V₁ = (0.5 · 10 mL) / 2=2.5 mL - Fresh medium:
V₂ − V₁=7.5 mL
Open this in the calculator to change the numbers.
Problem 2 OD600 / cell density
Convert the OD reading, or work out the back-dilution.
- OD600 (measured): 0.75
- Cells/mL per OD: 8e8
Show the answer and the working
That culture is about 6 × 10⁸ cells/mL.
- Multiply OD by the strain factor:
cells/mL = OD600 · factor - Substitute:
cells/mL = 0.75 · 800000000=600000000 cells/mL
Open this in the calculator to change the numbers.
Do it with your own numbers.
The OD600 / cell density calculator is free, shows every step, flags the hazards, and works offline once loaded. No account, no signup.
Open the calculator