Viral Titer Calculator
Compute viral titer using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Titer (PFU/mL) = Average Plaque Count / (Volume Plated x Dilution Factor)
Where Average Plaque Count is the mean number of plaques across replicate plates, Volume Plated is the amount of diluted virus added to each plate in mL, and Dilution Factor is the fold-dilution of the sample (e.g., 10^-6). The result gives plaque-forming units per mL of the original undiluted stock.
Worked Examples
Example 1: Standard Plaque Assay Titer
Problem:You count 42 and 38 plaques on duplicate plates from the 10^-6 dilution, with 0.1 mL plated per plate. What is the viral titer?
Solution:Average plaques = (42 + 38) / 2 = 40\nDilution factor = 10^-6 = 0.000001\nTiter = Average plaques / (Volume x Dilution)\nTiter = 40 / (0.1 mL x 10^-6)\nTiter = 40 / 1 x 10^-7\nTiter = 4 x 10^8 PFU/mL
Result:Viral titer = 4 x 10^8 PFU/mL (Log10 = 8.60)
Example 2: MOI Calculation from Titer
Problem:Your virus stock titer is 2 x 10^9 PFU/mL. You want to infect 1 x 10^6 cells at MOI = 5. How much virus stock do you need?
Solution:Total PFU needed = MOI x Number of cells\nTotal PFU = 5 x 1 x 10^6 = 5 x 10^6 PFU\nVolume of stock = Total PFU / Titer\nVolume = 5 x 10^6 / 2 x 10^9\nVolume = 2.5 x 10^-3 mL = 2.5 mcL
Result:Add 2.5 mcL of virus stock to 1 million cells for MOI = 5
Frequently Asked Questions
What is viral titer and why is it important?
Viral titer is a measurement of the concentration of infectious virus particles in a sample, typically expressed as plaque-forming units per milliliter (PFU/mL). It is a critical parameter in virology because it determines how much virus stock to use in experiments. Knowing the titer allows researchers to infect cells at a precise multiplicity of infection (MOI), ensure reproducible experimental conditions, compare virus stocks between preparations, and determine the potency of vaccine preparations. Without accurate titer determination, dose-response experiments, viral kinetics studies, and vaccine development cannot be properly conducted.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy