Virus Volume Calculator for a Target MOI
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/3/2026
The volume of virus needed is found by multiplying the target MOI by the number of cells, then dividing by the viral titer. Example: to infect 500,000 cells at an MOI of 10 with a stock titered at 1×10⁸ particles/mL, you need 0.05 mL (50 µL) of viral suspension.
Explanation
Multiplicity of infection (MOI) is the average number of infectious viral particles added per target cell, defined as MOI = number of viral particles ÷ number of cells. In practice, you usually know the viral stock's titer (in particles per mL, obtained from prior titration) and the number of cells to infect, and you're looking for the volume of viral suspension to draw to reach a target MOI: rearranging the formula, volume = (MOI × number of cells) ÷ viral titer. One important nuance: MOI is a statistical average, not an individual guarantee — even at MOI = 1, some cells receive several viral particles while others receive none at all, a distribution that follows a Poisson law. To estimate the number of DNA copies obtained after PCR amplification, see also our PCR amplification calculator; for the melting temperature of the primers used in that same PCR, our primer melting temperature calculator.
Example: infecting 500,000 cells at MOI 10
Inputs
Viral titer: 1×10⁸ particles/mL. Cells: 500,000. Target MOI: 10.
Calculation
Volume = (MOI × number of cells) ÷ viral titer = (10 × 500,000) ÷ 100,000,000 = 5,000,000 ÷ 100,000,000 = 0.05 mL.
Result
You need 0.05 mL (50 µL) of viral suspension for this infection.
Frequently asked questions
Does an MOI of 10 mean every cell receives exactly 10 particles?
No, it's an average across the entire cell population. The actual distribution of particles per cell follows a Poisson law: at MOI = 10, most cells receive a number of particles close to 10, but some receive far more and a small fraction receive none at all. The lower the MOI, the larger that fraction of uninfected cells.
Where does the viral titer used in this calculation come from?
The viral titer comes from prior titration of the stock, carried out using various methods (a serial dilution followed by a plaque count for a titer in PFU/mL, or a transduction readout for a titer in TU/mL, for example). This calculator assumes that titer is already known and provided as an input.
What MOI should I choose for a given experiment?
This depends entirely on the cell type, the viral vector used, and the goal of the experiment (transient infection at a low MOI to limit toxicity, or near-total infection at a high MOI for a knockdown or forced expression) — refer to the protocol specific to your experimental system rather than a universal value.