Multiplicity of Infection (MOI) Calculator
Find how much viral stock to add for a target MOI, and the Poisson fraction of cells that get infected.
How it works
Particles = MOI × cells · Volume = particles ÷ titer · Fraction infected = 1 − e^(−MOI)
Multiplicity of infection is the ratio of infectious particles to cells. The volume of stock to add is simply the particles you need divided by the titer of the stock. Because virus finds cells by chance, the actual number per cell follows a Poisson distribution: even at MOI 1 only about 63% of cells get at least one particle while the rest are missed, so high MOI is used when you need essentially every cell infected.
Worked example
To infect 1 × 10⁶ cells at MOI 1 you need 1 × 10⁶ PFU. From a 1 × 10⁸ PFU/mL stock that is 1 × 10⁶ ÷ 1 × 10⁸ = 0.01 mL = 10 µL, and 1 − e⁻¹ = 63.2% of cells will be infected.
Frequently asked questions
Why does MOI 1 not infect every cell?
Infection is random. At MOI 1 the average is one particle per cell, but by the Poisson distribution about 37% of cells receive none while others receive two or more. Only fractions of cells, not exact counts, are predictable.
What MOI should I use?
For near-complete infection use a high MOI (5–10 gives >99%). For single-hit conditions or to study infection kinetics, use a low MOI (0.01–0.1) so most cells get at most one particle.
Do I use PFU or particle count for titer?
Use infectious units (PFU or TCID50-derived) that match how MOI is defined. Total particle counts include non-infectious virus and would overstate the effective MOI.
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This calculator is for educational purposes. Double-check important results before acting on them.