What is multiplicity of infection?

MOI is the ratio of infectious viral particles to target cells in an infection. An MOI of 5 means five infectious particles per cell on average. It controls how heavily cells are infected and is a key variable in virology, transduction, and gene therapy experiments.

How is MOI calculated?

Multiply the titre in particles per mL by the volume added in mL to get the total infectious particles, then divide by the number of cells. So a 1e8 pfu/mL stock, 0.5 mL added to 500,000 cells, gives 5e7 particles over 5e5 cells, an MOI of 100.

Why use the Poisson distribution?

Particles do not divide evenly; they land on cells at random. The Poisson distribution describes that randomness, so the fraction of cells receiving at least one particle is 1 minus e to the minus MOI. This is why even at MOI 1 about a third of cells escape infection entirely.

What MOI gives near-complete infection?

Because the uninfected fraction is e to the minus MOI, you need a fairly high MOI for full coverage. MOI 3 leaves about 5 percent uninfected, MOI 5 about 0.7 percent, and MOI 10 around 0.005 percent. Choose the MOI from how complete you need the infection to be.

How do I find the volume for a target MOI?

Enter a target MOI and the tool back-calculates the volume to add. The particles needed equal the target MOI times the cell number, and the volume is that divided by the titre. This is the everyday calculation for setting up a transduction at a defined MOI.

What is the MOI (Multiplicity of Infection) Calculator?

Calculate multiplicity of infection from viral titre, volume added, and target cell number, then estimate the fraction of cells infected using the Poisson distribution. Also solves for the volume needed to hit a target MOI. Built for virology and gene therapy labs. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

MOI (Multiplicity of Infection) Calculator

Name: MOI (Multiplicity of Infection) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Multiplicity of infection sets how many viral particles each cell sees, and it determines everything from transduction efficiency to cytopathic effect. This calculator gives the MOI from your titre, volume, and cell number, and uses the Poisson distribution to estimate how many cells actually get infected.

How it works

The MOI is total infectious particles divided by target cells:

particles = titre (pfu/mL) x volume (mL)
MOI       = particles / cells

Because particles distribute over cells at random, the fraction of cells that receive at least one particle follows the Poisson distribution:

fraction infected = 1 - e^(-MOI)

To plan an experiment, enter a target MOI and the tool inverts this: the volume to add is target MOI x cells / titre.

Tips and example

A 1e8 pfu/mL stock, 0.5 mL added to 500,000 cells, gives 5e7 / 5e5 = MOI 100, essentially saturating infection. The Poisson behaviour is the part people forget: at MOI 1, e^-1 is about 0.37, so roughly 37 percent of cells stay uninfected even though there is one particle per cell on average. For near-complete infection aim for MOI 5 to 10; for single-copy integration in transduction studies use a low MOI, often below 0.3, so most infected cells receive just one particle.

The Poisson distribution: why MOI 1 is not 100% infection

The key insight that new researchers often miss is that MOI is an average — it says how many particles exist per cell, but particles do not know which cell to infect. They distribute randomly. The Poisson distribution describes this randomness:

At MOI 1: about 63% of cells receive at least one particle; 37% receive zero and escape infection.
At MOI 3: about 95% of cells are infected; 5% escape.
At MOI 5: about 99.3% are infected; 0.7% escape.
At MOI 10: approximately 99.995% are infected.

This means that if your assay requires essentially complete infection (for example, measuring cell death from a lytic virus), you need an MOI of at least 5 to 10, not MOI 1.

Choosing MOI for different experimental goals

Goal	Typical MOI	Rationale
Maximal cytopathic effect	5–10	Nearly complete infection of the monolayer
Transduction with single copy	0.1–0.3	Most infected cells receive exactly one vector
Reporter gene expression	1–3	Balance between infection rate and cell health
Phage infection kinetics	Variable	Depends on burst size and experiment design

Single-copy transduction

For stable cell line generation or gene therapy vector work, you often want most transduced cells to receive exactly one copy of the vector. At low MOI (below 0.3), the fraction of cells that receive two or more particles is small relative to the fraction receiving exactly one. This keeps the population genetically homogeneous. The trade-off is that most cells (over 70% at MOI 0.3) receive nothing at all, so you need a selection step after transduction.

Units and titre measurement

Titres can be expressed in different units depending on the assay used:

pfu/mL (plaque-forming units per mL) — measured by plaque assay; reflects infectious particles that form plaques on a cell monolayer.
TU/mL (transducing units per mL) — measured by the number of cells stably transduced; relevant for lentiviral and retroviral vectors.
vg/mL (vector genomes per mL) — measured by qPCR on the viral genome; counts physical particles including non-infectious ones.

Because these units measure different things, an MOI calculated with vg/mL titres will behave differently from one calculated with TU/mL. Use the same unit throughout a calculation and be explicit when reporting MOI in a methods section.