moi calculator virus

What Is MOI in Virology?

MOI (Multiplicity of Infection) is the average number of infectious viral particles added per target cell. It helps standardize experiments, compare transduction efficiency, and plan viral input volume from known titer values.

In simple terms, MOI tells you the intended “dose” per cell. An MOI of 1 means one infectious unit per cell on average, while MOI of 5 means five infectious units per cell on average.

How This MOI Calculator Works

Core equation

The required infectious units are calculated as:

• Required IU = Number of Cells × Desired MOI

Then the viral stock volume is:

• Volume (mL) = Required IU ÷ Effective Titer (IU/mL)

If your stock is diluted, the calculator adjusts with:

• Effective Titer = Stock Titer ÷ Dilution Factor

Poisson interpretation

Cell infection events are often modeled with a Poisson distribution. That gives useful estimates:

• Fraction uninfected: P(0) = e^-MOI
• Fraction infected at least once: 1 - e^-MOI
• Fraction with exactly one hit: MOI × e^-MOI
• Fraction with multiple hits: 1 - [P(0) + P(1)]

Why Researchers Care About MOI

• Consistency: Keeps experiments comparable across replicates.
• Optimization: Balances expression/infection rate versus cytotoxicity risk.
• Cost control: Prevents overuse of expensive viral prep.
• Interpretability: Helps explain single-hit vs multi-hit biology.

Example

If you have 1,000,000 cells, want MOI 2, and your titer is 1×10⁸ IU/mL:

• Required IU = 1,000,000 × 2 = 2,000,000 IU
• Volume = 2,000,000 ÷ 100,000,000 = 0.02 mL = 20 µL

The expected infected fraction is 1 - e^-2 ≈ 86.5%.

Common Input Mistakes to Avoid

• Using total particles/mL when your assay reports only infectious units/mL (or vice versa).
• Forgetting dilution effects before adding viral stock.
• Mixing up cell count at plating vs cell count at infection time.
• Assuming MOI equals exact hits per cell; it is an average, not a guarantee.