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Practical Infection

MOI calculator: multiplicity of infection, target volume, and infection-rate guide

Calculate MOI from cell count, infectious titer, and inoculum volume. You can also solve for the volume or titer needed for a target MOI and review the Poisson-based infected fraction.

All calculations run in your browser; data is not sent.

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Example (preset)

Choose an example to fill inputs and see results immediately.

Description
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How to use (3 steps)
  1. Select a mode (calculate / solve / multiwell planning).
  2. Enter cells, titer, and volume (or target MOI). Pay attention to units.
  3. MOI (or required amount) and a Poisson-based infection guide are shown.

MOI is a guide. Actual outcomes depend on the assay used for titer and the state of cells and conditions. Interpret results within your protocol definition.

Input (cells, titer, volume)

Mode (what to solve for)
Cells (cells)
Titer (infectious units recommended)

Examples: TU/mL, PFU/mL, IFU/mL. Since vg/mL is not an infectious unit, treat it as a reference metric.

Inoculum volume
Pipetting (optional)

Results

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Input summary (unit sanity check)
Cells
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Titer
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Volume
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Target MOI
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Wells
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Overage
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Min pipettable
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MOI is a guide. Actual outcomes depend on assay definition and conditions.

How it works (formulas)

MOI represents the average infectious units applied per cell.

Core equations
Infectious units = titer (units/mL) × volume (mL)
MOI = infectious units / cells
Unit conversions
  • µL → mL: µL ÷ 1000
  • nL → mL: nL ÷ 1,000,000
  • per µL → per mL: ×1000
This tool only calculates amounts. It does not provide biosafety or protocol instructions.

How to interpret MOI and infection rate

MOI is an average number of infectious units per cell, not a promise that every cell receives the same number of particles. That is why this page shows both the numeric MOI and a Poisson reference for the infected fraction.

Start with assay-compatible inputs

Use infectious titers such as TU/mL, PFU/mL, or IFU/mL when you want a true MOI estimate. If your source is reported as vg/mL or another noninfectious metric, the page can still do the arithmetic, but the result should be interpreted as a reference ratio rather than a protocol-ready MOI.

Read the Poisson guide correctly

The infected fraction is shown as 1 - exp(-MOI). This is useful for planning low-MOI vs high-MOI setups and for spotting when an apparently small MOI change can materially alter the expected fraction of infected cells.

When very small volumes appear

If the required inoculum volume drops below your practical pipetting range, do not treat the raw answer as a direct instruction. Use it as a signal to prepare an intermediate dilution, adjust cell count, or rethink the well format so the delivered volume is realistic.

Worked planning example

Suppose you have 2×105 cells and an infectious titer of 1×108 TU/mL. A target MOI of 0.3 requires 6×104 infectious units total, which is 0.6 µL of stock. The Poisson guide then shows an infected fraction of about 26%, helping you decide whether the setup matches your experiment goal or needs a dilution workflow.

FAQ

What is MOI (multiplicity of infection)?
It is a measure of how many infectious units are applied per cell on average. MOI=1 means an average of 1, not that every cell is infected exactly once.
Which titer should I use (TU/PFU/IFU, etc.)?
Use a titer defined as infectious units (TU/PFU/IFU, etc.). Values can differ by assay, so compare values measured with the same definition.
Can I calculate MOI with vg/mL (genome copies)?
You can compute the number, but vg is not an infectious unit. It is safer to treat it as a reference metric (vg/cell) rather than a true MOI. This tool performs numeric calculations only.
How does MOI relate to infection rate?
As a guide, a Poisson model approximates infected fraction as 1-exp(-MOI) (shown in the tool).
The volume is too small (e.g., 0.1 µL).
Very small volumes increase pipetting error. Consider an intermediate dilution or adjusting the scale of your setup.
What infection rate corresponds to MOI 0.3?
Using the Poisson guide, infected fraction is about 1-exp(-0.3) ≈ 26%. It varies by conditions, so use it as a rough guide.
Will this calculation guarantee the desired outcome?
No. This tool calculates amounts. Actual outcomes can vary with cell state, conditions, and measurement error.

Practical MOI planning tips

Keep the titer definition fixed

Do not compare TU/mL, PFU/mL, and IFU/mL as if they were interchangeable. If you are comparing vectors, lots, or historical runs, use the same assay definition on both sides before drawing conclusions from MOI alone.

Use volume warnings as workflow checks

A tiny calculated volume usually means the stock is concentrated enough that an intermediate dilution will reduce pipetting error. A very large calculated volume can mean the titer is too low for the planned well size or cell count.

Multiwell totals are planning values, not final protocol text

The multiwell planner is useful for estimating how much inoculum to prepare with overage, but it does not know your adsorption time, wash steps, or biosafety rules. Treat the totals as setup math that feeds into your own protocol review.