Calculate Multiplicity of Infection for viral transductions. Determine virus volume for target MOI or effective MOI from a given volume. Includes Poisson-based infection probability estimates.
Fill in the form and click “Calculate” to see your MOI calculations.
You harvested lentiviral supernatant, titered it at 2 × 10⁷ TU/mL, and now need to transduce 200,000 cells at an MOI of 5. How many microliters of virus do you add? An MOI calculator takes your viral titer (transducing units per mL), the number of target cells, and the desired multiplicity of infection, and returns the volume of virus stock to add to your well. MOI — multiplicity of infection — is the ratio of infectious particles to cells. An MOI of 5 means you are adding 5 infectious particles per cell.
The mistake that sinks most transductions: confusing total viral particles with infectious (transducing) units. A typical lentiviral prep has a particle-to-infectivity ratio of 100:1 to 1,000:1. If your titer reports total particles by p24 ELISA and you treat that number as TU/mL, your calculated MOI will be 100–1,000-fold too high on paper but the actual infectious MOI will be far lower. Always use functional titer (TU/mL from a transduction assay) for MOI calculations, not physical particle counts.
Virus particles do not infect cells one-per-cell in an orderly queue. They land on cells randomly, following a Poisson distribution. At MOI = 1, the average cell gets 1 particle, but some cells get 0, some get 2, and a few get 3 or more. The fraction of cells that get zero particles — and therefore remain uninfected — is e⁻ᵀᴾᴼᴼ, which at MOI = 1 is about 36.8%. So MOI = 1 infects only ~63% of cells, not 100%.
To infect >95% of cells, you need MOI ≥ 3 (e⁻³ = 0.05, so 5% uninfected). For >99%, you need MOI ≥ 5 (e⁻⁵ = 0.007). This is why protocols for stable cell line generation or functional screens call for MOI 3–10: they need nearly every cell transduced.
The Poisson model assumes every virus particle has an equal probability of infecting any cell and that infections are independent. In reality, not all cells are equally susceptible (cells in S-phase transduce more efficiently than G0 cells), so actual infection rates can be lower than Poisson predicts, especially in heterogeneous primary cell populations.
Sometimes you know how much virus you added (because you already pipetted it) and need to calculate the MOI after the fact. This happens when you are troubleshooting a failed transduction: “I added 50 µL of virus to 100,000 cells. The titer was 1 × 10⁶ TU/mL. What was the MOI?” The answer: 50 × 10⁻³ × 10⁶ / 10⁵ = 0.5. That is an MOI of 0.5 — half the cells got zero particles on average. No wonder the transduction rate was low.
The reverse calculation is also useful for library screens where you intentionally want low MOI (0.3–0.5) to ensure most infected cells receive only a single construct. If MOI is too high, cells get multiple library members and you cannot deconvolute which construct caused the phenotype.
When reporting MOI in papers or lab records, always include the titer, cell count, and volume alongside the MOI number. MOI alone is not reproducible — the same MOI from a 10⁷ TU/mL stock (small volume, less medium dilution) behaves differently from a 10⁵ TU/mL stock (large volume, significant medium dilution that changes effective concentration during incubation).
At MOI > 10, virtually every cell is infected (Poisson: <0.005% uninfected at MOI = 10). But most cells are now multiply infected — at MOI = 10, the average cell carries 10 integrations. This creates problems: transgene copy number is uncontrolled, insertional mutagenesis risk scales with copies, and transgene expression per cell can vary wildly.
High MOI also introduces cytotoxicity independent of the transgene. The viral envelope proteins, residual packaging plasmid DNA, and polybrene (if used) all contribute to cell stress. For lentivirus, MOI above 20–50 often causes significant cell death 24–48 hours post-transduction, especially in primary cells. If you see high toxicity, titrate down before assuming the transgene is toxic.
For adenovirus and AAV, the MOI scale is different because these are non-integrating. Adenoviral MOIs of 100–1,000 are routine for high-efficiency transient expression. AAV MOIs of 10,000–100,000 viral genomes per cell (vg/cell) are common because AAV transduction efficiency is inherently lower than lentivirus. Always specify the virus type when reporting MOI values.
I used MOI 10 but only 40% of cells are GFP-positive. What went wrong?
Most likely the titer is stale. Lentivirus loses ~50% infectivity per freeze-thaw cycle and decays even at −80°C over months. If the titer was measured 6 months ago, the effective titer today might be 5–10-fold lower. Re-titer on the same cell type you are transducing. Also check if polybrene was added — without it, lentiviral transduction drops 5–10-fold in many cell types.
My titer is in IU/mL from one lab and TU/mL from another. Are they the same?
IU (infectious units) and TU (transducing units) are functionally the same for MOI purposes — both measure infectious particles by functional assay. PFU (plaque-forming units) is also equivalent. The only titer unit that is NOT directly comparable is physical particle count (by p24 ELISA, NTA, or qPCR of viral genome copies), which overestimates infectivity by orders of magnitude.
Should I count cells at the time of seeding or at the time of transduction?
At the time of transduction. If you seeded 100,000 cells yesterday and they doubled overnight, you have 200,000 cells when you add virus. Using 100,000 in the MOI calculation gives an MOI that is twice the actual value. Count or estimate the cell number at the moment you add virus.
Does the volume of media in the well matter?
Yes. Virus particles need to physically contact cells. In a large volume, the virus is more dilute and random encounters take longer. Protocols for adherent cells recommend reducing the media volume during the transduction incubation (e.g., 0.5 mL in a 6-well instead of 2 mL) to increase effective concentration. After 4–6 hours, add fresh media back to normal volume.
Four equations cover MOI calculation and infection prediction:
Units note: titer must be in functional units per mL (TU, IU, or PFU). Volume in mL. Cell count is a raw number (not cells/mL). If titer is per µL, multiply by 1,000 first.
Scenario: You are transducing HEK293T cells in a 6-well plate with a GFP-expressing lentiviral vector. Cells were seeded yesterday at 300,000 per well and are now at ~500,000 (overnight doubling). Viral titer: 5 × 10⁷ TU/mL (titered last week on HEK293T with polybrene). Target MOI: 5.
Step 1 — Virus volume.
V = (5 × 500,000) / (5 × 10⁷) = 2,500,000 / 50,000,000 = 0.05 mL = 50 µL per well.
Step 2 — Predicted infection rate.
% Infected = (1 − e⁻⁵) × 100 = (1 − 0.0067) × 100 = 99.3%. Virtually all cells should express GFP.
Step 3 — Set up the transduction.
Remove media from each well. Add 0.5 mL fresh media + 50 µL virus + polybrene to 8 µg/mL final. Swirl gently. Incubate 4–6 hours at 37°C, then add 1.5 mL fresh media to bring the total to 2 mL.
Step 4 — Confirm at 48–72 hours.
Check GFP expression by flow cytometry or fluorescence microscopy. Expect >95% GFP-positive cells at MOI 5. If the actual rate is significantly lower (e.g., 60%), the titer may have dropped since last measurement — re-titer and recalculate.
Addgene — Lentiviral Titer and MOI Guide: Functional titering protocols and MOI calculation for lentiviral vectors.
Thermo Fisher — Lentiviral Transduction: Practical transduction protocols with MOI recommendations.
Nature Protocols — Lentiviral Vector Production: Titering methods and particle-to-infectivity ratio discussion.
NCBI — Poisson Statistics in Virology: Mathematical basis for MOI and infection probability calculations.
For bulk transductions, MOI 1-5 is typical, achieving 63-99% infection. For clonal selection where single integrations are desired, use MOI 0.1-0.3. Higher MOIs (>5) increase infection rate but also raise the risk of multiple integrations and cytotoxicity. Always optimize for your specific cell type and application. Understanding MOI selection helps you design experiments that achieve your goals while avoiding cytotoxicity and multiple integrations. The calculator shows Poisson predictions for different MOIs—use them to guide your selection.
Viral infection follows Poisson statistics, meaning particles distribute randomly among cells. At MOI 1, while on average each cell receives one particle, some cells receive zero while others receive two or more. The probability of a cell receiving zero particles at MOI 1 is e⁻¹ ≈ 36.8%. To infect >95% of cells, you need MOI ≥ 3. The fraction infected at least once follows: P(≥1) = 1 - e^(-MOI). Understanding Poisson statistics helps you see why MOI 1 doesn't infect all cells and why higher MOIs are needed for complete infection.
Use the formula: Volume (µL) = (MOI × Cell Count) / (Titer per mL) × 1000. For example, to achieve MOI 1 with 100,000 cells and a titer of 10⁸ TU/mL: Volume = (1 × 100,000) / (10⁸) × 1000 = 1 µL. The × 1000 factor converts from mL to µL. This calculator performs this calculation automatically when you select 'Volume from MOI' mode. Understanding this formula helps you determine how much virus stock to add to achieve your target MOI.
TU/mL (Transducing Units) measures functional, infectious particles and is commonly used for lentivirus. vg/mL (viral genomes) measures total viral DNA copies and is used for AAV. Physical particle counts (vg) are typically 10-1000× higher than infectious titers (TU) because not all particles are infectious. Use the titer that matches your experimental endpoint: use TU/mL for functional transduction, use vg/mL if measuring physical particles. Understanding this distinction helps you use the correct titer values in calculations and interpret experimental results correctly.
Adding more volume increases the amount of virus but also dilutes it in the culture medium. The key parameter is total infectious particles added, not volume alone. For small volumes, ensure accurate pipetting. For large volumes, consider that extended incubation in virus-containing medium may cause toxicity. Some protocols recommend concentrated virus in minimal volume. The formula shows that volume and titer are inversely related: higher titer requires less volume for the same MOI. Understanding this relationship helps you optimize transduction conditions.
Several factors affect actual transduction: cell type and receptor expression, cell health and cycle stage, virus pseudotype (VSV-G, native envelope, etc.), presence of transduction enhancers (polybrene, RetroNectin), virus quality and age (titers degrade over time), and incubation conditions (volume, time, temperature). Poisson predictions assume ideal conditions where all particles are equally infectious and all cells are equally susceptible. Always verify transduction efficiency with a reporter or marker. Understanding these factors helps you optimize conditions and interpret experimental results correctly.
Polybrene (hexadimethrine bromide) neutralizes charge repulsion between viral particles and cell membranes, often improving transduction 2-10×. Typical concentrations are 4-8 µg/mL. However, polybrene is toxic to some cell types (especially primary and stem cells). Test cytotoxicity first and consider alternatives like RetroNectin for sensitive cells. Understanding when to use polybrene helps you optimize transduction efficiency while avoiding cytotoxicity. The calculator provides theoretical predictions—polybrene may improve actual efficiency beyond these predictions.
Use low MOI (0.1-0.3) where most infected cells receive only one viral particle. At MOI 0.1, ~90% of cells are uninfected, ~9% receive one particle, and <1% receive multiple. After transduction, select for transduced cells (e.g., with antibiotic resistance or fluorescence), then expand clones and verify integration copy number by qPCR or Southern blot. Understanding low MOI helps you minimize multiple integrations, which is important for gene therapy and research applications where single-copy integration is desired.
Store lentivirus at -80°C in small aliquots to avoid repeated freeze-thaw cycles. Each cycle can reduce titer by 10-50%. For short-term storage (days), 4°C is acceptable but expect gradual loss. AAV is more stable and can tolerate more handling. Always aliquot fresh virus before initial freeze and record the date of preparation. Understanding storage requirements helps you maintain virus titer, which is critical for accurate MOI calculations. Degraded titer leads to lower actual MOI than calculated.
Yes, the Poisson model applies to any system where particles randomly encounter targets. For bacteriophage, MOI is typically much higher (10-100+) for complete lysis. For bacterial pathogens infecting mammalian cells, MOI varies widely by pathogen (1-1000). The core formula and statistics remain the same regardless of the infectious agent: MOI = Particles / Cells, and infection follows Poisson distribution. Understanding this universality helps you see that MOI concepts apply broadly across virology, microbiology, and biotechnology.
Build essential skills in virology, gene delivery, and infection modeling for your research
Explore All Biology Calculators