Viral Titer Calculator

A plaque assay counts visible zones of cell death (plaques) on a monolayer after infecting it with a diluted virus stock, and this calculator turns that raw count into titer using the standard relationship: titer (PFU/mL) equals the average plaque count across your replicate plates, divided by the dilution factor times the volume plated. Because a ten-fold dilution series is typical, the dilution is entered as a power of ten (enter 6 for a 1:1,000,000 dilution) rather than the fraction itself. The titer is reported both as a raw PFU/mL value and as its log10, since viral stocks commonly span many orders of magnitude and researchers routinely compare and report titers on a log scale for that reason. From the titer, the calculator solves the multiplicity of infection (MOI) equation for volume: MOI equals the number of infectious particles added divided by the number of target cells, so rearranging gives the microliter or milliliter volume of stock you need to pipette to hit a chosen MOI against a chosen cell count.

It also reports the MOI you'd get from a single microliter of undiluted stock as a quick reference. The key assumption is that your plate counts fall in the countable range (roughly 30-300 plaques per plate) — outside that range, Poisson statistics make the count unreliable regardless of what the calculator outputs. This tool computes titer from plaque (PFU) data specifically; it does not perform a TCID50 endpoint-dilution calculation, so don't feed it Reed-Muench-style partial-infection data expecting an equivalent conversion.