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Most Probable Number (MPN) Calculator

Tool to estimate the concentration of viable microorganisms in a sample

The Most Probable Number (MPN) Calculator is an easy-to-use tool developed for bacteriologists to estimate microorganisms in a sample by computing the MPN value estimate using probability formulas. The calculator was developed to help public drinking water utilities monitor their water for bacterial indicators and pathogens. Subsequently, it was modified to provide MPN calculations for viral contaminants.

Software Downloads and Platform

The MPN Calculator is public domain, Windows®-based program that can be freely copied and distributed. Installation requires administrative rights, so users may need to have the software installed by information technology personnel with these rights. The program may require prior installation of Microsoft's .NET Framework, especially for any Windows version prior to version 10. The program also requires prior installation of Microsoft SQL Server Compact version 3.0 or an earlier version. These programs can be downloaded freely and installed by an individual with administrative rights by following the directions on the Microsoft websites. For Windows 10, download and save the 64-bit version of the Calculator. For earlier versions of Windows that have a 32-bit architecture, download and save the 32-bit (x86) version of the Calculator. After the download is complete, click on the installed file and follow the installation instructions.

Software Downloads

Date Description
08/01/2013 32-bit (x86) Installer Package for MPN Calculator v2.0 (ZIP)(3 MB)
08/01/2013 64-bit Installer Package for MPN Calculator v2.0 (ZIP)(3 MB)

Microsoft Downloads

.NET FrameworkExit
SQL Server CompactExit

Installation Problems

While the latest version of the .NET Framework is backwards compatible with many Windows versions prior to Windows 10, it is recommended that users install the .NET Framework version more closely matching the installed Windows version. A list of .NET Framework versions (includes previous versions) and dependencies is found on Microsoft's .NET Framework versions and dependencies webpage. Some computers running early Microsoft Windows version or non-Microsoft platforms may not run files with an "msi" extension. Should this problem occur, an executable file and instructions for installing the MPN program are available upon request (see Support section).

Disclaimer: Any mention of trade names, products, services, or enterprises does not imply an endorsement by the U.S. Government or EPA.

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Date Description
08/01/2013 MPN Calculator v2.0 User's Manual  (Updated manual coming soon)

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Most Probable Number Calc ScreenshotThe MPN Calculator is a useful tool for estimating the population density of viable microorganisms in a test sample and is useful for low concentrations of organism. It computes estimates using a probability algorithm, the Spearman-Karber estimates, the Salama correction for bias, the approximate confidence limits of Cornish & Fisher, and the exact confidence limits of Loyer & Hamilton. The MPN Calculator allow users to save computed results into text, word, and excel file formats. It also provides a bias correction for problems associated with the calculation of MPN values with abnormal results.

The MPN Calculator allows users to select standard 2-fold, 5-fold, or 10-fold dilution defaults, but users also can compute MPN values based on non-standard dilutions. The MPN Calculator saves all computed results and associated dilution data sets into its database file by default. Calculations stored in the database can be retrieved using various search criteria.

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The algorithms used in the MPN Calculator were originally developed by EPA's Office of Research and Development as a DOS-based program using the FORTRAN programing language. A modified DOS version was also developed for use during EPA's Information Collection Rule (ICR) in the mid 1990's. The ICR required large public drinking water utilities to monitor their water for contaminants, including viruses. The ICR method used for virus measurements required that utilities use the modified DOS version for their MPN calculations.

When new Windows versions made the DOS versions obsolete, EPA developed the MPN Calculator for the new Windows platforms. This new version combined the features of the original DOS and modified DOS programs so that it could be used by both bacteriologists and virologists. Subsequently, the MPN Calculator was used during the third monitoring period of the Unregulated Contaminant Monitoring Rule. It also is used by many microbiologists at university, industry, and government laboratories throughout the world.

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For questions, installation problems, or suggestions for improvement, please send an email to Eric Villegas.

The MPN Calculator was developed in partnership with Avineon, Inc.

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