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Water Quantity and Quality

The E.coli Assay Calculator

You may determine the "most probable number" or MPN with our new E. Coli assay calculator.

The Partnership for the Umpqua Rivers (PUR) tests, among other water quality parameters, the amount of coliform bacteria and E. coli in water samples. To do this, the IDEXX Quanti-Tray©/2000 is filled with a water sample which has been mixed with chemicals providing the bacteria with food and an indicator. After incubating for 24 hours PUR determines the amount of coliform bacteria present by how many wells, in the tray, turn yellow. The amount of E. coli present is determined by counting the cells that glow under a black light. You can see these glowing cells in the photo above. You count the number of large wells that are positive and the number of small wells that are also positive.

Once you have these two numbers you look up the final result, or "Most Probable Number", using the IDEXX Quanti-Tray©/2000 MPN Table (PDF) (per 100ml) - by looking across one row and down the appropriate column until you find the correct result. Looking at this table is slow and a little frustrating to ensure you do not make a mistake. PUR decided to create a computer program to simplify the process. With PUR's new software, called MPN Calculator 3.1, (available as a free download) all one has to do is enter two numbers, hit enter, and you instantly have the correct MPN/100ml!

DISCLAIMER: The MPN Calculator 3.1 is provided for informational purposes only. This calculator is shared with permission from IDEXX Laboratories, Inc. The IDEXX Quanti-Tray©/2000 MPN Table, used to create this program, is a product of IDEXX Laboratories, Inc. IDEXX Laboratories, Inc. in no way endorses or recommends this product's use. The Partnership for the Umpqua Rivers has no affiliation or relation with IDEXX Laboratories. While this tool has been checked for accuracy, no guarantees are expressed or implied. The MPN Calculator 3.1 is provided as is.

For IDEXX's MPN Generator software program (mpn32.exe), which calculates the most probable number and lower and upper 95% confidence limits, please go to  The program is available as a free download.