What is the MPN method?

The Most Probable Number (MPN) method estimates the concentration of viable coliform bacteria in a water sample by inoculating multiple tubes at each of several dilutions and recording which tubes show positive growth (gas production). Statistical tables derived from Poisson distribution convert the pattern of positives into a bacterial count per 100 mL.

Which MPN table does this calculator use?

This calculator uses the standard MPN tables from Standard Methods for the Examination of Water and Wastewater (APHA/AWWA/WEF), which are also referenced by EPA Method 9221 and WHO Guidelines for Drinking-water Quality. Confidence limits are 95% per Thomas (1942) approximation.

What do the three dilution levels represent?

Typically the three levels represent 10 mL, 1 mL, and 0.1 mL inocula per tube (or equivalently 1:1, 1:10, 1:100 dilutions). The exact volumes depend on your protocol, but the tube count combination is what the table uses.

What does it mean when the result shows ">"?

When all tubes at all dilutions are positive (e.g., 3-3-3 or 5-5-5), the true MPN exceeds the upper bound of the table. This indicates the sample is heavily contaminated and should be re-tested at higher dilutions.

Is MPN the same as a plate count?

No. MPN is a statistical estimate based on presence/absence of growth in tubes. Membrane filtration plate counts give a direct colony count. Both methods are accepted for coliform testing, but MPN has wider utility for turbid or heavily polluted samples where filtration is impractical.

Water Coliform MPN Calculator

Enter positive tube counts from a 3-tube or 5-tube dilution series to get the MPN of coliforms per 100 mL with 95% confidence limits.

Tube Series & Positive Counts

Dilution series

3-tube 5-tube

Positive tubes — Dilution 1 (largest volume / lowest dilution)

Positive tubes — Dilution 2

Positive tubes — Dilution 3 (smallest volume / highest dilution)

Enter tube counts and click Calculate

MPN per 100 mL — 3-tube series (10 mL / 1 mL / 0.1 mL inocula). Source: APHA Standard Methods, Table 9221.IV.

Combination	MPN	Lower 95%	Upper 95%

Summary

This tool calculates the Most Probable Number (MPN) of coliform bacteria in water samples using the standard multiple-tube fermentation method described in Standard Methods for the Examination of Water and Wastewater (APHA) and adopted by EPA and WHO. Enter the number of positive tubes at each dilution level and the calculator returns the MPN index per 100 mL along with 95% confidence limits. Both 3-tube and 5-tube series configurations are supported.

How it works

Select whether you used a 3-tube or 5-tube dilution series.
Enter the number of positive (gas-producing) tubes at each of the three dilution levels.
The calculator looks up the combination code in the official MPN table.
The MPN index per 100 mL is displayed alongside the 95% lower and upper confidence limits.
Results outside the standard table are flagged so you know when to re-test.

Use cases

Drinking water quality verification by public health labs.
Wastewater effluent compliance testing.
Surface water and recreational water monitoring.
Environmental engineering field studies.
University microbiology lab coursework and reports.
Food processing water safety audits.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu