How to Calculate the Volume of
Liquid Held by an Inoculating Loop

Inoculating loops can be used to tranfer tiny volumes of liquid.
This page shows how to use bacteriophage suspension to calculate the volume.

If you have a liquid culture of bacteriophage, bacteria, radioisotope or a suitable chemical of known concentration, you can calculate the volume of liquid held by an inoculating loop. This experiment can be done most easily a culture of bacteria, but you must also determine the number of bacteria per ml by plating at the same time the loop was used to plate some bacteria.

Drawing of an Inoculating Loop

An inoculating loop is a piece of stainless steel, nichrome, platinum, or other wire mounted in a handle. Most people use 26 guage platinum or 25 guage nichrome wire. To make a loop, bend the tip of the wire into a small circle having an inside diameter of 1 mm or whatever size you want. When you dip the loop into a liquid, it should retain a droplet of water. The loop shown on the left will not hold a droplet of liquid bacteria culture because the open section of the loop is too wide. The tip of the loop must bend around and touch the metal or it will not hold water.

A Quick Simple Example

Suppose you have a liquid culture of bacteria which has 10,000 bacteria per mL (1 x 104 cells per mL). You know there are 1 x 104 cells per mL because you assayed the number of bacteria per mL using the methods taught on page b038.htm. You must do that assay at the same instant you test the loop because bacteria may be multiplying.


Suppose you count 100 colonies in the plate. Calculation: 100/10,000 reduces to 1/100 or 0.01. The volume of the loop is 0.01 mL. If you can't understand this, send an e-mail to the address below and this example will be improved.

This method assumes bacteria are not adsorbed to the loop.

An Easy Example Using Radioisotope

Suppose you have a solution of a radioisotope which gives 50,000 counts per minute per mL.

Suppose you get 500 counts per minute. 500/50,000 = 0.01 indicating the volume of the loop is 0.01 mL. The two methods gave the same volume for the loop.

Method for Testing a Loop

ice chilled bacteria and shaken

Calculation of the Volume of the Loop

First installed Sep 1999      Revision #0 1999 Sep 29                H2SO4     1022m  90o C
Bacteria Index Page | INDIANA BIOLAB Web Sites |
Written by Harold Eddleman, Ph. D., President, Indiana Biolab, 14045 Huff St., Palmyra IN 47164