by Dr. Shipman, USDA

Relative values for cell size using various popular units of measure.
# cells/dm2 (2 sides)# cells/inch# cells/in2(2 sides)cell widthin mm2
Cells per square decimeter (dm2) is a useful unit of measure for understanding colony population dynamics and for developing management strategies, but, for practical field diagnostics it is easier to measure the width of a row of 10 cells measured side to side. A dm2 is an area 10 cm by 10 cm; cells on both sides of the comb area are counted. To convert cells per dm2 to cells per linear dm and then to mm per cell use equation 1:

Equation 1:
cells/dm2 = 2.31XN2 (where N is the number of cells per linear dm)
Example: 850 = 2.31 x (19.18)2
Divide 100 by cells per linear dm to obtain mm per cell
Example: 100 divided 19.2 = 5.2

To convert cells per in2 to inches per cell:
Equation 2:
cells/in2 = 2.31XN2 (where N = number of cells per inch)
Example: 54.8 = 2.31 x (4.87)2

Equation 3:
cells/dm2 = 15.5 X cells/in2
Example: 850 = 15.5 x 54.8

The values in this column represent the width of a single cell as measured between centers of opposing cell walls. The actual cell interior width is one cell wall thickness less than this value.