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 width in mm
650	4.3	41.9	6.0
700	4.4	45.2	5.7
725	4.5	46.8	5.6
750	4.6	48.4	5.5
800	4.7	51.6	5.4
830	4.8	53.5	5.3
850	4.9	54.8	5.2
920	5.1	59.4	5.0
950	5.2	61.3	4.9
1050	5.4	67.7	4.7

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 = 2.31 x N² (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 = 2.31 x N² (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.