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Diameter of the field of view (mm) = F / M, where F is the number of field of view (FOV) of the eyepiece, and M is the magnification (mag.) of the objective.

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Most modern microscopes have eyepieces with the number of field of view at 20 or 22. The diameters of the field of view are listed:

Most people use put a transparent plastic ruler under microscope to perform this task. Actually, the microscope itself provides pretty accurate measurements ranging from 0.2 - 10 mm. Based on optical physics, the diameter of the field of view can be reliably derived by a simple formula:

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For example: Diameter of  the field of view (mm) = 20 / 40 = 0.50, where 20 is the field number of eyepiece, and 40 = objective mag.

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The field area (A) is calculated by . If the cell is used as unit (instead of mm), the total number of cells in the field of view can be derived from the number of cells on the diameter line with the formula. This is an excellent approximation for most of our histological samples.  For example, if 16 positive cells and 28 total cells are on the line across diameter, respectively. Then Positive cells (Pi x 82 =201) divided by total cells number (Pi x 142 = 616) in entire field would be % of positive cells (201/ 616= 32.6%) (Figure).