Square the value of the diameter in mm of the indenter ball and the value of the diameter in mm of the indentation. Subtract the indentation squared from the indenter squared. Take the square root of the difference. For example, if the indenter ball has a diameter of 10 mm and the indentation has a diameter of 5 mm, then the value would be sqrt(10^2-5^2) or 8.66 mm.
Subtract the value from Step 1 from the diameter of the indenter. Following the same example, 10 mm - 8.66 mm, or 1.34 mm.
Multiply the value from Step 2 by the diameter of the indenter ball and pi/2. In the example this would be 1.34 mm * 10 mm * pi/2 or 21.04 mm ^2.
Divide the force in kgf applied to the indenter ball by the value from Step 3. If the force is 100 kgf, then 100 kgf / 21.04 mm^2" is 4.75 kgf/mm^2. The unit of kgf/mm^2 is the Brinell hardness unit, or HB.