The concept of mud volume increase due to adding Hematite is as same as the mud volume increase by adding barite and calcium carbonate. The formula for calculating volume increment is just different only one factor which is 40 for hematite but 22.5 is usedfor calcium carbonate and 35 is used for barite. Please follow the formula below for determining volume increase because of adding Hematite.
Volume increase per 100 bbl of mud = 100 x (W2 – W1) ÷ (40 – W2)
Where; W1 = current mud weight
W2 = new mud weight
Please read and follow the example below for more understanding.
Example: Determine the volume increase when increasing the density with hematite from 10.0 ppg (W1) to 13.0 ppg (W2):
Volume increase per 100 bbl of mud =100 x (13.0 – 10.0) ÷ (40 – 13.0)
Volume increase per 100 bbl of mud = 11.11 bbl
If you have total volume of 500 bbl of mud, the volume increase due to adding calcium carbonate will be equal to 55.56 bbl (11.11 x 500 ÷ 100).
Please find the Excel sheet used to calculate how much volume increased due to adding Hematite.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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