After adding barite to increase mud weight in the system, total mud volume will increase due to volume of dry barite. This formula below shows you how to determine volume of mud increase in bbl per 100 bbl of original mud.
Volume increase per 100 bbl of mud = 100 x (W2 – W1) ÷ (35 – W2)
Where;
W1 = current mud weight
W2 = new mud weight
Example: Determine the volume increase when increasing the density with barite from 10.0 ppg (W1) to 13.0 ppg (W2):
Volume increase per 100 bbl of mud = 100 x (13.0 – 10.0) ÷ (35 – 13.0)
Volume increase per 100 bbl of mud = 13.64 bbl
If you have total volume of 500 bbl of mud, the volume increase due to adding barite will be equal to 68.2 bbl (13.64 x 500 ÷ 100).
Please find the Excel sheet for calculating volume of mud in bbl increase due to adding Barite.
Ref: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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