From the previous post, you learn how to **calculate annular capacity** and this post shows you how to use the same principle to calculate inner capacity of open hole / inside cylindrical objects such as tubulars, drill pipe, drill collars, tubing, casing etc.

There are several formulas to calculate inner capacity depending on unit of inner capacity required. Please read and understand the formulas below:

## Formula#1) Calculate inner capacity in bbl/ft

Inner Capacity in bbl/ft = (ID in.)^{2} ÷1029.4

Example: Determine inner capacity in bbl/ft of a 6-1/8 in. hole:

Inner Capacity in bbl/ft = 6.125^{2}÷1029.4

Inner Capacity in bbl/ft = 0. 0364 bbl/ft

## Formula#2) Calculate inner capacity in ft/bbl

**Inner Capacity in bbl/ft = (ID in.) ^{2} ÷1029.4**

Example: Determine inner capacity in ft/bbl of 6-1/8 in. hole:

Inner Capacity in ft/bbl = 1029.4 ÷ 6.125^{2}

Inner Capacity in = 27.439 ft/bbl

Inner Capacity in ft/bbl = 1029.4 ÷ (ID in.)

## Formula#3) Calculate inner capacity in gal/ft

**Inner Capacity in gal/ft = (ID in.) ^{2} ÷24.51**

Example: Determine inner capacity in gal/ft of 6-1/8 in. hole:

Inner Capacity in gal/ft = 6.125

^{2}÷ 24.51

Inner Capacity in = 1.53 gal/ft

## Formula#4) Calculate inner capacity in ft/gal

**Inner Capacity in ft/gal = 24.51 ÷ (ID in.)**^{2
}

Example: Determine inner capacity in ft/gal of 6-1/8 in. hole:

Inner Capacity in ft/gal = 24.51 ÷ 6.125^{2}

Inner Capacity in ft/gal = 0.6533 ft/gal

**Determine the volume of mud to fill up the inner of the cylindrical objects by the following equation.**

**Inner Volume = Inner Capacity x ****Length**

Example: Inner capacity = 0. 0364 bbl/ft

Length = 3000 ft

Volume = 0. 0364 x 3,000 = 109.2 bbl.

**Please find the ****excel sheet on how to calculate inner capacity.**

**Ref books: **Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.