Pressure across a bit occurs when the drilling mud passing through the jet nozzles. This pressure drop is important for drilling hydraulic optimization (maximum hydraulic horse power or impact force) and hole cleaning. The pressure drop at the nozzle area is a function of total flow area of the bit, flow rate and mud density.

Pressure drop across the bit can be determined by the following equation:

**Oilfield Unit**

Pb = (Q²×W) ÷ (12031 × A²)

**Where:**

Pb = pressure drop across a bit, psi

Q = flow rate, gpm

W = mud weight, ppg

A = total flow area, square inch

**Please use the following information to determine pressure drop across a bit**

Flow rate = 800 gpm

Mud weight = 9.0 ppg

Total Flow Area = 0.3728 square inch

Pb = (800²×9.0) ÷ (12031 × 0.3728²)

Pb = 3,446 psi

**Metric Unit**

Pb = (Q²×W) ÷ (7.191 × A²)

**Where:**

Pb = pressure drop across a bit, KPa

Q = flow rate, lpm

W = mud weight, kg/m³

A = total flow area, mm²

**Please use the following information to determine pressure drop across a bit**

Flow rate = 3,000 l/m

Mud weight = 1,100 kg/m³

Total Flow Area = 240 mm²

Pb = (3,000²×1,100) ÷ (7.191 × 240²)

Pb = 23,901 KPa

The new version of spreadsheet can be downloaded by clicking the image below.

**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.