Total Flow Area (TFA)

Total Flow Area (TFA) is summation of nozzle areas which fluid can pass through a bit. When you consider about the TFA, you need to count all nozzles that you have in a bit or a reamer.

Basically, you can determine flow area with a simple circle area formula.

Flow Area = (π × D2) ÷ 4


Area in square inch

π is a constant which approximately equates to 3.14159.

D is diameter in inch

Let’s make it easier for our life. Normally, a diameter of nozzle is reported in xx/32 inch. For example, a bit has 3 nozzles and each one of them has size of 20/32 inch.

The formula above can be simplified like this.

Oilfield Unit

Flow Area = N2 ÷ 1303.8


Flow Area for each nozzle in square inch

N is nozzle size in number/32 inch.

In order to find the total flow area of each bit or reamer, you must add all area of each nozzle.


The bit that has a total of 5 nozzles. Three nozzles have a diameter of 10/32 inch and other 2 nozzles are 12/32 inch diameter.

Determine the total flow area (TFA) of the bit.

By the definition, you must sum every nozzle together in order to get the TFA; therefore, you can apply the formula above into this form.

Total Flow Area = (102 + 102 + 102 + 122 + 122 )÷ 1303.8

Total flow area = 0.451 square inch

Metric Unit

Flow Area = N2 × π ÷  4


Flow Area for each nozzle in square millimeter 

N is nozzle size in millimeter .


The bit that has a total of 5 nozzles. Three nozzles have a diameter of 7.94 mm and other 2 nozzles are 9.53 mm diameter.

Determine the total flow area (TFA) of the bit.

Total Flow Area = (7.942 + 7.942 + 7.942 + 9.532 + 9.53 2 ) × π ÷  4

Total flow area =  291 square mm

Download Excel Spreadsheet for Total Flow Area Table in both oilfield and metric unit.

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.

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14 Responses to Total Flow Area (TFA)

  1. Andeem Malik says:

    It realy very good.

  2. Andeem Malik says:

    its useful.

  3. louis hilliard says:

    how do you calculate the best TFA for a given bit size.

  4. mike says:

    Theres a minor typo up there, Pi = 3.14159 not 3.1429.
    Usefull info here non the less.

  5. habib says:

    TFA information

  6. dinesh says:

    v saying in bit nozzele dia 10/32 and 12/32 .in that what represent 32..can u tel me…

  7. Juan says:

    How is a flow restrictor calculated into the TFA? The flow restrictor would be at the mud motor therefore what is the true Pressure Drop Across the Bit behind the flow restrictor? i believe you have to take the Bit TFA into account, but how are both the bit nozzles and the flow restrictor calculated into it all?

    • In this post, the calculation is based on the bit nozzle area. If you want to know the flow restrictor for each particular equipment, you need to contact service company to get it. They will have the technical information showing you pressure loss with specified flow rate, mud type, etc.

  8. mark says:

    how is the formula simplifies i didnt get it

  9. Brett says:

    Mark (D/32)^2 = D^2/1024

    so (pi*D^2)/(4*1024)=TFA


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