# How to compute flow rate?

**URL:** https://discourse.paraview.org/t/how-to-compute-flow-rate/5300
**Category:** ParaView Support
**Created:** [September 9, 2020, 9:41am UTC](https://discourse.paraview.org/t/how-to-compute-flow-rate/5300 "2020-09-09T09:41:07Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![frodooon](https://discourse.paraview.org/user_avatar/discourse.paraview.org/frodooon/32/4870_2.png) [@frodooon](https://discourse.paraview.org/u/frodooon)
#### Post date: [September 9, 2020, 9:41am UTC](https://discourse.paraview.org/t/how-to-compute-flow-rate/5300/1 "2020-09-09T09:41:07Z")

</div>

Hello,  
I noticed that this question has been asked several times, but I still haven’t seen an answer that would help me really understand what is going on, and if I am doing something wrong.  
I would like to compute the volume or mass flow rate through a surface (typically, a surface obtained by contour of given variable).  
In order to get my ideas straight, I thought I’d do that on a very simple case (the openFOAM “windAroundBuildings” tutorial, to be accurate), taking the boundaries of a steady incompressible CFD computation. Since I have then an enclosed region (a box, basically), I would expect that the net flow rate through all boundaries would be close to zero.  
Well, I tried both using the SurfaceFlow filter on these boundaries and using GenerateSurfaceNormals \> Calculator with (U dotproduct Normals) \> IntegrateVariables. Both operations give more or less the same result, a value of around 9300.

Now, I get that being a quite coarse and simple tutorial case, it might not be converged, but my question is: Is my understanding correct that I _should_ obtain a zero value instead? (_if_ perfectly converged) In other words, are the operations I am performing within Paraview the correct way to obtain the flow rate through a surface?

Thanks a lot,  
Arnaud