# Neumann condition
**URL:** https://community.freefem.org/t/neumann-condition/1903
**Category:** General Discussion
**Created:** [July 20, 2022, 10:18am UTC](https://community.freefem.org/t/neumann-condition/1903 "2022-07-20T10:18:50Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Jamal](https://community.freefem.org/u/Jamal)
#### Post date: [July 20, 2022, 10:18am UTC](https://community.freefem.org/t/neumann-condition/1903/1 "2022-07-20T10:18:50Z")
Hello everyone.
how to calculate the outward normal vector.
Because I want to find the value of Neumann condition.
When I use :
Neumann= dx(Uex)\*N.x+dy(Uex)\*N.y
I found
Neumann=
000000
000000
…
NB: Uex is the exact solution
Thanks.
---
### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [July 22, 2022, 9:42am UTC](https://community.freefem.org/t/neumann-condition/1903/2 "2022-07-22T09:42:31Z")
The problem is by default N.X and N.Y is not defined correctly in general.
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### Author:  [@Jamal](https://community.freefem.org/u/Jamal)
#### Post date: [July 22, 2022, 3:13pm UTC](https://community.freefem.org/t/neumann-condition/1903/3 "2022-07-22T15:13:46Z")
Merci cher professeur Frédéric pour votre réponse
Donc il n’existe pas un macro qui permet de calculer la condition de Neumann.
Cordialement.
---
### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [July 23, 2022, 10:16am UTC](https://community.freefem.org/t/neumann-condition/1903/4 "2022-07-23T10:16:00Z")
You can lnear form to evaluate Neuman Condition on Poisson equation.
```auto
varf vNeuman(used,v) = int1d(Th)(v*(dx(Uex)*N.x+dy(Uex)*N.y)/lenEdge);
Vh vn; // Finite element function to store the Normal derivative
vn[]= vNeuman(0,Vh);
plot(vn,wait=1);
```