# Problem understanding the unit of flux **URL:** \ **Category:** User Support\ **Created:** [February 27, 2020, 6:40am UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560 "2020-02-27T06:40:00Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Yasmin\_Prity](https://avatars.discourse-cdn.com/v4/letter/y/3ab097/32.png) [@Yasmin\_Prity](https://openmc.discourse.group/u/Yasmin_Prity)\ **Post date:** [February 27, 2020, 6:40am UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/1 "2020-02-27T06:40:00Z")
What is the unit of flux given in tallies.out? ---
**Author:** ![Pranto](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/pranto/32/489_2.png) [@Pranto](https://openmc.discourse.group/u/Pranto)\ **Post date:** [February 27, 2020, 12:46pm UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/2 "2020-02-27T12:46:49Z")
Score unit in OpenMC is " per source particle". And the unit of flux in tallies.out is “neutrons-cm/source”. For further information , go to openmc manual in details. Best regards Pranto ---
**Author:** ![tanzim\_ttanzim](https://avatars.discourse-cdn.com/v4/letter/t/f4b2a3/32.png) [@tanzim\_ttanzim](https://openmc.discourse.group/u/tanzim_ttanzim)\ **Post date:** [February 28, 2020, 9:37am UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/3 "2020-02-28T09:37:19Z")
[https://groups.google.com/forum/#!msg/openmc-users/r5JtjyH9BIM/Cyg-djYABQAJ](https://groups.google.com/forum/#!msg/openmc-users/r5JtjyH9BIM/Cyg-djYABQAJ) You can also check this post.May I ask where you are from ---
**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\ **Post date:** [February 28, 2020, 12:36pm UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/4 "2020-02-28T12:36:49Z")
Here’s a link to the relevant section of the user’s manual that outlines what units are used for each score. [https://docs.openmc.org/en/stable/usersguide/tallies.html#scores](https://docs.openmc.org/en/stable/usersguide/tallies.html#scores) Best, Paul ---
**Author:** ![Yasmin\_Prity](https://avatars.discourse-cdn.com/v4/letter/y/3ab097/32.png) [@Yasmin\_Prity](https://openmc.discourse.group/u/Yasmin_Prity)\ **Post date:** [February 29, 2020, 12:03pm UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/5 "2020-02-29T12:03:39Z")
When i draw flux vs z position curve using spyder, it shows unit n/src.What does it mean? Also since neutrons/cm2.sec is the unit of flux, how could neutrons.cm/src be the unit? ---
**Author:** ![Pranto](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/pranto/32/489_2.png) [@Pranto](https://openmc.discourse.group/u/Pranto)\ **Post date:** [February 29, 2020, 12:31pm UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/6 "2020-02-29T12:31:22Z")
OpenMC calculate flux using " Collision estimator Tally". In monte carlo, history of neutrons are weighted by " per source particle". OpenMC accumulates 1/Sigma\_t which has units of cm, so units are like " neutrons.cm/source". Read theory and methodology part from openmc manual [https://docs.openmc.org/en/stable/methods/index.html](https://docs.openmc.org/en/stable/methods/index.html) Best regards Pranto ---
**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\ **Post date:** [March 11, 2020, 4:00pm UTC](https://openmc.discourse.group/t/problem-understanding-the-unit-of-flux/560/7 "2020-03-11T16:00:54Z")
Just to add on to the discussion here – OpenMC, like all other neutron transport codes, can not give you an **absolute** value of a flux (in units of neutrons/cm^2-sec as was suggested) without knowing what the actual source in the problem is. In a k-eigenvalue calculation where the source comes from fission, the actual source rate is really determined by the power level, which is something that the code doesn’t know about. Another way to think about this is from textbook reactor physics; if you solve the diffusion equation in a slab, everyone knows [you get a cosine shape](https://www.nuclear-power.net/nuclear-power/reactor-physics/neutron-diffusion-theory/infinite-slab-reactor/), but there is also a constant out front that determines the magnitude. The analogy here is that OpenMC will give you that cosine shape, but it can’t give you the magnitude. There are other posts in this mailing list discussing how to go from the units that OpenMC gives you to “real” units (neutrons/cm^2-sec). Have a look [here](https://groups.google.com/forum/#!searchin/openmc-users/units%7Csort:date/openmc-users/r5JtjyH9BIM/Cyg-djYABQAJ), for example. Best regards, Paul