# Significant Difference b/w dose of Cell Tally and Mesh Voxel Result

**URL:** <https://openmc.discourse.group/t/significant-difference-b-w-dose-of-cell-tally-and-mesh-voxel-result/5962>\
**Category:** User Support\
**Created:** [August 21, 2025, 6:51am UTC](https://openmc.discourse.group/t/significant-difference-b-w-dose-of-cell-tally-and-mesh-voxel-result/5962 "2025-08-21T06:51:48Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![NrnAktr](https://avatars.discourse-cdn.com/v4/letter/n/dbc845/32.png) [@NrnAktr](https://openmc.discourse.group/u/NrnAktr)\
**Post date:** [August 21, 2025, 6:51am UTC](https://openmc.discourse.group/t/significant-difference-b-w-dose-of-cell-tally-and-mesh-voxel-result/5962/1 "2025-08-21T06:51:48Z")

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i am modeling a shielding problem and my model includes tallies both with cell filter and mesh filter. but the difference of dose is about order of 3. Probably I am missing something while calulating dose using Mesh filter.

**My Mesh tallies:** Dose\_tally\_2 = openmc.Tally(name=“dose tally\_2”)  
Dose\_tally\_2.filters = [mesh\_filter, energy\_function\_filter\_p ,particle\_filter]  
Dose\_tally\_2.scores = [“flux”]  
Dose\_tally\_2.estimator = ‘tracklength’  
Dose\_tally\_2.id = 5

**My Cell tallies:** photon\_filter = openmc.ParticleFilter([‘photon’])  
dose\_tally.filters = [cells\_filter, energy\_function\_filter\_p,photon\_filter]  
dose\_tally.scores = [‘flux’]  
dose\_tally.estimator = ‘tracklength’  
dose\_tally.id = 6

For final result I am dividing cell-tally by respective volume. The Cell filter results matches with reference values.

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**Author:** ![yajat](https://avatars.discourse-cdn.com/v4/letter/y/c67d28/32.png) [@yajat](https://openmc.discourse.group/u/yajat)\
**Post date:** [August 25, 2025, 5:39am UTC](https://openmc.discourse.group/t/significant-difference-b-w-dose-of-cell-tally-and-mesh-voxel-result/5962/3 "2025-08-25T05:39:07Z")

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Hello @Ghotme

I think the default tally unit in OpenMC is not divide by volume as this is also written in the documentation that unit of flux tally is n-cm. Can you please clarify if the flux tally is indeed divided by volume inherently

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**Author:** ![yajat](https://avatars.discourse-cdn.com/v4/letter/y/c67d28/32.png) [@yajat](https://openmc.discourse.group/u/yajat)\
**Post date:** [August 25, 2025, 5:40am UTC](https://openmc.discourse.group/t/significant-difference-b-w-dose-of-cell-tally-and-mesh-voxel-result/5962/4 "2025-08-25T05:40:05Z")

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Hello @NrnAktr

How did you calcualate the final flux tally in mesh filter, i.e. did you divide by the individual mesh volume?

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**Author:** ![NrnAktr](https://avatars.discourse-cdn.com/v4/letter/n/dbc845/32.png) [@NrnAktr](https://openmc.discourse.group/u/NrnAktr)\
**Post date:** [August 27, 2025, 9:21am UTC](https://openmc.discourse.group/t/significant-difference-b-w-dose-of-cell-tally-and-mesh-voxel-result/5962/6 "2025-08-27T09:21:47Z")

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Thanks @yajat and @Ghotme . I got the error. I was extracting results in wrong way.

@Ghotme However, **OpenMC tallies are not already normalized by the region or voxel volume like in other codes.**