# Great difference between ExecutionTime and ClockTime

**URL:** <https://www.simscale.com/forum/t/great-difference-between-executiontime-and-clocktime/81280>\
**Category:** Fluid Flow / CFD\
**Created:** [December 16, 2017, 3:49pm UTC](https://www.simscale.com/forum/t/great-difference-between-executiontime-and-clocktime/81280 "2017-12-16T15:49:02Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![mjosic](https://www.simscale.com/forum/letter_avatar/mjosic/32/5_5575768a8748004e209b776fc1b2916d.png) [@mjosic](https://www.simscale.com/forum/u/mjosic)\
**Post date:** [December 16, 2017, 3:49pm UTC](https://www.simscale.com/forum/t/great-difference-between-executiontime-and-clocktime/81280/1 "2017-12-16T15:49:02Z")

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Hello SimScaler’s

in my Simulation the difference between ExecutionTime and clockTime is around 40%?

ExecutionTime = 664.71 s ClockTime = 1130 s\_  
End\_

i used 1 processor for a mesh with 13k cells and the solver was  
rhoCentralFoam.

And which time will be used for calculating the cpu-hours?

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**Author:** ![Get\_Barried](https://www.simscale.com/forum/user_avatar/www.simscale.com/get_barried/32/20599_2.png) [@Get\_Barried](https://www.simscale.com/forum/u/Get_Barried)\
**Post date:** [December 16, 2017, 6:23pm UTC](https://www.simscale.com/forum/t/great-difference-between-executiontime-and-clocktime/81280/2 "2017-12-16T18:23:55Z")

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Hi @mjosic!

Maybe this **[link](https://www.simscale.com/forum/t/what-is-the-definition-of-a-core-hour/237)** might help you better understand the definition of what core hours are.

Cheers.

Regards,  
Barry