How to calculate Epic progress based on children's statuses?

[Coco S Chaussée]

I need to calculate the % complete of epics based on the total number of children divided by the number of completed (Done) children.

I would like the result to be entered into a custom field that I can pass on to an integrated application.

How do I do this?  

No plug-ins please.

Thank you in advance!

[Somesh Sahoo]

Calculating Epic Progress Percentage 
https://youtu.be/C6vZR0M1ScQ

[Mohanraj Thangamuthu]

Hello, Good day. Instead of adding the value to the custom field, have you explore "Epic report" and "Epic burndown report" ?

[Coco S Chaussée]

Thanks, but I need the result in a field in order to map it to an external system.

I got the response on another thread

https://community.atlassian.com/t5/Jira-questions/Re-Re-Automation-Rule-calculate-progress-based-on-chi/qaq-p/2547122/comment-id/599442#M599442