Ability to create custom fields which automatically calculate their values based on other fields.
For example Field A = Field B * 0.2
or Field A = sum all values in field 'X' for all issues of 'issue type' in project ABC

There are plugins that do this for self-hosted JIRA.

Workarounds

Automation

In Cloud, you can use custom fields in mathematical functions but it will require setting up some Automation rules. We document this in the following articles and Community posts:

• Jira smart values - math expressions
• Increment Custom Field on Transition
• Examples of using math expression smart values

You will just need to keep in mind the following limitations on executions in Automation for Jira Cloud:

In Jira Cloud, automation functionality is included out of the box. However, there are limits to how many times rules can be executed per month, depending on which plan you’re on:

• Free plan: There is a limit of 100 executions for all global and multi-project rules.

• Standard plan: There is a limit of 500 executions for all global and multi-project rules.
• Premium plan: There is a limit of 1,000 executions per user per month, pooled across all users and products. For example, if you have 200 users in your Jira Cloud Premium license, then you’ll have a limit of 200,000 global/multi-project rule executions per month.

Forms

Add-ons

There are also several add-ons in our Marketplace that you can use for performing maths functions on custom field values:

• sumUp for Jira
• Abacus - Calculate Jira 
• ScriptRunner for Jira and specifically their feature which allows you to Calculate custom field on issue update
• You can browse other similar add-ons here

Please note that the above add-ons are neither built nor maintained by Atlassian. If you have product-specific questions about them, we will ask that you contact each add-on's Support team, which you can find on the above Marketplace pages.