How to set new value in bulk using formulas?

The following is a step-by-step guide that explain how to set a new field value in bulk using a custom formula on a subset of your records / all records (without coding).
The following example will show you how to set a new field value in mass by using a custom formula on a subset of your records / all your records (across all the list-pages). The following process can work on any object & field – we built the following example for a custom “SLA” field located under the standard Account object.
Creating a custom formula and using it in Mass:
1. Optional Step: Backup your data via Setup>Data Management>Data Export (you will be able to revert / compare)
2. Create an update-action-process using Salesforce’s process builder and add the desired formula to populate your field (setup>create>Workflow & Approvals>Process Builder> use the update record action):

3. Go to smart-mass-update>accounts tab & click on the mass update button:
Before the update:

Choose the same field you chose in step#2 & mass update all the rows (grid filters will be respected):

The result:

5. Disable the process/flow – in case you don’t need that for all the rows.
Mass update in action: