What is the ideal bulk size formula in ElasticSearch?

Read ES bulk API doc carefully: https://www.elastic.co/guide/en/elasticsearch/guide/current/indexing-performance.html#_using_and_sizing_bulk_requests

• Try with 1 KiB, try with 20 KiB, then with 10 KiB, ... dichotomy
• Use bulk size in KiB (or equivalent), not document count !
• Send data in bulk (no streaming), pass redundant info API url if you can
• Remove superfluous whitespace in your data if possible
• Disable search index updates, activate it back later
• Round-robin across all your data nodes

I derived this information from the Java API's BulkProcessor class. It defaults to 1000 actions or 5MB, it also allows you to set a flush interval but this is not set by default. I'm just using the default settings.

I'd suggest using BulkProcessor if you are using the Java API.

I was searching about it and i found your question :) i found this in elastic documentation .. so i will investigate the size of my documents.

It is often useful to keep an eye on the physical size of your bulk requests. One thousand 1KB documents is very different from one thousand 1MB documents. A good bulk size to start playing with is around 5-15MB in size

There is no golden rule for this. Extracted from the doc:

There is no “correct” number of actions to perform in a single bulk call. You should experiment with different settings to find the optimum size for your particular workload.