# What is the BM 25's way to disable field length norm as in TF/IDF

**URL:** <https://discuss.elastic.co/t/what-is-the-bm-25s-way-to-disable-field-length-norm-as-in-tf-idf/76907>\
**Category:** Elasticsearch\
**Created:** [March 1, 2017, 3:44am UTC](https://discuss.elastic.co/t/what-is-the-bm-25s-way-to-disable-field-length-norm-as-in-tf-idf/76907 "2017-03-01T03:44:03Z")\
**Posts on this page:** 1\
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**Author:** ![Youxu](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/youxu/32/117375_2.png) [@Youxu](https://discuss.elastic.co/u/Youxu)\
**Post date:** [March 1, 2017, 10:06am UTC](https://discuss.elastic.co/t/what-is-the-bm-25s-way-to-disable-field-length-norm-as-in-tf-idf/76907/2 "2017-03-01T10:06:55Z")

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I found this [https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html](https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html)

b: Controls to what degree document length normalizes tf values. The default value is 0.75.

I think this is what I want.

Come back to the "norms" setting approach, seems it not only disable field length norm, but also disable all other normalization factors. So if I just want to disable field length norm, I should use custom similarity with b=0.

Can any expert confirm?

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