---
id: rank-bm25
version: "0.2.2"
license: Apache2.0
license_treatment: permissive
maintenance: active
---

# rank-bm25 — Various BM25 algorithms for document ranking

License: permissive · Maintenance: active · Downloads: 8.9M/mo

## What it is and what it does

Rank-BM25 provides implementations of the BM25 family of ranking algorithms—Okapi BM25, BM25L, and BM25+—for scoring how relevant documents are to a search query. It takes a corpus of pre-tokenized documents, builds an index, and then scores or ranks documents against tokenized queries using probabilistic relevance models. The package is intentionally minimal: it does not handle text preprocessing like lowercasing, stemming, or stopword removal, leaving those decisions to the caller.

The typical workflow is to tokenize your document corpus and query using your chosen preprocessing pipeline, initialize a BM25 class with the tokenized corpus, then call get_scores() to retrieve relevance scores or get_top_n() to retrieve the highest-ranking documents. It's commonly used to build search engines or to rank candidate documents in information retrieval pipelines.

Use it for:

- Build a lightweight full-text search engine for a document collection without external infrastructure.
- Rank candidate documents in a retrieval-augmented generation (RAG) pipeline before passing to a language model.
- Score document relevance in a question-answering system to find the most relevant passages.
- Implement search functionality in a web application where you control the preprocessing and indexing.
- Benchmark BM25 variants against each other on your own corpus to evaluate ranking quality.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Implements BM25 ranking algorithms (Okapi BM25, BM25L, BM25+) to score and rank documents by relevance to a query.

Yes. Low install friction, no security vulnerabilities, permissive license, active maintenance, and a focused, well-documented implementation of a standard algorithm. Install if you need BM25 ranking and want to control preprocessing yourself; skip if you need a full-featured search engine with built-in text processing.

## Install

pip install rank-bm25
uv add rank-bm25
poetry add rank-bm25

## Installing rank-bm25

Before you install: Low friction install with a single runtime dependency (numpy). Repository is active with recent commits and steady maintenance since 2019.

License in practice: Apache2.0 permissive license allows commercial and private use with minimal restrictions.

Quickstart:

pip install rank-bm25

from rank_bm25 import BM25Okapi

corpus = ["Hello there good man!", "It is quite windy in London"]
tokenized_corpus = [doc.split(" ") for doc in corpus]
bm25 = BM25Okapi(tokenized_corpus)

query = "windy London"
tokenized_query = query.split(" ")
scores = bm25.get_scores(tokenized_query)

Package expects pre-tokenized input (list of token lists); you must handle text preprocessing (lowercasing, stopword removal, stemming) yourself before passing to BM25.

Verify before relying:

- Whether BM25-Adpt and BM25T algorithms mentioned as unimplemented are planned for future releases.
- Current test coverage and benchmarking against other ranking libraries.
- Performance characteristics on large corpora (memory usage, query latency).

## Package facts

- License: Apache2.0 (permissive)
- Python support: unspecified
- Install friction: low
- Maintenance: active
- Downloads: 8.9M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

BM25 document ranking, search relevance scoring, information retrieval algorithms, query document matching, text search ranking, okapi bm25 implementation, document similarity scoring, information-retrieval, search-ranking, bm25

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