Developer & Tech
RAG Chunking Calculator
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How to use it
Using the rag chunking calculator
- 01
Measure the corpus in tokens
Use our token counter on representative documents rather than word counts.
- 02
Choose chunk and overlap
512/64 is a common starting point; retrieval quality should tune it, not habit.
- 03
Price the embedding run
Stored tokens include duplication; the honest figure for API budgeting.
Good to know
Why overlap exists
Fixed cuts slice sentences and ideas in half. Overlap gives each chunk a running start into its neighbor’s territory, so a fact straddling a boundary lives fully inside at least one embedded chunk. The cost is duplicate storage; visible here as an overhead percentage.
Bigger chunks vs better retrieval
Larger chunks preserve more context per hit but blur relevance signals and burn query-time context. Smaller chunks retrieve precisely but fragment reasoning. Practitioners commonly sweep 256–1024 tokens against real queries instead of trusting defaults.
How it's calculated
The math behind this calculator
chunks = ⌈(doc − overlap) ÷ (chunk − overlap)⌉
stride = chunk − overlap stored = chunks × chunkOverlapping windows advance by the stride (chunk minus overlap) each step, so a document of D tokens with chunk C and overlap O yields ceil((D − O) ÷ (C − O)) chunks. Every chunk stores its full size including duplicated context, and multiplying stored tokens by your embedding price prices the indexing run.
Assumptions & limitations
- Sliding fixed-size windows; semantic/sentence-aware splitters differ.
- Overlap must be strictly smaller than chunk size.
- Embeddings billed per token at your entered rate.
Worked example
A 10,000-token document split into 512-token chunks with 64-token overlap yields 23 chunks (ceil((10000 − 64) / (512 − 64))); about $0.0002 of embeddings at $0.02/Mtok.
FAQ
Frequently asked questions
- Why was my equal overlap rejected?
- Overlap ≥ chunk size means zero stride; the window never advances and chunking never terminates. Progress requires strict overlap < chunk.
- Does chunking affect query cost too?
- Yes; retrieved chunks enter the prompt. More, smaller chunks raise per-query input tokens even though they sharpen retrieval.
- Is fixed-size splitting optimal?
- Rarely perfect. Sentence- and section-aware splitters respect structure; this calculator still approximates their output well for planning.
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