Embedding Index Storage Cost Calculator

How the Embedding Storage Calculator Works

The calculator models an embedding index as a dense matrix of size N × D, where N is the number of vectors and D is the embedding dimension. Each entry in this matrix is stored with a given numeric precision, expressed as bits per value. On top of this raw matrix, most practical vector indexes add overhead for structures such as graphs or inverted lists, plus metadata for IDs and bookkeeping.

At a high level, the computation proceeds in four steps:

1. Compute raw storage needed for the dense embeddings in bytes.
2. Apply an index overhead percentage to capture additional index structures.
3. Convert the result to gigabytes (GB).
4. Multiply by the per-GB monthly storage price and the number of replicas.

Formulas

Let:

• N = number of vectors
• D = embedding dimension
• b = bits per value
• o = index overhead percentage
• c = storage cost per GB per month
• r = number of replicas

First, compute the raw storage in bytes:

Next, apply the index overhead percentage:

Convert bytes to gigabytes. The calculator uses 1 GB = 1,073,741,824 bytes (2³⁰):

Finally, compute total monthly cost including replicas:

Field-by-Field Explanation

The calculator inputs map directly to common design choices in embedding-based systems.

• Number of Vectors: the total number of items you plan to index, such as documents, passages, products, or user profiles. For passage-level indexes, this can be much larger than the number of source documents.
• Embedding Dimension: the length of each embedding vector. Typical values are 384, 512, 768, or 1024, depending on the model. Higher dimensions usually improve recall to a point but increase memory usage linearly.
• Bits per Value: numeric precision for each vector component. Common options include:
  • 32-bit float (standard single-precision FP32)
  • 16-bit float (half-precision FP16 or bfloat16)
  • 8-bit quantized values (aggressive compression)
  Lowering precision reduces memory but may slightly degrade similarity accuracy depending on your workload and quantization scheme.
• Index Overhead (%): percentage that approximates additional memory beyond the raw vectors. This captures graph edges (HNSW), inverted lists (IVF), posting lists, and metadata. In practice, overhead can range from under 10% to well over 100% depending on the index type and configuration.
• Storage Cost per GB per Month ($): your infrastructure price for storing 1 GB of data for one month. For cloud object storage this is often in the range of $0.01–$0.03/GB, while high-performance SSD or RAM-backed caches are substantially more expensive.
• Number of Replicas: how many full copies of the index you maintain. Extra replicas are used for high availability, fault tolerance, or parallel query throughput. Doubling replicas roughly doubles both storage and cost.

Worked Example

Consider an index with the following parameters:

• Number of vectors: 1,000,000
• Embedding dimension: 768
• Bits per value: 16
• Index overhead: 10%
• Storage cost: $0.02 per GB per month
• Replicas: 1

Raw storage in bytes:

M = 1,000,000 × 768 × (16 / 8) = 1,000,000 × 768 × 2 = 1,536,000,000 bytes

Apply 10% overhead:

Mt = 1,536,000,000 × (1 + 10 / 100) = 1,689,600,000 bytes

Convert to GB (using 1,073,741,824 bytes per GB):

GperReplica ≈ 1,689,600,000 / 1,073,741,824 ≈ 1.57 GB

Monthly cost for one replica:

Cost ≈ 1.57 × 0.02 × 1 ≈ $0.03 per month

This example shows that even a million 16-bit, 768-dimensional vectors can be surprisingly cheap to store on commodity storage. However, as you scale to hundreds of millions or billions of vectors, or use more expensive storage tiers, costs grow quickly.

Precision and Cost Comparison

The table below illustrates how vector precision affects storage and cost for a fixed index size. It assumes:

• Number of vectors: 1,000,000
• Embedding dimension: 768
• Index overhead: 10%
• Storage cost: $0.02 per GB per month
• Replicas: 1

In practice, you would also consider the impact on recall and ranking quality. For many workloads, 16-bit embeddings are a good balance between accuracy and memory efficiency. Aggressive 8-bit quantization provides substantial savings but usually requires experimentation to verify that end-to-end metrics remain acceptable.

Interpreting the Results

When you run the calculator, you will typically see at least two outputs:

• Total storage per replica (GB)
• Total monthly storage cost across replicas

Use these values to:

• Decide whether your index can fit entirely in RAM, needs to spill to SSD, or must use object storage.
• Compare alternative embedding dimensions or precision schemes (e.g., 768-d at 16-bit vs 1024-d at 8-bit).
• Estimate the impact of adding replicas for availability or latency (e.g., deploying in multiple regions).
• Sanity-check infrastructure quotes or internal storage allocations for upcoming projects.

If the numbers are higher than expected, experiment by reducing the number of stored items (e.g., indexing chunks instead of full documents), tightening retention windows, or lowering the embedding precision. Conversely, if storage is inexpensive relative to your budget, you may prioritize higher recall by using larger models or more generous index configurations.

Assumptions and Limitations

This calculator is intentionally simplified. It is designed to be transparent and easy to reason about rather than perfectly match every vendor implementation. Key assumptions and limitations include:

• Dense vectors only: the model assumes every vector has D non-zero components. Sparse or hybrid representations are not modeled.
• No compression beyond bits-per-value: techniques like product quantization, vector compression codecs, or delta encoding are approximated only through your choice of bits per value and overhead percentage.
• Simplified overhead: index overhead is treated as a single percentage applied to raw storage. Real-world overhead depends heavily on the index type (e.g., HNSW vs IVF), tuning parameters, and implementation details.
• Storage cost only: bandwidth, request charges, compute costs for queries, and background maintenance (rebuilds, compaction) are not included. For many deployments, these can exceed pure storage cost.
• Static snapshot: the calculator assumes a steady-state number of vectors. Growth over time, churn, and temporary duplication during reindexing are not explicitly modeled.
• Vendor-specific features ignored: tiered storage, automatic replication, backups, and retention policies vary widely across providers, so the numbers here should be treated as approximate guidance only.

Always cross-check the results against your vendor’s documentation and pricing calculators, and consider adding a safety margin when using these estimates for budgeting or capacity planning.

Practical Usage Tips

• Start with rough estimates for N and D based on your current data, then scale them up to cover 6–12 months of expected growth.
• Try multiple bits-per-value settings to see how much memory you can save by lowering precision, and then evaluate the impact on your quality metrics in a separate experiment.
• Run the calculator for different replica counts to understand the storage implications of active-active multi-region designs versus a single-region setup.
• Use a conservative overhead percentage the first time you plan a new index type, then refine it using actual memory consumption from a prototype deployment.