---
title: "Compute a molecular formula's weight and composition"
method: POST
path: "/v1/chemistry/formula/weight"
tags: ["chemistry"]
---

# Compute a molecular formula's weight and composition

`POST /v1/chemistry/formula/weight`

## Request

- Base URL: `https://api.snipget.ai` (the document declares no server; this is the origin it was published from)
- URL: `https://api.snipget.ai/v1/chemistry/formula/weight`
- Auth: none declared

Parses a molecular formula (with nested parentheses/brackets and multipliers, e.g. 'C9H8O4', 'Ca(OH)2', 'Fe2(SO4)3') and returns its molecular weight, monoisotopic weight, per-element counts, and mass composition. Fully local — bundled IUPAC/CODATA atomic weights, no upstream. Unparseable formula or unknown element -> confidence 0.0. Charges 1 cost_unit.

## Query parameters

- `include_receipt` boolean

## Headers

- `authorization` string, nullable
- `X-API-Key` string, nullable

## Request body

- FormulaWeightRequest
  - `formula` string, required: The molecular formula to weigh (e.g. 'C9H8O4', 'Ca(OH)2').
  - `include_trace` boolean

## Response `200`

Successful Response

- ChemistryFormulaWeightResponse: Success envelope for `/v1/chemistry/formula/weight` — `result` carries this utility's precise shape.
  - `status` string
  - `confidence` number, required
  - `result` object, required: Result of :func:`weight`. ``input``, ``formula`` and ``is_valid`` are always present; the computed fields appear on a clean parse and ``why_invalid`` on a failure. ``monoisotopic_weight`` is present only when every element in the formula has a stable isotope.
    - `input` string, required
    - `formula` string, required
    - `is_valid` boolean, required
    - `why_invalid` string
    - `molecular_weight` number
    - `monoisotopic_weight` number
    - `element_counts` object
    - `element_composition_pct` object
  - `meta` ResponseMeta: Metadata attached to every successful response.
    - `version` string
    - `build` string
    - `elapsed_ms` integer
    - `cost_units` integer: Cost units charged for this call: one per item processed (a batch of N items = N; live-registry lookups bill 2 per item). A call is one request; cost units are the billing measure.
    - `request_id` string, nullable
    - `trace` string[], nullable
    - `rate_limit_remaining` integer, nullable
    - `rate_limit_reset` integer, nullable
    - `quota_remaining` integer, nullable: Cost units left in the plan's monthly included bucket before this call's charge lands (None on unlimited tiers).
    - `quota_reset` integer, nullable: Unix timestamp when the monthly included bucket resets.
    - `credit_remaining_usd` number, nullable: Prepaid overage-allowance balance in USD, populated once the included cost units are used up and this call is drawing on it.
    - `receipt` ReceiptMeta: Opt-in execution receipt (ADR #68 B1) — caller-verifiable hashes. Requested with the ``include_receipt=true`` query parameter (free, like ``include_trace``). Reproduce either hash with any RFC 8785 (JCS) library: ``input_hash = sha256(jcs({"tool": <request path>, "args": <your JSON body minus include_trace/include_receipt>}))``; ``result_hash = sha256(jcs(result))``. For deterministic tools (everything except the generators) the same input on the same ``build`` reproduces ``result_hash`` — disputes are honored by replay.
      - `receipt_id` string, required
      - `build` string, required
      - `input_hash` string, nullable
      - `result_hash` string, nullable
      - `error` string, nullable
      - `message` string, nullable

## Other responses

- `400`: Input was syntactically valid but semantically unusable (`INVALID_INPUT`). Not retryable without changing the input.
- `401`: Missing or unrecognized API key. Send it as `Authorization: Bearer <key>` or `X-API-Key: <key>`. Keys are issued at https://snipget.ai.
- `403`: The key is valid but this caller may not use it: the request IP is outside the key's allowlist (`IP_NOT_ALLOWED`) or the account is deactivated (`ACCOUNT_DEACTIVATED`).
- `422`: The request body failed schema validation (`INVALID_REQUEST`). `details` lists the per-field errors. Not retryable without changing the request.
- `429`: Two distinct conditions share this status — branch on `error_code`. **`RATE_LIMITED`** is a per-second throughput throttle: retry after `retry_after_seconds` (the precise float; the `Retry-After` header is the same value rounded up). **`QUOTA_EXCEEDED`** is monthly capacity and is NOT retryable: recovery depends on `limit_type` — upgrade (`monthly_quota`), buy an overage allowance (`included_exhausted`), or top up (`overage_balance_exhausted`).
- `503`: A transient, retryable service condition — branch on `error_code`. **`MAINTENANCE_MODE`**: an admin-toggled maintenance window (auth, portal, and webhook surfaces stay up). **`UPSTREAM_UNAVAILABLE`**: a utility's external data source (PubChem, RxNorm, ClinicalTrials.gov, the FX feed, NPPES…) is down, blocking us, or timed out. **`UPSTREAM_RATE_LIMITED`**: that external source is throttling Snipget (it returned a 429) — the caller's own rate is fine, so this is NOT our `RATE_LIMITED` (429); it's a service-side throttle. All three are retryable; wait for `retry_after_seconds` / the `Retry-After` header when present.

---

[API](https://skmtc.dev/snipget/apis/snipget-api.md) · [All operations](https://skmtc.dev/snipget/apis/snipget-api/llms.txt) · [OpenAPI document](https://skmtc.dev/snipget/apis/snipget-api/revisions/ed1077fb1bcb?raw)